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Cookie Clicker

Cookie Clicker: Simple, stupid, and yet strangely addictive. In it you must click a cookie (hence the title). Once you have enough cookies, you can spend them on items that will produce cookies for you. Eventually you will be getting hundreds, then thousands, then millions of cookies per second.

There are a few different items that you can buy for cookies: A clicker (that clicks the cookies for you), a grandma (that bakes the cookies for you), a farm (that grows cookies for you), a factory (that mass produces cookies for you), a mine (that will mine and process veins of dough for you), a shipment (that ships cookies from other planets to you), an alchemy lab (that transforms gold into cookies), and others that we won't worry about.

Let's golf a simplified Cookie Clicker.


Write a full program. Your program should always display the number of cookies as a whole integer. Every second, your program should add the current cookies per second (defaulted to 0) to the cookie count.

  • When the spacebar is pressed, it adds the base click amount (defaulted to 1) to your cookie count.

  • When the key "1" is pressed, if there are 10 or more cookies in the cookie count, subtracts the cookie count by 10 and adds 0.1 to the current cookies per second.

  • When the key "2" is pressed, if there are 100 or more cookies in the cookie count, subtracts the cookie count by 100 and adds 0.5 to the current cookies per second.

  • When the key "3" is pressed, if there are 500 or more cookies in the cookie count, subtracts the cookie count by 500 and adds 4 to the current cookies per second.

  • When the key "4" is pressed, if there are 3,000 or more cookies in the cookie count, subtracts the cookie count by 3,000 and adds 10 to the current cookies per second.

  • When the key "5" is pressed, if there are 10,000 or more cookies in the cookie count, subtracts the cookie count by 10,000 and adds 40 to the current cookies per second.

  • When the key "6" is pressed, if there are 40,000 or more cookies in the cookie count, subtracts the cookie count by 40,000 and adds 100 to the current cookies per second.

  • When the key "7" is pressed, if there are 200,000 or more cookies in the cookie count, subtracts the cookie count by 200,000 and adds 400 to the current cookies per second.

  • When the key "0" is pressed, if there are 50 or more cookies in the cookie count, subtracts the cookie count by 50 and adds 1 to the base click amount.

There is no input, only output, being changed every second to update the cookie count. No other key should do anything, so you cannot press enter after every key and have it do something.

Other information

  • This is code golf so shortest program in bytes wins.

Thoughts for sandbox

  • There are many, many more features I could add to this challenge if it is too simple. I feel that challenge entries for this will already be long enough.

  • I do not see many challenges that ask for constant input. Does this mean that this challenge is a bad idea?

  • Have I crossed a line?

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    \$\begingroup\$ Requiring real-time user input is definitely rare. One big reason is that it's fairly hard to do and (probably) often requires a library. Many if not most esolangs won't be able to do it at all. \$\endgroup\$ – El'endia Starman Oct 3 '15 at 3:08
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    \$\begingroup\$ A lot of people will ask if they can require the player to press enter after typing each number, so you should be explicit that this is not allowed. \$\endgroup\$ – feersum Oct 3 '15 at 3:30
  • \$\begingroup\$ It's a real shame that non-blocking terminal reading isn't easier to work around, although it has been done a few times: 1, 2, 3 it probably would be a bit of a barrier. I do like the sound of this though! \$\endgroup\$ – Dom Hastings Oct 8 '15 at 9:35
  • \$\begingroup\$ The list should show what is being bought here (instead you can skim down the introductory paragraph). \$\endgroup\$ – Paŭlo Ebermann Oct 11 '15 at 8:19

This is a raw draft about an idea for a popularity contest. Any input would be appreciated.

My watch, it has two buttons

I have this watch with two buttons and a display that can show six characters split into groups of two by colons like this


Each character is displayed by a 5x7 LCD-Matrix, so arbitrary ASCII-characters can be displayed.

I'd like to call the buttons "select" and "modify".

The problem is that the watch is dead. It needs a new operating system.

Since I'm not very trained at designing operating systems I want you to write an emulator for my watch. The emulator should be programmable using the following commands.

  • big letters A-Z represent short presses of "select" the corresponding number of times.
  • small letters a-z have the same meaning for the modify button.
  • < represents keeping "select" pressed for half a second (or something like that).
  • > represents the same for the modify button.
  • numbers in the code mean to wait for that number of hundredth of seconds.

You're free to program any kind of functionality into my watch, but it should at least be usable as a watch showing the time and as a stopwatch showing minutes, seconds and hundredth of seconds.

One thing I know about my watch is that it can be programmed to receive data from my stdin and send data to my stdout. So once the operating system is installed I could send data and a program to the watch and print the results of the execution to my console.


Spot the differences

Little Timmy is waking you up on this Saturday morning once again to help him solve his puzzles. You love the little bugger, but those Spot the Differences games are starting to undermine your patience. Like always, you plan to delegate this tedious task to Robotic Dad™ so you can better spend your time... planning for your child education? Yeah, I think that was the plan.
Anyway, you tell Timmy not to worry, that you're going to help him soon enough, grab a beer and sit in front of your computer to help your child solve those puzzles, once and for all.

Your task is to write code that will take two similar pictures which differ in a few spots and somehow output the differences between them. The format of the output is free, however a 5 year-old child should be able to get it.

Here are examples of input :

a glorious abstract realisation of mine, 1 a glorious abstract realisation of mine, 2

wikipedia's example, 1 wikipedia's example, 2

Since there is no formally defined output, this is a .
Please also keep in mind that you'd like to spend a little time sipping your beer calmly in front of your computer. In this regard, built-in solution should be regarded less highly.

Meta : I plan to post a community answer as an example output, linking to the http://franklinta.com/2014/11/30/image-diffing-using-css/ article which made me think of this challenge and using a snippet to illustrate it. Is it enough?

  • \$\begingroup\$ I don't understand the last sentence. If I want to sip beer calmly, surely a built-in solution is the best? \$\endgroup\$ – Peter Taylor Oct 11 '15 at 20:33
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    \$\begingroup\$ It immediately occurs to me that the easiest way of doing this is to XOR the two images together. BTW What is the input format? \$\endgroup\$ – Level River St Oct 11 '15 at 23:35
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    \$\begingroup\$ The technical side seems like it'd simply be subtracting or xor'ing the two arrays, then the popularity side is very open-ended (just draw freehand circles around them?) \$\endgroup\$ – Nick T Oct 12 '15 at 0:18
  • \$\begingroup\$ @PeterTaylor the father described in this question seems to enjoy his time in front of his computer more than with his child. He still wants to improve his child's future but will use any excuse to do it in front of his computer. Disclaimer : I do not encourage bad parenting ;) \$\endgroup\$ – Aaron Oct 12 '15 at 8:47
  • \$\begingroup\$ @NickT & steveverrill I do not know the first thing about image processing so my challenge may indeed be too way too easy. Do you know how I could avoid simple XOR answers? \$\endgroup\$ – Aaron Oct 12 '15 at 8:51

Sudoku with handicap

Note: I've completely reworked this, as the comments convinced me that there's not a good way to describe the restrictions I originally was after in a language-independent way without unreasonably restricting languages. Thanks to all the commenters.

I now reworked the question in a way that also inhibits traditional recursive solving (at least doing so in a straightforward way), and at the same time even allows to add a metrics about the "efficiency" of the algorithm. The basic idea being that your program is called not once, but many times, each time only having limited information about the field.

Also note that this new version requires me to write a driver program; so the question cannot go live until the driver program is written.

Questions are set in italics inside the text

The goal of this challenge is to solve a given Sudoku. However there's a twist: The program cannot access the full board at any time. Instead it is called repeatedly, and each time it has only limited information about the board. I'll refer to the totality of all calls as the "calling loop". The program can then request different information for the next run, or declare that it is finished (that is, request to not be called again; the call loop is terminated).

The only way to pass information between different runs is through the Sudoku board, and a small amount of scratch space. The Sudoku board is initialized before the first call with the Sudoku to solve (obviously) and is then checked after the call loop terminated. During the call loop, the Sudoku board is not checked, so you may "abuse" it to store additional information, as long as at the end, a valid result is generated.

Since it may not be possible to completely solve all Sudokus using such an algorithm, the only hard requirement is that the call loop is guaranteed to eventually terminate, the Sudoku field after termination is in a valid state. The rest is covered by scoring.

Standard loopholes are explicitly disallowed.

The stored data

The data that is stored outside the program consists of 90 nine-it unsigned numerical values (that is, minimal nmumber 0, maximal number 511), 81 of which represent the Sudoku field, and 9 values are scratch space. The values of the field are interpreted as bit fields, as described below.

In the following I'll use as example the Sudoku field


where dots contain fields that have not been filled.

Initially, the data gets filled as follows:

  • Each field pre-filled with number $n$ is represented by the value $2^{n-1}$, that is, the bit corresponding to that number is set, and all other bits are unset.

  • The unfilled fields are represented by the value $511$ (that is, all nine bits are set).

  • The scratch space is filled with $0$.

After the run loop terminates, each pre-filled field needs to have the same value as initially, and each initially empty field must have at least the bit corresponding to the correct solution set. That is, every zero bit represents a value that your program excluded for that field, and a program that excludes the correct solution is disqualified.

The contents of the field is only evaluated at the end of the call loop. So in between your program is free to make creative use of the storage space given.

The input

The program receives its data through standard input of the following form:

The first line contains a description of which data is given to/set by the program in this run. It consists of one to three space-separated words from the following list. On the first run, it is just "S". At later runs, it is exactly what the program requested at its previous run.

The possible values and corresponding interpretation are:

  • R1 to R9: The indicated row of the Sudoku, 1 being the uppermost row.
  • C1 to C9: The indicated column of the Sudoku, 1 being the leftmost column.
  • F1 to F9: The indicated $3\times 3$ subfield of the Sudoku, numbered left to right, up to down. So for example 1 denotes the upper left subfield, 6 denotes the middle right subfield.
  • S: The scratch space.

