Insertion Sort in Python Basic implementation of insertion sort algorithm
Insertion Sort in Python
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- What I'm going to do in this video is
- attempt to create an implementation
- of the insertion sort algorithm
- that we talked about in the last video.
- I'll do it as a Python function.
- So I'll call the Python function insertion_sort()
- and it will take in a list—
- so list is its parameter in the function definition—
- so we'll have to pass in a list as an argument.
- And let's—what we're going to do is
- we're going to step through each of the slots in the list.
- I guess we could call it that way.
- So let's say "for index in range()".
- We could start at the leftmost slot in the list.
- So, we could just say "len()"—
- "len" just means—it's short for "length"—
- so length of the list.
- So what this would do (is)
- this would start index—
- so let's say the list has four elements in it—
- then "len(list)", this right here,
- would be 4, would evaluate to 4.
- "range(4)" would produce a list
- that has elements [0,1,2,3] in it.
- And so, index would be able to step through
- the different indices for this list right over here.
- And we could do it that way,
- but you might remember from the previous video
- that when you're doing the insertion sort
- it doesn't actually make sense
- to start at the very leftmost element.
- Because there's nothing to compare it to the left.
- So we can actually just start
- at the second to the leftmost element.
- And the leftmost element is the 0th item,
- so we can start at the first item.
- So now, if the list has a length of 4,
- this right here will produce [1,2,3].
- So, 1 would be the second to the leftmost element.
- 2 would be the next one to the right.
- 3 would be the last one.
- Remember, we always start our indices at 0—
- the 0th term is the leftmost element in a list.
- So fine, we can step through that.
- Let's get the value of—at that point in time—
- of the element that is at that index.
- So, that way we don't have to keep finding it,
- value is equal to list[index].
- And this by no means is going to be
- the most efficient implementation
- of even insertion sort.
- This is going to be my best try,
- writing it in real time,
- and in a way that hopefully...
- you might be able to understand it.
- So "value" is just the item in the list
- at each of those indexes
- that we're now going to compare to
- all of the items to the left of it.
- And what I want to do is—
- compare a value—I want to compare a value
- to each item to the left of it.
- So let's define a variable "i"
- and let's let this be the index of the things
- that I want to compare value to.
- And right from the get-go
- I want to compare value to the thing that is left of it.
- So "i" should have
- one lower of an index than "index".
- So "index - 1".
- So this is the index of the item
- that is directly left to it.
- But we're going to keep taking "i" lower and lower.
- So we can keep comparing value
- to things further and further to the left.
- And so what we want to do is we're going to—
- we want to keep comparing further and further left
- until "i" has gotten all the way to the beginning of the list.
- And "i" has gotten all the way to the beginning of the list
- when it is equal to 0.
- So what we want to do is,
- we want to perform this
- while "i" is greater than or equal to 0.
- Because if we keep taking "i" lower and lower and lower,
- we're going further and further
- to the left of the list.
- We don't want to try to perform it
- when "i" is—you know—further—is a negative number—
- that'll just start doing crazy things.
- So while “i” is greater than or equal to 0—
- what I'm going to do is
- I'm going to keep pushing "i" further and further to the left.
- So let's try it this way, so the first thing I want to do—
- we've already pushed it to the left once.
- So let's compare—
- so if "value" is less than the—
- this thing keeps syntax error
- because (of) waiting for me to keep typing—
- if "value" is less than the item that is now at index "i".
- So the item at index "i"—list[i]—
- the item at index "i" is this right here.
- So if it is less than that,
- let's shift the item that's over here—
- let's shift it one to the right.
- So the right slot is "i + 1"
- and I can't just say it's "index",
- because remember I'm going to keep pulling "i"
- lower and lower and lower.
- Because right now "i" is "index - 1"
- in the first pass through this while loop,
- but I'm going to—as you'll see in a second—
- I'm going to keep lowering "i",
- so it always won't be one left of "index".
- So I'm going to say, wherever "i" is,
- let's take the spot one to the right of "i"—
- one to the spot of—one to the right of that—
- so that's "i + 1" is its index.
- And let's replace that with whatever is at list[i],
- whatever is at "i", at slot "i".
- So we've essentially taken this thing right over here
- whatever number was there.
- And we're putting it in the slot
- that is one to the right of it. And then,
- and actually the way we were setting up that algorithm.
- Whatever's there is going to be...
- Well...I won't talk about that.
- We'll step through it and see how it all plays out.
- And then we can shift "value" to the left.
- So whatever was in this slot right over here
- will be replaced by "value".
- So list[i] will equal "value".
- So one way to think about it
- --Let me write a comment here--
- shift what was...
- shift number in slot "i" to slot "i + 1"
- or actually bucket "i + 1"
- --I guess is one way to think about it--
- And then you could say,
- shift number right—let me call it this way—
- shift number right—in slot
- --we're going to write this way--
- shift number in slot "i" right to slot "i + 1"
- And then over here, we are shifting...
- shift value left into slot "i".
- And so if you remember what we did in the last video,
- it's exactly describing it.
- We're comparing "value" to the thing to the left of it.
- If it's less than it,
- then whatever number was in that box/slot to the left of it,
- shift it to the right, and then shift "value" to the left.
- And now let's compare value
- to something one lower than that.
- So we want to decrement "i", we want to lower "i"—
- decrement is just increment down.
- So "i" is equal to "i - 1",
- and then we'll perform the loop over again.
- And now "value" will be compared—
- now "i" is two to the left of "index"
- --compared to that--
- if it is less than it, shift that to the right
- and shift "value" again to the left
- Now what if we have the situation
- where "value" is not less than
- the item that you are comparing it to?
- Well if it's not less than the item you're comparing it to,
- that means "value" is already going to be in the right place.
- It also means that you're done,
- and that you don't need to shift "