The next one to three lines contain the corresponding data, from left to right, and from up to down, as space separated decimal numbers.

So at the first run, your program will receive the input

0 0 0 0 0 0 0 0 0

At the second run with the example Sudoku field, the input to your program might be:

R2 C3 F4
511 511 2 511 16 511 32 511 511
16 2 64 4 511 8 128 32 256
511 511 4 511 511 511 32 128 8


The first one to three lines are the new values to replace the ones given in the input. The number of the lines must be the same as the number of fields in the first input line, and each line must contain nine values separated by whitespace (leading/trailing whitespace gets ignored).

If some field appears in more than one data line, the corresponding values are bitwise anded together. For example, if the initial line of your program's input was

R1 C1

and the first two line of your output read (with question marks replacing values that are irrelevant for this example — of course your code may not actually output question marks here)

3 ? ? ? ? ? ? ? ?
5 ? ? ? ? ? ? ? ?

then the upper left value us the Sudoku storage field will be 3 & 5, that is, 1

Following those data lines, there will be a single line containing either the single word STOP, in which case the run loop is terminated and the resulting field is created, or a line containing one to three whitespace separated words requesting data to be served in the next run, that is, the words to be presented in the first line of the next run of the program.


The score for qualifying entries is calculated as follows (lower score is better):

  • You get 1 score point for each run of your program.
  • You get 5 score points for each set bit in the final representation of your Sudoku field
  • At the end, subtract 45 (because a perfectly solved Sudoku will have nine bits set; if your program leaves less bits set, it will be disqualified anyway).

The total score is then calculated as weighted mean of the test cases, where the difficulty is used as weight, rounded up to the next integer. That is, if $d_k$ is the difficulty assigned to test case $k$, and $S_k$ is the score you achieved at test case $k$, your total score is $$S = \left\lceil \frac{\sum_k d_k S_k}{\sum_k d_k}\right\rceil$$

Sandbox question: Should I change the relative weight of program runs versus unsolved fields? And is the difficulty weighting a good idea, or should I simply add up all scores?

Test cases:

(Hardness as reported by GNOME Sudoku)

Test case 1: Easy (0.17)




Test case 2: Hard (0.63)




Test case 3: Very hard (0.96)




Sandbox question: Should I add more test cases?

  • \$\begingroup\$ Pretty sure you meant code golf not gode golf. \$\endgroup\$ – Blue Sep 2 '15 at 10:49
  • \$\begingroup\$ Is using constraint programming libraries/capabilities of a language allowed, since I'm only calling them and not writing them? \$\endgroup\$ – Fatalize Sep 2 '15 at 14:06
  • \$\begingroup\$ Also do you intend to add a time limit constraint to the challenge? I could write an answer that tries every possible grid until one is valid, without recursion or stacks \$\endgroup\$ – Fatalize Sep 2 '15 at 14:09
  • \$\begingroup\$ @muddyfish: Definitely. Thanks, fixed. \$\endgroup\$ – celtschk Sep 2 '15 at 20:12
  • \$\begingroup\$ @Fatalize: I don't know constrained programming libraries; it might be something I also want to ban. Maybe ban every built-in library that could not be written without recursion? Also, good point on the brute force method. I don't really like time limits, because they are too vague (different computers have different speed), maybe limitations on loops would be an alternative. Or limitations on how often the same variable/memory location may be changed. \$\endgroup\$ – celtschk Sep 2 '15 at 20:25
  • \$\begingroup\$ Limitations on how often a variable can change would be useless in python because you can setattr globals. \$\endgroup\$ – Blue Sep 2 '15 at 22:05
  • \$\begingroup\$ @muddyfish: setattr also changes a variable (by adding attributes to it), doesn't it? \$\endgroup\$ – celtschk Sep 3 '15 at 7:38
  • \$\begingroup\$ Yes but if you're saying you can't do that, you're saying you can only have a certain number of variables. \$\endgroup\$ – Blue Sep 3 '15 at 7:59
  • \$\begingroup\$ I really don't know what's banned as a recursive technique. For instance, what is a stack data structures? Can I use a list and extract the last element? What if I used dynamic programming instead of recursion? \$\endgroup\$ – xnor Sep 4 '15 at 7:46
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    \$\begingroup\$ Are stack-based languages (CJam, GolfScript, PostScript, FORTH, etc) banned? If so, are languages which use a stack for function/method calls (C, Java, etc.) also banned? Would a better approach be to forget talking about stacks and instead allow only a certain number of memory locations to be used, and limit each of them to 8-bit values? Then C-like language programmers can use a single global array for all their memory, or split it between a few global arrays and some loop variables; stack-based language programmers can work with a limited maximum stack depth; etc. \$\endgroup\$ – Peter Taylor Sep 4 '15 at 14:07
  • \$\begingroup\$ Basically what I want to prevent is algorithms trying to insert some numbers, and when it fails, track back and try something different. \$\endgroup\$ – celtschk Sep 5 '15 at 5:43

The Drunken Knight


  • a: Starting location of the knight, e.g. A2
  • b: Target location of the knight, e.g. B4. The starting and target locations may be the same.
  • n: An integer equal to or greater than 0.


The probability that a knight starting at a, moving at random for n turns on an 8x8 board, ends at b.


  • The knight has equal probability to move to any of the squares which it can access.
  • There are no other pieces on the board that could block any squares from the knight.
  • \$\begingroup\$ Any time constraints? \$\endgroup\$ – Sp3000 Oct 7 '15 at 14:21
  • \$\begingroup\$ @Sp3000 I haven't really thought about that yet. Do you think it would be more interesting if I tried to disallow brute force solutions via a time constraint? \$\endgroup\$ – absinthe Oct 7 '15 at 14:24
  • \$\begingroup\$ From a simple test (assuming my implementation is right), caching seems to be all you need if you want to bypass a time constraint (brute force ~10 moves in a lot of secs, caching > 100 in less than a sec). So I guess it might be better off without a time constraint after all... \$\endgroup\$ – Sp3000 Oct 7 '15 at 14:51
  • \$\begingroup\$ 1. Is the knight moving on an 8x8 board? 2. Please tag markov-chain \$\endgroup\$ – Peter Taylor Oct 11 '15 at 20:50
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    \$\begingroup\$ Ah, another thought which occurs: valid output formats? The obvious three are floating point to a certain precision and accuracy; exact rational; and exact rational reduced to simplest form. \$\endgroup\$ – Peter Taylor Oct 13 '15 at 13:31

Time Series Analysis

Time series analysis comprises methods for analyzing time series data in order to extract meaningful statistics and other characteristics of the data (from here).

The challenge

Write a program or function that take a time series vector ts and output a the coefficient of determination of a simple linear regression model estimated by the ordinary least squares.

The coefficient of determination should be the squared correlation between the predicted values of the model and the real values of the input, in such way that any perfectly linear time series should have 1 as it's coefficient.


  • ts is a vector of racional numbers of length .
  • To make the model assume ts is time sucessive and use time as its regressor.
  • You can assume that ts is already loaded.
  • You can't use build-in modeling functions such as lm() or similars.
  • The input should be in any reasonable format.


# input
# output                      

enter image description here

# output    

enter image description here

Let the shortest code win!

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    \$\begingroup\$ This could use either a link to a page that clearly explains the terms and method used, or preferably a better explanation within the post. \$\endgroup\$ – Geobits Oct 14 '15 at 18:42
  • \$\begingroup\$ Good idea, I will link it. \$\endgroup\$ – Mutador Oct 14 '15 at 18:43
  • \$\begingroup\$ The fact that this is time series doesn't change anything since it's just OLS estimation, which is not specific to time series. OLS requires two dimensions, so x is essentially position in the series and y is the value of the series? \$\endgroup\$ – Alex A. Oct 14 '15 at 18:43
  • \$\begingroup\$ I thought about saying time series to add some context, but yeah, it is just that, not sure about the output though. \$\endgroup\$ – Mutador Oct 14 '15 at 18:46
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    \$\begingroup\$ What does lm() do? \$\endgroup\$ – flawr Oct 14 '15 at 18:53
  • \$\begingroup\$ @flawr lm() is a function in R for fitting linear models. \$\endgroup\$ – Alex A. Oct 14 '15 at 19:33

Cops and Robbers: Text Transformations

Cops' challenge

The cops must write a fully deterministic program that reads input from STDIN and writes output to STDOUT as its only side effects. The mapping from strings to strings performed by this program will be called f.

A cop's post consists of such a program's source code, along with its length and the name of the language the program is written in. The poster must also prepare a possible crack (see below), and release it when their post is safe. A cop's post is safe when it remains uncracked for exactly two weeks.

A safe post where the original code is n bytes long is worth 1/n² points. The author with the highest point total wins. The tiebreaker is popularity (sum of votes of answers in the robbers' thread.)

A single author may not use the same language twice in two different cop answers.

Robbers' challenge

To crack a cop's post, a robber must figure out which transformation f the program in the post is performing, and write a program P in the language used by the cop, so that both P and f(P) perform the transformation f.

The length, method, or complexity of P are irrelevant; as long as is produces the same output as the cop's original code for any input you pass it, the solution is valid.

Successfully cracking a cop's post is worth one point. The author with the highest point total wins. The tiebreaker is popularity (sum of votes of answers in the robbers' thread.)

This is a bit hard to conceptualize, so here's a very simple example.

If the cop's post is:

Python 3, 20 bytes


(i.e., reverse STDIN and remove every other character) The robber's answer might be:

print(input()[::-2])# ) ] 2 - : : [ ) ( t u p n i ( t n i r p

as passing this program as input to itself yields a new program that does the same thing:


As another example, if a cop writes a C++ program that rotates lines on STDIN by 90 degrees, a valid solution is a C++ program that also rotates lines by 90 degrees, and does the very same thing if you rotate it by 90 degrees.

The difficulty for cops is to come up with transformations that are short to express, but difficult to code around (and, of course, they essentially have to crack their own post -- but at least they know f in advance.)

The difficulty for robbers is to decipher the cops' solutions to find out which transformation f they're performing, and then write any program P such that both P and f(P) perform f.

  • 1
    \$\begingroup\$ "A single author may not use the same language twice in two different cop answers." Not a fan of rules like that. Anyway, is there anything to prevent a cop from using a function like "Return the (n/2)nd character of the string." In that case it's pretty much impossible for f(P) to compute this function. Or should the mapping be surjective? Also do cops and robbers have to use the same language? Finally, how do we prove that a robber's implementation computes the exact same function as the cop's, especially if the cop's code is obfuscated and undocumented? \$\endgroup\$ – Martin Ender Oct 19 '15 at 12:07
  • \$\begingroup\$ If it's impossible for f(P) to compute f, the original post is invalid anyway -- cops must be able to crack their own cop answers and release the solutions if they go uncracked. Also, yeah, they would use the same language. \$\endgroup\$ – Lynn Oct 19 '15 at 13:16
  • \$\begingroup\$ @feersum pointed out a more serious problem in chat: cops can post something like x = readline(); if (md5(x) == 'f0a92b8efc0...') print x, which is nearly impossible for other people to solve. \$\endgroup\$ – Lynn Oct 19 '15 at 13:18

Watermelon Contest

You and your buddies are contesting a lone piece of watermelon left in the middle of the table. You decide to make a program to contest for you.

The Goal

You want to be the last program standing. Then you get the watermelon.

The Process

Every iteration 1 program will be eliminated from the watermelon contest. This will be decided by a vote among all the remaining programs. This means that your program will have 1 and only 1 vote to spend on the elimination of another program. Whichever program ends up with the most votes is eliminated.

This continues until there are only 1 program left, the winner. This entire process is considered a "round".

After there is a winner, another round will be started with a new piece of watermelon. All programs will be re-entered. When 10,000 rounds have been completed, the program with the most "wins" will be considered the "grand champion". All the rounds combined is considered the "tournament".

The Catch

Every program will have an opportunity to send a message to all the other programs. The message must be the same for every program. The message is a string, up to 500 bytes long.

You may have a file in which you may store any data you wish from previous rounds. This will persist over the entire tournament.

The Program

Write a program or function that accepts the following input in any (convenient) form:

[program-name], [message], "The Slug", "hey! don't vote for me!", "Chucknorium II", "a2TEI5ds#" ...

and outputs the name of the program that you vote for:

Chucknorium II


  • In the likely event of a tie, one of the high scoring programs will be randomly eliminated
  • Messages can be anything that doesn't mess with stuff it's not supposed to (e.g. don't mess with the controller or other people's programs). This is what makes the challenge interesting.
  • You may not hard-code program names into your program! In other words, numbering the programs randomly at the beginning of the game should produce the same output. Names are just more fun.
  • For observation purposes, your program will still be run even if it has been eliminated. It will not, however, have a chance to vote that round.
  • 3
    \$\begingroup\$ This seems to reward out-of-band collaboration (or posting multiple answers, which is effectively the same thing). \$\endgroup\$ – Peter Taylor Oct 20 '15 at 8:20
  • \$\begingroup\$ So the winner between the last two is basically always random (since I assume neither will vote themselves out)? \$\endgroup\$ – Geobits Oct 20 '15 at 18:34
  • \$\begingroup\$ @Geobits correct. Over several thousand rounds this should even out. \$\endgroup\$ – Stretch Maniac Oct 20 '15 at 19:49
  • \$\begingroup\$ I see. There's nothing about how many rounds will be played, so I was wondering about that. If you add this, you should make sure to distinguish turns/rounds or rounds/games, etc. \$\endgroup\$ – Geobits Oct 20 '15 at 19:53
  • \$\begingroup\$ Also, do programs continue to be called once they're "out"? If I want to track who did well each round, for instance, I don't think I can (since I think I only get input up to when I get voted out). \$\endgroup\$ – Geobits Oct 20 '15 at 19:56
  • \$\begingroup\$ Good point. Yes. You just won't be able to vote. I'll change that (and other things) when I get to a computer. \$\endgroup\$ – Stretch Maniac Oct 20 '15 at 19:59
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    \$\begingroup\$ I don't need to hard-code program names: I just need to agree a signature algorithm whereby the combination of name and message means that either the program is a collaborator or they're piggybacking on our agreement. \$\endgroup\$ – Peter Taylor Oct 20 '15 at 20:37
  • \$\begingroup\$ I see... How about a statement that prevents engineering to specific programs? Would that prevent pre-determined collaboration? \$\endgroup\$ – Stretch Maniac Oct 20 '15 at 20:42
  • \$\begingroup\$ Can you clarify the format of the input \$\endgroup\$ – user193661 Oct 21 '15 at 9:05
  • \$\begingroup\$ I hate to be a party pooper, but I honestly don't see the point of this KOTH since it seems to be more politics than programming. Then again I didn't get the cake cutting one either and look at how that one turned out... \$\endgroup\$ – Sp3000 Oct 23 '15 at 12:36

"The" Gaidhlig Challenge

The Gaidhlig language has some non-trivial rules when it comes to putting "the" in front of a word.

You're challenge is to create a program that takes two inputs, the first input is a string of text, a real word or made up that we can pretend is a noun. The second input is either the letter 'f' or the letter 'b' to denote whether the word is masculine or feminine.

The type of delimiter between these two inputs is your choice but must not be the letters a to z, a dash, or an apostrophe.

  1. The first input is always assumed to be a noun.
  2. The second input denotes whether the noun is masculine (f) or feminine (b).
  3. We will always assume all inputs is valid.

You're output will be the the first input, modified for the following rules:

Masculine Nouns (where 'f' is supplied.)

  1. Before vowels: An t-

  2. Before b f m p: Am

  3. Before all other instances: An

Feminine Nouns (where 'b' is supplied.)

  1. Before sl sr sn so se si su: An t-

  2. Before b m p c g : A' [with lentition]

  3. Before f: An [with lentition]

Before all other instances: An

Whether the word is masculine or feminine:

Words that start with l n r sg sm sp st always start with: An


When lentition is asked for, you must add the letter h after the first letter of your word in cases where the word starts with b c d f g m p s t. Otherwise the word remains unchanged. Further, you must not add an additional letter h if there is already a h in place.


Lentition of Aran: Aran

Lentition of Ghoul: Ghoul

Lentition of Goul: Ghoul

Lentition of House: House


Cat f An Cat

Cat b A' Chat

fear f Am fear

fear b A' fhear

Obair f An t-Obair

Obair b An Obair

snow f An snow

snow b An t-snow

Shortest code in bytes wins.

  • \$\begingroup\$ I think you have a consistent typo: it should be lenition, right? \$\endgroup\$ – Peter Taylor Oct 23 '15 at 21:54

I would love to hear your thoughts about the following challenge. Too difficult or contrived? Or should we actually want more complicated and challenging tasks? I'll provide an example implementation in MATLAB by the time I post the challenge.

Concert Harp: Pedal Meddle

Sometimes I hear people say, ‘Why would anyone outside the ICT business learn how to program?’ Often, they get replies like ‘well, sometimes there are problems to which there is no software available’, but when asked about what kind of problems these would be, they’re forced to admit that all they really wanted was to make Conway’s Game Of Life for their own entertainment.

However, I recently found a problem that I think the home-and-garden programmer could face in reality. It considers a harp (a side effect sharing an apartment with a significant other) and a completely dumbfound pianist/programmer, who’s struggling enough with one pedal as it is. The harp in question has seven.

Now, for some background music/harp theory. You may skip as much as your musical background, or lack thereof, allows.

Music theory (a very condensed version)

Both in a harp and a piano, the strings/keys are laid out as follows:

… C | D | E F | G | A | B C | D | E F | G A | B …

There are seven root notes, [A-G], with at some locations a | in between to signify that there’s a note in between. These |’s are address by making a note higher by appending a #, or lower by appending a b. For example, C#==Db, F#==Gb (and also, Fb==E). Using these notes, we can make a scale. The difference between D and D# is called a half note, and between D and E a whole note.

Scales are made as follows: 1: take the root note 1: Find the next notes by going up a whole or half number of notes in the following pattern (last step in () because that makes you end up at the root note again)

Major: 1 1 ½ 1 1 1 (½)
Minor: 1 ½ 1 1 1 1 (½)

For example, D major and A# minor D E F# G A B C# (D) A# B# C# D# E# F# G# A#

Of course, these notations are not unique, since for example E#==F.


A harp has seven pedals, each responsible for one note. This note, they can either raise half a note, or lower half a note. For example, the C pedal can either make all C’s sound like C#(==Db) or like Cb(==B). Let’s designate raising by a pedal setting of +1 and lowering by -1, and leaving it as-is as 0. Given an input scale, write a program or function that outputs how each of the pedals should be set to achieve all of the tones in that scale.


A scale designation. Scales are designated as follows: R[m][k]

  • R: Root. [A-G]
  • [m]: Optional: modification. Either flat b or sharp #
  • [k]: Optional: minor key, designated as m.

Valid inputs would be for example

  • C C major
  • Dm D minor
  • Fbm F flat minor


The pedal setting -1, 0 or 1 for each of the pedals, in the following order, reflecting the actual location of the pedals on a harp:

D C B | E F G A

Test cases: (not exhaustive; i.e., there may be more solutions, I only included a double solution to one input)

C , B#, Am -> 0 0 0 | 0 0 0 0
Cm, B#m    -> 0 0 0 | -1 0 0 0
F#m, Gbm   -> 1 1 0 | 1 1 1 1
        or -> -1 -1 -1 | -1 0 -1 -1
  • \$\begingroup\$ I'm confused: you say "Given an input scale" and then describe the input as "A chord designation". Which is it? \$\endgroup\$ – Peter Taylor Oct 25 '15 at 7:38
  • \$\begingroup\$ @PeterTaylor thanks, I was doubting between two versions of this challenge so that must've slipped through, edited now. What do you think of the challenge itself though? \$\endgroup\$ – Sanchises Oct 25 '15 at 10:18

Stupid leaks

Considering how immensely successful my last two challenges have been, I'll do a different style this time.

Drip, drip, drip, drip...

It's the year 3000. Due to clean air shortages, a system was created that turns water into air. However, that caused a water shortage (don't you love progress?). Therefore, a new (expensive!) system was created to convert the air back into water.

All that to say that water prices have skyrocketed.

And yet, here you are, stuck with a leaky faucet. Plumbers are expensive, but if you do it yourself, you have to order the parts online and wait for them to get here. You need a way to determine what is cheaper: calling a plumber and getting it fixed in a day, or buying the parts online but having water leak until they get here.

The input

You need to take seven positive numbers as input:

  • The price of water per gallon g.
  • The number of drops leaking per hour d.
  • The number of gallons wasted per drop z. This will always be a floating-point number less than 1.
  • The price of calling a plumber to fix it p.
  • The price of ordering the parts online o.
  • The number of hours it takes for the plumber to fix the leak l.
  • The number of hours it takes for the parts to get here s.

Only g, z, p, and o can be floats; all the rest will be counting numbers (integers greater than 0).

The task

The gallons of water wasted per hour from the leak is d*z*g. For the sake of brevity, let's call that rate R. If R*l+p is less than R*s+o, then you should print/return DIY!. If greater than, print/return Call the plumber!. If equal, print/return Whatever....


Floating-point precision through calculations is very...weird. Basically, your program can use whatever your language's default is. If you're using a language where the default floating-point type has infinite precision. If your language's default floating-point type can lose precision throughout calculations (like in Python, where .1+.1+.1+.1+.1+.1+.1+.1 != .8), then that works, too.

Short version: floating-point semantics and precision are however your language is by default.


Code-golf. Shortest wins. Standard loopholes banned.

  • \$\begingroup\$ What is a "counting number"? Does it include 0? You also might want to specify at what precision the equality will be checked (presumably hundredths?), and how rounding should be handled for the "Whatever..." case. \$\endgroup\$ – FryAmTheEggman Nov 2 '15 at 18:24
  • \$\begingroup\$ @FryAmTheEggman Is this better? \$\endgroup\$ – kirbyfan64sos Nov 2 '15 at 21:03
  • 2
    \$\begingroup\$ My language doesn't support floating point at all, so can I always just output Whatever...? More seriously, I'm failing to see the point of this question. Do some trivial arithmetic and then output one of three strings which between them take up 90% of the code? \$\endgroup\$ – Peter Taylor Nov 2 '15 at 21:15
  • \$\begingroup\$ I think it is better, but I think you do have to spend a bit addressing Peter's comment about languages that don't support floats. \$\endgroup\$ – FryAmTheEggman Nov 2 '15 at 23:55

Oh, no! There's been a fire at Claus HQ, and it's destroyed Santa's flight route! He has called on you to come up with a route that has him arriving at every home between the hours of 9PM Christmas Eve and 7AM Christmas Day. He'd also like to finish his deliveries in as little time as possible.


Your program will take data for as many geographical areas as you or Santa chooses to enter. For every geographical area to be added, your program will accept:the name of the area, and the geographical coordinates of the area's center and the number of "nice" children who live in that geographical area.

Output In a .csv file your program will place:

1) Each geographical area's name, 1 per line, listed in order, with the area to be visited first placed first, and the area to be visited last placed last.

2) Next to the name, an ETA to the area in local time, and estimated time of departure, assuming Santa takes about 1/6100 seconds per child.

3) At the last line of the file, the total number of miles travelled, as determined by the sum of the great-circle distances - determined using the Vincenty formula, assuming an oblate spheroid Earth - from each stop to the next. Other than as stated above, I don't care what your file looks like. Rules

-You may not use any external library to perform any task except the following:

--Converting from one timezone to another.

-You may give your output file whatever name you choose.

-Estimated departure times must be no later than 7:01 AM.

Scoring For simplicity, scoring will be done using US states as areas. The population inputted will be the number of 14-and-under Christians residing in each state.

You get 1 point for every thousand miles travelled and an additional point for every hour of travel.

  • \$\begingroup\$ 1. "Vincenty formula" should be hyperlinked to a clear explanation, and geode parameters should be supplied. 2. Converting between timezones isn't hard, and doesn't need an external library, but the input does need to include the timezones. It would also be good to state explicitly whether or not the International Date Line needs to be taken into consideration. 3. We have no way of calculating travel times, because you haven't given us a speed. We also have no way of scoring our programs, because you haven't supplied the test data. \$\endgroup\$ – Peter Taylor Nov 8 '15 at 8:11

Convert your Language to Turing Machine Code

You are locked into a room, with only a laptop and a single-bidirectionallyinfinite-tape, two symbols Turing Machine (Therefore supports only 0s and 1, and it has a tape which is infinite in both directions). Your perverted captor set you a task: he will set you free only if solve all of the problems on the Project Euler page.

However, there is a catch. You are not allowed to solve the problem using your laptop, but you'll need to use the Turing Machine instead.

Since you think it will be incredibly tedious to convert your code to Turing Machine Code, you decided to write an source-to-source compiler on your computer, and since you're incredibly eager to get out, you decide to write your code in the shortest form possible.


  • Write in your language of choice an compiler that converts your language into your Turing Machine Code.
  • Your language may not need to be completely transformed, but at least the basic operations needed for mathematical computations need to be implemented, therefore you will need to implement at least three of the following, with mandatorily being able to translate a looping construct of your language, then for each more operator implemented you will get a 10% bonus:

    • Addition
    • Subtraction
    • Division
    • Multiplication
    • Modulus
    • Looping (Mandatory)
    • Bitwise operators: &(AND) |(OR) ^(XOR) !(NOT) (they count as 3 distinct ones)

    In practice your code should be able to translate at least a primality testing algorithm into Turing Machine Code.

  • When I refer to Turing Machine Code, I refer to code for TML (Syntax explained later here)


  • The turing machine does not support decimal numbers, only binary, so you may (or may not if you have a better method) write numbers in unary. e.g 11111011 are respectively 5 and 2

  • Since the Turing Machine does not have a predefined IO, you may consider leaving the return value on the tape and halting as returning a value. e.g 111110000 and halting will return 5.

  • For the Input, you have full access to the starting tape, according that you don't do any other operation rather than initializing the variables. e.g if you implemented add(a,b) and run add(5,7) you may initialize the tape to this: 1111101111111 or 11111001111111 or 111110001111111 exc. but you may not initialize the tape to: 111111111111

  • TML Language description. TML which is the language your are interpreting your code to, uses a systems of cards, in this form 0{0-1}{0-1}{Integer}-1{0-1}{0-1}{Integer} where the first value determines which piece of code to execute (reading a value from the tape and comparing it, if it is 0 it will execute the code after the 0 until the dash, else the other piece of code), the second one tells what to write on the tape(0, 1), the third one finally tells the tape whether to go left(1) or right(0). The last value tells us to which card to go next, with the card 0 reserved for halting.

Note that TML is not 100% complete, so if your code follows the specs, but doesn't actually work just let me know, so I can fix the Language interpreter (if it's actually broken)

This is Code Golf, so the shortest code wins!

  • \$\begingroup\$ 1. The mention of interpreters is confusing: what you're asking for is a source-to-source compiler. 2. It would be useful to mention in the first sentence that the TM only supports two symbols. 3. What does "when to execute the card" mean? 4. In what circumstances could n be useful? It just seems to overcomplicate the explanation. 5. Is the tape infinite in both directions or just one? 6. What are "the basic operations needed for mathematical computations"? \$\endgroup\$ – Peter Taylor Nov 7 '15 at 12:31
  • \$\begingroup\$ @PeterTaylor I've modified the question a bit, I think it should be clearer this way, the only thing I haven't changed is the use of n, which I think could be useful if one were to implement something like a "pass" card (just speculating). What do you think? \$\endgroup\$ – WizardOfMenlo Nov 7 '15 at 13:04
  • \$\begingroup\$ That's rather a long list of mathematical operations! Is the intention to handicap higher levels languages because they support more operations and so will have more cases to compile? Write nothing is equivalent to writing what you read; move nothing is "Enter an infinite loop" if you wrote the symbol you read, or "See the other side of this card" otherwise. So all ns can be eliminated trivially except the infinite loop case, which can be eliminated by adding two cards x and y with instructions 000y-100y and 001x-101x respectively. \$\endgroup\$ – Peter Taylor Nov 7 '15 at 13:19
  • \$\begingroup\$ @PeterTaylor. The long list of operations I've introduced as sample operations, that I think a standard programming language should have, however I am no Programming Language expert, and I'm quite ignorant (yet) of the variety that they present, especially regarding the operations. Regarding the "n", I think that your logic is more than valid, and I will also project this change to the language itself. Thank you a lot! P.s If you have some more effective ideas for the operations to implement, please be welcome! \$\endgroup\$ – WizardOfMenlo Nov 7 '15 at 23:17
  • \$\begingroup\$ There are a lot of astandard languages on this site! The obvious language to use to answer this challenge is BF, which doesn't have anything more than increment and decrement. (And even a language as mainstream as Java doesn't have an operator for exponentiation or integer square root). \$\endgroup\$ – Peter Taylor Nov 7 '15 at 23:39
  • \$\begingroup\$ @PeterTaylor I've now added a system of bonuses, what do you think? Do you think the question is ready to be asked? \$\endgroup\$ – WizardOfMenlo Nov 8 '15 at 8:51
  • \$\begingroup\$ In addition to the issue of languages not having all of the operations on that list built-in, I find long lists of bonuses unappealing in general. I suggest that you scrap the list of operations, and instead require that the source language has to be able to translate all of its own functionality. This would create an interesting tradeoff between languages which are powerful, but have too many functions to implement, and ones that have the advantage of being minimal, but are difficult to program in. If you go with this, make sure to require the source language be Turing-complete. \$\endgroup\$ – feersum Nov 9 '15 at 8:10

First 100 Twin Primes

What Are Twin Primes?

Twin primes are two prime numbers that has a gap of 2 between them; i.e. 3-5, 5-7, 11-13...


  • Take no input and print first 100 twin primes to STDOUT.
  • Shortest code in bytes wins.


  • Your submission should be an executable complete program.
  • Every prime couple should be on a new line.
  • Printed twins should have a space between them.
  • All standard rules are applied.


  • No usage of built-in or external methods or functions that returns a prime number.
  • No hardcoded prime numbers except 2 (as number, not count).

Any suggestions? I looked to similar questions but i couldn't see an identical one.

  • 1
    \$\begingroup\$ Unfortunately, we consider questions for which most solutions can be easily transferred duplicates, so without the restriction on builtins (and probably even with) your question would be closed as a duplicate of codegolf.stackexchange.com/questions/31822/… \$\endgroup\$ – lirtosiast Nov 10 '15 at 1:14

Rounding Fractions

Back in the old days of game programming, before FPUs were the norm, games predominantly used fixed-point math to represent non-integer values. Typically, the lower 8 or 12 bits of a 32-bit word are used as fractional parts, and the rest are treated as the integral part. Sometimes when looking at fixed-point constants in old game code, I get confused trying to figure out what they were actually trying to approximate, particularly if it's not a number between 0 and 1 (0x4C = 0.3, 0x119 = 1.1?, 0x73 = ???).

Since just rounding 1/256ths and 1/4096ths has a limited range of applications, the challenge here is to take any integer ratio a/b, and output the simplest fraction that rounds down to it. More specifically, output the ratio p/q with lowest denominator such that a/b ≤ p/q < (a+1)/b.

This code should support any non-negative a and positive b up to at least 10,000, and should run in a reasonable time for anything in that range (nothing on the order of minutes, at least). Answers should be correct, i.e., no rounding errors due to floating-point. Answers can be in the form of a full program or function, and use any convenient input / output (a string '1/2', an ordered pair (1, 2), a list of two integers {1, 2}, etc).

This challenge is code-golf, lowest score in bytes wins.

Some test cases:

1/3 -> 1/3
4/10 -> 2/5
33/100 -> 1/3
66/100 -> 2/3
67/100 -> 19/28
115/256 -> 9/20  (who knew?)
0/417 -> 0/1
653/654 -> 653/654
1404/702 -> 2/1

Sandbox Questions

Hey! I'm a long-time lurker, first-time-question-asker; I'm pretty sure this hasn't been asked before (more general than "Un-round fractions", not quite "Closest fraction"). Not sure what to set for a deadline before accepting an answer, since this is my first time actually participating. Anything else obvious I missed?

  • 1
    \$\begingroup\$ Don't set a deadline before accepting an answer, at least in the sense of putting a date in the question. As a rule of thumb, wait a week, accept the winning answer, and if someone later posts an improved answer then change the accepted answer. \$\endgroup\$ – Peter Taylor Nov 14 '15 at 18:19
  • \$\begingroup\$ Sort of duplicate? The differences seem to be normal rounding vs rounding down and the odd restriction to a single loop in the existing challenge. \$\endgroup\$ – Martin Ender Nov 14 '15 at 23:46
  • \$\begingroup\$ That challenge is only about decimals, whereas this starts with an arbitrary rational number. Not sure if that's enough of a differentiation, but this one's broader, I guess. \$\endgroup\$ – Jonathan Aldrich Nov 15 '15 at 0:02
  • \$\begingroup\$ @MartinBüttner, I agree it's borderline, but some of the answers to the older question couldn't easily be adapted because they rely on converting float to string, and that only works in base 10. \$\endgroup\$ – Peter Taylor Nov 15 '15 at 8:47
  • \$\begingroup\$ @PeterTaylor It is a duplicate. floor(x)=round(x-0.5) \$\endgroup\$ – Akangka Nov 17 '15 at 12:04
  • \$\begingroup\$ @ChristianIrwan, why is that relevant? \$\endgroup\$ – Peter Taylor Nov 17 '15 at 14:32
  • \$\begingroup\$ @PeterTaylor Oh, sorry, misread. \$\endgroup\$ – Akangka Nov 18 '15 at 9:45

Nondeterministic Turing Machine


We all know the concept of Turing machines, if not let's reiterate the concept. We have the following things that define a Turing machine:

  • A tape that is divided into cells and is (potentially) infinite to the right.

  • A read/write head that moves along the tape and reads and writes from it

  • A state that changes according to what the head read

  • A transition function that defines state changes and the direction in which the head shall move, based on the current state and the input read.

We can now have to supply some input, and the definition of the transition function (the set of states is implicit, and contains all states defined in the function). Additionally we assume that the alphabet is [0-9a-zA-Z!?()^+-] and space is the blank symbol. The tape head is then postioned over the leftmost character on the tape, which in our case is the first character of the input tape. The machine then starts applying the transition function. The computation continues until one of the following happens:

  • The machine reaches the HALT state
  • There is no transition defined in the transition function for the current state with the given input.

If the first case occurs, we say that the machine "accepts" the word. If the second case occurs we say that the machine "rejects" the word.

We can now extend this definition, to obtain a nondeterministic Turing Machine. To do this we allow the transition function to define more than one "next" state for each state/input combination. The machine can then choose which "execution path" to take. We then say that the machine accepts the word if it reaches the "HALT" state in any execution path, and it rejects it if it does not reach this state in all exection pats.

Problem definition

You must supply a program or function that accepts a string and returns a truth-ish value (either true/false, or 0/1, or anything else, at long as the meaning is clear) indicating wheter the word is accepted for at least one computation path or not. The input has the following for:


All the parts of the tuple are provided as strings where

  • <input read> is a string of length 1, which can contain any character except ","
  • <output> is also a string of length 1
  • <move_direction> is either "l" (move left) or "r" (move right)

You may assume the following:

  • The machine will always halt (i.e. no infite loops)
  • There is only one state on which the machine halts which is HALT
  • The alphabet is [0-9a-zA-Z!?()^+-] plus space as the blank symbol
  • The leftmost character is always a blank, to indicate the ending of the tape on the left side.
  • States are defined implicitly by the tranistion function. So the only states the machine knows are the one that occure during the definition of the transition function and there is no explicit definition of the states.
  • The initial state is always s_i

The input has the following form


All lines end with a newline character (\n) and the input string is not under double quotes.

Standard loopholes are disallowed! Shortest answer in byte wins.


Working example is still missing, I'll update that in the following days

  • \$\begingroup\$ "The machine then starts the computation at the beginning of the input": I suggest adding "i.e. with the first character under the tape head, and the others to the right". I'm not sure what you mean by "States are defined implicitly by the tranistion function". \$\endgroup\$ – Peter Taylor Nov 13 '15 at 13:59
  • \$\begingroup\$ Followup questions: 1. What characters can appear in the name of a state? 2. What delimiters occur between the tuples in the input? 3. What delimits the end of the initial tape contents and the start of the tuples? \$\endgroup\$ – Peter Taylor Nov 13 '15 at 18:20
  • \$\begingroup\$ Thank you very much for your feedback. I just realized that I didnt think the input part through, I'll need some time to rethink that. \$\endgroup\$ – wastl Nov 13 '15 at 18:54
  • 1
    \$\begingroup\$ I realised today that there's another thing which needs specifying in the input: what is the starting state? This could done implicitly by saying that it's the <current_state> of the first tuple. \$\endgroup\$ – Peter Taylor Nov 14 '15 at 18:20
  • \$\begingroup\$ I added input specification. Do you think this is an adequate method to provide the input? \$\endgroup\$ – wastl Nov 16 '15 at 11:39
  • \$\begingroup\$ Yes, that works. \$\endgroup\$ – Peter Taylor Nov 16 '15 at 12:16

DND Level Tables

In 3rd edition DND, there a lot of stats that you have to keep track for when you level up. Let's fix that by making some nice tables for them.

Crash Course on 3rd edition DND

3rd edition DND is very complex since it does not have "classes" in the traditional sense of most RPG games. Instead of picking a class when you create a character and more or less sticking to it, in 3rd edition you can actually "opt in" into any class you want at any time. Essentially, when you level up, you can choose to put that level in any class you want. This means that you can potentially have three levels in Rogue, two levels in Fighter, etc.

In addition to this, 3rd edition has special things called skills, which are unique to each class. When you level up you get some skill points that you can spend on skills. You can spend one skill point to get one "rank" in a skill. However, if that skill does not belong to your class you only get one-half of a "rank" in that skill. There are also feats, which are essentially general skills that don't belong to any class and have their own "skill points", and stat increases, which are little stat bonuses you get on level up.

All together, this is probably enough to make your head swim. Luckily, this challenge doesn't require you to get too much into the nitty gritty of this.


Given an number representing a level as input, print out a table starting from level 1 up to and including the specified level of the following stats:

  • The total amount of XP you've gained to reach this level

  • Maximum class skill rank you've gained

  • Maximum cross-class skill rank you've gained

  • Whether you've gained a feat or stat increase (they are not gained every level) and the number of it.

The stats should be in this exact order. The table should be formatted with columns for each stat, with at least 4 spaces between each column. The level, feat and status increase numbers should have their appropriate suffixes (like 3rd). If you do not get a status increase or feat at a level, write -- where the feat or status increase number would go.

Now, without further ado, the math for all of this. Assume x is the level.

Total XP:

Class Rank:

Cross-Class Rank:

Feats: If

(x divisible by 3), then you have gained your

feat. You also automatically gain a feat at 1st level.

Stat Increases: If

then you have gained your

stat increase. Unlike feats, you do not get one of these at 1st level.


Sample input: 40

Sample output:

1st    0    4    2.0    1st    --
2nd    1000    5    2.5    --    --
3rd    3000    6    3.0    2nd    --
4th    6000    7    3.5    --    1st
5th    10000    8    4.0    --    --
6th    15000    9    4.5    3rd    --
7th    21000    10    5.0    --    --
8th    28000    11    5.5    --    2nd
9th    36000    12    6.0    4th    --
10th    45000    13    6.5    --    --
11th    55000    14    7.0    --    --
12th    66000    15    7.5    5th    3rd
13th    78000    16    8.0    --    --
14th    91000    17    8.5    --    --
15th    105000    18    9.0    6th    --
16th    120000    19    9.5    --    4th
17th    136000    20    10.0    --    --
18th    153000    21    10.5    7th    --
19th    171000    22    11.0    --    --
20th    190000    23    11.5    --    5th
21st    210000    24    12.0    8th    --
22nd    231000    25    12.5    --    --
23rd    253000    26    13.0    --    --
24th    276000    27    13.5    9th    6th
25th    300000    28    14.0    --    --
26th    325000    29    14.5    --    --
27th    351000    30    15.0    10th    --
28th    378000    31    15.5    --    7th
29th    406000    32    16.0    --    --
30th    435000    33    16.5    11th    --
31st    465000    34    17.0    --    --
32nd    496000    35    17.5    --    8th
33rd    528000    36    18.0    12th    --
34th    561000    37    18.5    --    --
35th    595000    38    19.0    --    --
36th    630000    39    19.5    13th    9th
37th    666000    40    20.0    --    --
38th    703000    41    20.5    --    --
39th    741000    42    21.0    14th    --
40th    780000    43    21.5    --    10th

Reference Implementation


sub formatNum {
  $first = substr $_,-2,-1;
  if($first == "1" and length $_ > 1) { return $_."th"; }
  $digit = substr $_,-1;
  if($digit == "1") { return $_."st"; }
  if($digit == "2") { return $_."nd"; }
  if($digit == "3") { return $_."rd"; }
  else { return $_."th" };

sub space { 
  print " "x4; 

while(<>) {
  $limit = $_;
  for(my $c = 1; $c <= $limit; $c++) {
    print formatNum $c;
    print ($c*($c-1)*500);
    print ($c + 3);
    printf ("%.1f", ($c - 1)/2 + 2);
    if($c % 3 == 0) { print formatNum $c/3+1; }
    elsif($c == 1) { print "1st"; }
    else { print "--"; }
    if($c % 4 == 0) { print formatNum $c/4; }
    else { print "--"; }
    print "\n";

Try it online.


coreutils default behavior stdin/stdout

We have a lot of challenges to implement just one of these operations, but a lot more are missing. Instead of adding a challenge for each of them, I thought I'd see if I could make a multiple-holes challenge that's complex enough to inspire some code re-use. This challenge is to reproduce a small subset of what Busybox does, namely to implement the default behavior of [almost] all of the GNU coreutils that (usually) read input from stdin or a file and send output to stdout or a file.

The utilities to reproduce are as follows:

  1. cat copy stdin to stdout
  2. tac copy stdin to stdout, reversing the order of the input lines (last line first)
  3. nl copy stdin to stdout, adding a line number to the start of each line. Start at 1, use spaces to pad each number to a width of 6, and add two spaces between the line number and the original line
  4. od I can't come up with a concise unambiguous way to describe the default output from od. I might skip it.
  5. base64 for every 3 bytes of stdin, split into groups of 6 bits, look those 6-bit values up in the base64 alphabet, and output 4 such bytes to stdout. wrap output lines at 76 characters. pad missing bits with 0s, and output an all-padding 0b000000 as "="

I'll finish filling out descriptions for some subset of the following if this idea proves popular enough to proceed with.

fmt, pr, fold, head, tail, split, csplit, wc, sum, sort, shuf, uniq, ptx, tsort, cut, tr, expand, unexpand, yes

The format of an entry would be either one program or one function, which can perform all of these tasks, just like busybox can. As a program, it might read its own process name to decide which tool to run. As a program or a function, the first parameter might be which tool to run. As a program, input should come from stdin and go to stdout. As a function, input should be a single string parameter, and return a single string.

  • \$\begingroup\$ I think it would be better to say "These ones have already been implemented" with links to the appropriate questions, and to then ask for a multi-tool which implements the non-dupes. That keeps things clear and, frankly, the ones which have already been done are probably mainly the less interesting ones. \$\endgroup\$ – Peter Taylor Nov 21 '15 at 23:07
  • \$\begingroup\$ @PeterTaylor fewer holes means less likelihood of code reuse. I'm annoyed at other challenges where code reuse is possible but it's not useful. I hope to see it in winning entries here. \$\endgroup\$ – Sparr Nov 22 '15 at 2:06

Find the nested source codes

A cops and robbers challenge where the cops write between 2 and 8 programs that produce output in the same language and interweave the programs together. WLOG, let's discuss this action being performed on two programs. By interweaving, I mean adding the characters of the second program to the first program so that when the characters of either the (WLOG) first program are removed, the second program can be seen. Obviously, commenting in any program is not allowed.

The cops will post their combined codes, the number of different programs in what they post, and the language the codes are written in, and what the programs print as output. To get credit for cracking the submission, the robbers must post the split codes and what each one outputs.

As a general rule, cops cannot use a language more than once.


A cop will receive points if their submission is safe for one week from the time of posting. Their score will be the sum of the two following.

  • The first value is 256 divided by 2 to the power of the number of different programs used.
  • Round the number of bytes in the combined codes up to the nearest power of 2 and call this number x. The second value is 1024 divided by x.

A cop will lose 10 points for every code that is cracked before the one-week period is up.

Robbers will receive a number of points equal to the sum of the following for each cracked submission.

  • The first value is 2 to the power of the number of different programs used in the cop's answer.
  • Round the number of bytes in the combined codes up to the nearest power of 2 and call this number y. The second value is 1024 divided by y.


Python, 2 codes


Prints llama and h

Codes: print("llama") and print("hello"[0])

  • \$\begingroup\$ Can I see an example? \$\endgroup\$ – Conor O'Brien Nov 22 '15 at 1:46
  • \$\begingroup\$ @CᴏɴᴏʀO'Bʀɪᴇɴ Updated. \$\endgroup\$ – Arcturus Nov 22 '15 at 1:54
  • \$\begingroup\$ On interweaving: does one have to say which method they used to interweave? \$\endgroup\$ – Conor O'Brien Nov 22 '15 at 1:57
  • \$\begingroup\$ Interweaving is done in order. The example given would be prPRiIntN("T**"(HlElLamLaO")"[0]**, where bold and capitalization represent the second code. \$\endgroup\$ – Arcturus Nov 22 '15 at 2:03
  • \$\begingroup\$ @CᴏɴᴏʀO'Bʀɪᴇɴ Think this is good enough to post officially? \$\endgroup\$ – Arcturus Nov 22 '15 at 2:36
  • \$\begingroup\$ I'd wait a day or two, and ask some other people in chat. \$\endgroup\$ – Conor O'Brien Nov 22 '15 at 3:03

Complete the Digit Sequence

We have a string of digits with some elements missing and marked with .s.


We want to fill the missing parts in a way that they would form an arithmetic sequence with the previous on next elements. E.g. 8...2 becomes 8642. If this kind of filling is not possible with single digits, mark the positions with ?. E.g. 7..3 would become 7??3.

With this rules our original example becomes


You should write a program or function which receives a digit string as input and outputs or returns the filled sequence.







This is code golf so the shortest entry wins.

Sandbox note: this seems boring so ideas are welcomed.

  • 1
    \$\begingroup\$ Idea: turn the entire string into an arithmetic sequence of as many integers as possible (after inserting spaces). e.g. 7....2 would become 765432, i.e. 7 6 5 4 3 2, but 7....3 would have to become 791113, i.e. 7 9 11 13, whereas 7....4 would have to become 727374 (72 73 74) or 767574 (76 75 74) \$\endgroup\$ – Martin Ender Nov 23 '15 at 11:38
  • \$\begingroup\$ @MartinBüttner I like it although I would like to avoid the extensive brute-force approaches. \$\endgroup\$ – randomra Nov 23 '15 at 12:37
  • \$\begingroup\$ I just noticed there are a lot more possibilities for my last test case, like 72 53 44. But yeah, I'm not sure how much one could optimise solutions for this. (Also, I'd still keep the possibility of given digits within the sequence, I just used the x....y format for simplicity.) \$\endgroup\$ – Martin Ender Nov 23 '15 at 12:40

The Language Relay!

A typical relay race only has four competitors per team, but where's the fun in that? Instead, let's see how many teammates you can cram onto the bus to the stadium and still finish the race. (This analogy is starting to break down, so I'll cut to the chase.)

Your task is to write a program or function in 256 bytes or less. It will take no input, and its output will be a program or function in another language. That program or function will also take no input, and its output will be a program or function in another language... and so on and so forth, until the last program, which will output the following:

.     \O/      .
|===== |_ =====|
|    _/  |     |

The winner is whoever manages to use the most languages. If there is a tie, the winner is the one with the shortest code in bytes.

You cannot use the same language twice, and different versions of the same language do not count as different languages. The output must also be different every time (no using languages that leave the program or function untouched.)

Here's a small example:

int main(){printf("print(\".     \\O/      .\\n|===== |_ =====|\\n|    _/  |     |\")");}

This C code produces this Python code:

print(".     \O/      .\n|===== |_ =====|\n|    _/  |     |")

And the Python produces the final output. I've used two languages, so my score is 2, and I'm going to lose terribly.

  • 1
    \$\begingroup\$ I'd implement some way to incorporate byte counts into the score because otherwise I'm sure there will be plenty of ties. \$\endgroup\$ – Downgoat Nov 25 '15 at 17:45
  • \$\begingroup\$ Good idea, thanks. \$\endgroup\$ – Luke Nov 25 '15 at 17:46

This might already exist, but through my search I couldn't find anything like it, so here it goes.

Find My Number

My friends and I have made are playing a game where we have a variable N that represents a number from 0 to 10. Using an expression they give us containing + - * / for addition subtraction multiplication and division, > < = for greater than, less than, and equal to, and the integers 0-10, we must find there number or what their number can possibly be.

I’m lazy, so I want you to write a Program that takes an input from STDIN or an acceptable alternative and return the possible numbers to STDOUT or an acceptable alternative.





1, 2, 3, 4









Because I might be caught, I want the program to be a small as possible in characters to avoid me friends seeing it, so the shortest solution wins!

  • \$\begingroup\$ What it is impossible such as N<N or the result is all real numbers such as N=N? \$\endgroup\$ – Downgoat Nov 26 '15 at 16:18
  • \$\begingroup\$ Well in the first one is impossible, so nothing works, so it doesn't output anything. The second one since anything works it prints all numbers 0-10 \$\endgroup\$ – Generic User Nov 26 '15 at 20:12
  • \$\begingroup\$ I assume that the n in one of the examples is a typo for N. This is not really an interesting question. The addition of < and > makes it marginally different to existing "evaluate this expression" questions, but it's still trivial to handle with eval in languages which use infix operators. \$\endgroup\$ – Peter Taylor Nov 26 '15 at 21:14
  • \$\begingroup\$ Thanks for pointing out the typo, I think expression still works since I explained what I mean, but I will try to find a way to reword the question. Anyone have suggestions? \$\endgroup\$ – Generic User Nov 26 '15 at 21:43

I'm thinking I want to make a KOTH challenge that has bots play the classic game of Mafia. Bots will be placed in groups of seven, roles randomly assigned, and they play the game!

Explanation of Game Mechanics The game proceeds in multiple turns, each turn consisting of Night followed by Day. At night, each person completes his role as will be described below. During the daytime, players (in real life) discuss the events of the night (communicated by God, a separate person who does not participate in the game besides prompting people to their roles.) Just before night, the players have the option of voting to lynch another player, the objective being to lynch a mafia member to help the town win. Choosing not to lynch is always an option if you think it is too risky to lynch someone who might be innocent.

In real life, God would tell everyone to sleep, then ask mafia to wake up and silently agree on who to kill, tell them to sleep, and continue with the other roles in a similar manner. Town wins if all the mafia are dead; mafia wins if they at least outnumber the town.

There are four roles in this game, but I have played many games with more than seven people with more ridiculous roles that are very fun to play. Each player bot is assigned an ID number indicating his role.

  • Mafia - There will be 2 mafia in this game. The two mafia will come to a consensus on who to kill. If one is dead the other decides on his own. The two mafia can not choose one of them to be killed. Mafia has a role of either 1 or 2, which are identical in function.

  • Inspector - There will be 1 in this game. At night he visits a person, and receives a report on that person's innocence. Both Mafia members will appear as guilty and the rest will appear as innocent. He can not inspect himself nor dead people. His number is 3.

  • Doctor - There will be 1 in this game. At night he visits a person, and that person will not die that night. Doctor can not save himself nor dead people. His number is 4.

  • Townsperson - Does nothing at night. Numbers 5, 6, 7.

The Challenge

You will write a Java player class who will have (at least) three important methods: night, claim, and vote. All of these three methods will return the player to 'act' on (of course, depending on your role), given an ArrayList of information that the other players have claimed in public.

I am not very clear on how to run a KOTH challenge and what github is supposed to do and how to use it so it would be very helpful if someone could point me in the right direction. As of now I'm still working out the exact details of a Java class to run the game, as well as making a generic Player class that the answers will have to extend and use methods from.

Right now, I have a generic Player class that each answer will extend, and each Player object has two Identity objects: a public, claimed Identity and a private Identity. The private Identity will hold the players actual role (kept secret, of course), as well as a doctor arraylist of Integers and a cop arraylist of Strings. If the player happens to be a cop, the controller program will add an entry to the private cop arraylist which will serve as his way of receiving a verdict. I'll make a method called verdict() which returns the last element to make the programming easier. An example string is "4G" which means that player 4 is guilty. This player 4 may be mafia 1; player numbers are just given for discussion purposes and to identify a player based on his claims and is independent on role. Doctor is the same except integers because you only need to store who was saved that night.

The public, claimed Identity works in much the same way except that the controller will never modify it, only the player can. An identity object contains a role (an integer representing the role number), an ArrayList of strings called 'visits' (to be used if you are a cop or doctor, to store a list of people visited and any outcomes), and an ArrayList of suspicions to indicate that your bot suspects or is guessing that another player has a given role. It will also have a lynch value which can be changed during the day, and this is where a player decides on who to lynch (if at all). Finally, it will have a boolean for if the player is dead or alive. You had better not mess with this. The idea of having two is that you can claim whatever you want; so a Mafia can claim to be Cop or a townsperson and the doctor can claim to be a cop.

Initially your public claim ID is 0, indicating that you have not claimed. The program will give you your private ID, which will contain your actual role and and is not necessarily meant to be made public just yet. The program controller will only modify your private ID object, and only then if your are Doctor or Cop. It will append to the visits ArrayList in the private identity object the result of the visit; the doctor will have access to everyone he has visited, and the cop will have access to everyone he has inspected and their alignment.

The controller, each night, will ask Mafia 1 for his choice then Mafia 2 for his choice, then repeat, say, 25 times. If at any point the two mafia agree, then it stops because the Mafia have just made their choice. The number of times attempted thus far will be given to the Mafia's night method, so the code will look like M1.night(<other players info>, 1) then M1.night(<other players info>, 2), etc. is that arraylist of public identities previously mentioned. If no consensus, no kill. Similarly prompt cop and doctor once for their choice on who to visit.

In the day time, every player's lynch value will be 0, indicating that they do not vote yet; each player will be given 25 chances to act. An action can consist of adding to the list of suspected values, claiming a role, changing a role, changing the list of suspected values, voting to lynch someone, changing a lynch vote, etc., or more. Some actions are smarter than others, and your goal is to have a good action strategy. Upon each call of a players act() method, he will be given an arraylist of everyone else's public identity objects, as well as how many times before he has been prompted. This gives him access to what other players claim to be, who other players claim to have visited and outcomes, who they suspect as who, and who others are voting to lynch. If at any point there is a majority (more than half but not exactly half) of people wanting to lynch a person (No lynch, indicated by the value -1, counts too), then day time ends, that person is lynched and his role revealed. If majority voted -1 (no lynch) then nobody dies and day time ends.

Again, please guide me on how I should design my classes, controller, and what files I should put up and how people are going to test their bot at home. I would be glad to show you what I have so far, which includes a controller program and a dummy player class.

  • \$\begingroup\$ Note that we already have a Mafia KOTH here in the sandbox. \$\endgroup\$ – Alex A. Dec 5 '15 at 1:45
  • \$\begingroup\$ @AlexA. Note also that he allowed me to take over. \$\endgroup\$ – Faraz Masroor Dec 5 '15 at 2:17
  • \$\begingroup\$ Some important notes: The messages from the past post were a really good idea. Use them. Also, I think that its a good idea to have people submit a bot that plays a single role. Then, from round to round, simply switch in/out the bots for that role. \$\endgroup\$ – Nathan Merrill Dec 7 '15 at 4:40
  • \$\begingroup\$ I am not very clear on how to run a KOTH challenge and what github is supposed to do and how to use it so it would be very helpful if someone could point me in the right direction I like this KoTH, and I've got quite a few of them under my belt. If you pop into chat I'd be happy to chat about it with you (and help with the code, if you'd like) \$\endgroup\$ – Nathan Merrill Dec 7 '15 at 4:49
  • \$\begingroup\$ @FarazMasroor are you still planning on doing this? \$\endgroup\$ – Nathan Merrill Dec 27 '15 at 13:12

Generate a graphical representation of a Stern–Brocot tree of depth n

enter image description here

I am aware of this challenge. Would this be considered as a dupe?


Gilbert-Shannon-Reeds Shuffle


The Gilbert-Shannon-Reeds (GSR) shuffle is a simple model of riffle shuffling close to how real humans shuffle a deck of cards. The well-known rule of thumb to riffle-shuffle a deck of 52 cards seven times for sufficient randomness is based on the GSR shuffle.


  • Cut the deck at a position k, 0≤k≤n. If there are n cards, the probability that any given k is chosen is (n nCr k)/(2^n).

  • Put the first k cards into one pile, and the other n-k in a second pile.

  • Until all the cards are gone:

    • Where the sizes of the two piles are x and y, choose the first pile with probability x/(x+y) and the second pile with probability y/(x+y).

    • Move the first card in that pile to the new array.


Input: An array of positive integers, of length n<1000.

Output: The array shuffled once.


You may use any algorithm that gives equivalent results to the GSR shuffle.

  • \$\begingroup\$ Just to clarify: We have to shuffle once, yes? \$\endgroup\$ – Dennis Dec 4 '15 at 19:45
  • 1
    \$\begingroup\$ Closely related. This is one of the rare cases where I would actually favour closing an older question as a duplicate of a newer one. \$\endgroup\$ – Peter Taylor Dec 4 '15 at 20:27

Play Chess with a One-Move Lookahead

"I see only one move ahead, but it is always the correct one."

– Jose R. Capablanca, World Chess Champion 1921-1927

This is a chess tournament with a twist: your chess engine is only allowed to look 1 move (2 ply) ahead. In order to succeed, you must create the best board evaluation algorithm.

Additional Rules

  • En passant, castling, and under-promotion will be allowed. Of course, it is up to you if you want to bother adding those capabilities to your AI. I personally find these to be some of the best rules of chess.

I am considering making a "template" bot which implements alpha-beta pruning, and requiring users to just fill in the method for board evaluation. Otherwise, there may be ways to stretch what it means to "look ahead." The benefit for users would be that they don't have to write their own getLegalMoves() method.

  • 1
    \$\begingroup\$ How is the algorithm scored? \$\endgroup\$ – Doorknob Dec 7 '15 at 2:24
  • 1
    \$\begingroup\$ I don't think there's any way to specify "look ahead" which isn't vulnerable to stretching. Even if you just ask for the board evaluation. \$\endgroup\$ – Peter Taylor Dec 7 '15 at 9:44
  • 1
    \$\begingroup\$ Why would you need pruning for a bot that only looks 1 move ahead?? \$\endgroup\$ – feersum Dec 7 '15 at 11:50

Rearrangement Inequality: The Sequel

Read the previous version here.

Your mathematics teacher looked at your test results on inequalities. Not good.

Hence he decided to give you some homework on inequalities.

Since this is about inequalities (how unfair), he decided to give each student a differing amount of homework. The exact amounts do not matter to him.

After allocating the homework, he had a bunch of complaints. Apparently, some students have some neighbouring students who have less homework than them.

Hence, he has decided to rearrange the students such that there will be fewer complaints. As long as each student sees that at most one adjacent student has less homework than them, they will not complain. (The students are quite reasonable.)

Note that he can swap the position of two students, but he cannot move a student into an empty spot as the empty spot has no chairs and the chairs in this classroom cannot be moved around due to safety concerns.


Here are different forms of input you can consider:

Optional: You can include integers in your input for the size of the classroom for convenience. This is , so input and output does not matter too much.

Function input: Container with truthy and falsy values representing if a student is there or not. All truthy values must be the same, similarly, all falsy values must be the same.

Standard Input: A grid of 2 different characters, which can be separated by spaces. Possible input:

0 1 1 0 1
1 1 1 1 1
1 0 1 1 1
1 1 1 1 1




The teacher wants to know how to arrange the students. Each student can be represented as an integer from 1 to n from the least amount of homework to the most amount of homework. For the input:

0 1 1 0 1
1 1 1 1 1
1 0 1 1 1
1 1 1 1 1

where 1 means there is a student and 0 means there isn't any students, a possible output is:

0  12 13 0  4
10 11 16 15 3
9  0  14 17 2
8  7  6  5  1

where the students labelled 16 and 17 are not satisfied.

Your score for every test case will be the number of students which are satisfied over the total number of students in that test case. It is guaranteed that there is at least one student in each test case. Your final score will the average of your scores over all test cases.

If you want to use another form of input and output, please clearly state it in your answers.

The teacher has some concerns:

Please ensure that your code is deterministic, as the teacher may test it several times. You are allowed to code your own psuedorandom number generator if you need random numbers.

Please ensure that your code terminates within a minute for each test case, as the teacher is impatient.

The teacher has been to PPCG for a long time, so he knows all the standard loopholes.

The teacher has a very large classroom (100 by 100), and would be worried if your program fails to give an answer for such a large classroom.


This is , and the winner will be the person with the largest score.

Sandbox Meta:

I still need to generate some large test cases, so I'm leaving this in the Sandbox until I have large test cases. How should I show people large test cases?

  • \$\begingroup\$ Is it acceptable to use a built in pseudorandom number generator provided it is always run with the same seed? \$\endgroup\$ – trichoplax Dec 29 '15 at 15:37
  • \$\begingroup\$ @trichoplax I may need to be able to test the code multiple times, so that may require all compilers/interpreters to implement the same pseudorandom number generator, which I am not sure if that is the case. \$\endgroup\$ – Element118 Dec 29 '15 at 15:41
  • \$\begingroup\$ Ah, I see. That is definitely guaranteed for some languages, and definitely not for others... Banning all built in RNGs does at least seem fair to all languages. \$\endgroup\$ – trichoplax Dec 29 '15 at 15:58

Approximate an image using all colours

I know that this is basically a subset of the then very popular American Gothic in the palette of Mona Lisa: Rearrange the pixels but I think it is worth making it a new challenge.

As @MartinBüttner pointed out, there has also been the Images with all colors challenge, but in my opinion it has very little in common with the proposed challenge.


Given a image of 2^12 x 2^12 pixels as input, your program should recreate this image, but you have to use each of 8-bit RGB colour exactly once.


These are roughyl 12MP images, quite large. So one could just restrict it to 7-bit RGB then the pictures would only have to be 2^18 pixels, that means e.g. 512 x 512 pixels which would be way more suitable for the challenges here.

Or does anyone know a convenient other colour representation?

  • 1
    \$\begingroup\$ There's also this. I'm really not sure you'll get any better approaches than those used for CH's palette challenge. \$\endgroup\$ – Martin Ender Dec 22 '15 at 19:11
  • \$\begingroup\$ I was wondering that too on the other hand you have the advantage of exactly knowing the pallete and at the same time the disadvantage of having to use all the colours, even those that do not follow a "natural distribution" (a palette that is similar to "natural" images.) \$\endgroup\$ – flawr Dec 22 '15 at 19:15
  • \$\begingroup\$ I lean towards thinking this is too much of a duplicate of "in the palette of", but if it turns out to be accepted I'd recommend changing the title to "Approximate an image using all colours" to make it clear it's not "Images with all colors" again. \$\endgroup\$ – trichoplax Dec 29 '15 at 15:52
  • \$\begingroup\$ As noted in my answer to "in the palette of", it was directly adapted from a program which does this, so as far as I'm concerned it's a dupe. \$\endgroup\$ – Peter Taylor Dec 30 '15 at 10:54

Make a New Year countdown

Editor's note: This challenge is cancelled because it is the year 2016, and I'm too late. XP It is kept here for posterity.

It is New Year coming soon. As such, it would be a great idea to make a New Year countdown. That is your challenge today!

Happy New Year!

The full rules

  • Create a program taking from STDIN the current time and outputting a countdown until New Year into STDOUT.

  • The program should be flexible - after Year 2015 has concluded, the program should count down until New Year 2017, and so on. See examples for more information.

  • The program should count down until it is taken down by external means (using Ctrl-C, the Task Manager, the reset button, et cetera).

  • The countdown may be formatted however you like.

  • At the null second in the new year (YYYY-01-01 00:00:00), it should output 0 days, 0 hours, 0 minutes 0 seconds in your chosen format.

  • This is , so shortest answer wins.

  • The last rule: Have fun!


Given the input after from: the following countdown should be given, in any format you like (after to:).

2015-12-31 06:00:00
0 days 18:00:00

2015-12-31 23:59:57
0 days 00:00:03

2016-01-01 00:00:00
0 days 00:00:00

2016-01-01 00:00:01
364 days 23:59:59
  • 1
    \$\begingroup\$ Three things: 1. IMO this is close enough to this question that it's a borderline dupe. 2. 2016 is a leap year, so from 2016-01-01 00:00:01 to 2017-01-01 00:00:00 is 365 days 23:59:59 (assuming no leap seconds are added, but that can't be predicted in advance). 3. Because of points such as the previous one, for testing date questions it's usually better to take the current time on stdin rather than read it from the clock. \$\endgroup\$ – Peter Taylor Dec 31 '15 at 10:59
  • \$\begingroup\$ For 3; changed the rules to get the time from STDIN \$\endgroup\$ – user48538 Dec 31 '15 at 16:40

Sig-fig calculator

As a scientist, sig-figs are definitely one of the most important parts of measurements and calculatations (unless you are a theoretical scientist, where everything must be exact!). Sig-figs is an important way of measuring uncertainty and accuracy of a value. To calculate the number of sig-figs for a value, use the following to help you (from here):

  1. ALL non-zero numbers (1,2,3,4,5,6,7,8,9) are ALWAYS significant.

  2. ALL zeroes between non-zero numbers are ALWAYS significant.

  3. ALL zeroes which are SIMULTANEOUSLY to the right of the decimal point AND at the end of the number are ALWAYS significant.

  4. ALL zeroes which are to the left of a written decimal point and are in a number >= 10 are ALWAYS significant.

  5. Exact numbers in an equation have infinite sig-figs. For example, in the equation $A=pi*r^2$, $pi$ has infinite sig-figs and the exponent of 2 also has infinite sig-figs. (not included in the above link)

To calculate the number of sig-figs from a calculation, use the following rules:

  1. For multiplication and division, the number of sig-figs for the result is equal to the number of sig-figs for the least accurate value (i.e. the value with the least number of sig-figs). For example, 2.000*5.00 = 10.0 and 4.5*6.00 = 27

  2. For addition and subtraction, the result has as many decimal places as the one with the least decimal places. For example, 5.00-2 = 3 while 6.0-3.000 = 3.0

  3. logs (including natural logs) have as many decimal places as the number of sig-figs of the value of whose log is being taken (I am not sure if this sentence of English is correct!). For example, log(2.45) = 0.389.

  4. Other functions, such as square roots, exponents, sines, cosines, etc. can be assumed to have the same number of sig-figs as its argument. So the sin(3.14) = 0.00159


The challenge is to write a sig-fig calculator with the following functions:

  1. addition, subtraction, multiplication, division
  2. sine, cosine, tan, and their inverses
  3. sinh, cosh, tanh, and their inverses
  4. Exponents, including shortcuts for e^x and 10^x, along with sqrt. The exponents are assumed to be exact and has infinite sig-figs
  5. log of base 10 and natural log.

The result of each calculation should give you the correct result, but insert a "\" before the last sig fig. If there is infinite sig figs, there should be no "\" at all. Also, undefined and infinite values should give an error or NaN or print "infinity" or "undefined". For example,

sin(3.14) = 0.0015\9265292
log(1.01) = 0.00\432137378
5.00*2 =\10

This is a so the shortest code wins!

  • \$\begingroup\$ The positioning of the backslash in the output spec is rather confusing, but I think that one of the examples disagree with the spec. 5.00 * 2 is 10 with one sig fig according to the rule for multiplication that you take the number of sig figs of the least accurate value. Of course, this runs into the problem that your (unconventional, I believe) rule 4 does not provide any way to represent the number 10 with only 1sf, but even so I think the correct output should be \10. \$\endgroup\$ – Peter Taylor Nov 24 '15 at 22:06
  • \$\begingroup\$ @PeterTaylor thanks! It is now fixed \$\endgroup\$ – TanMath Nov 24 '15 at 22:59

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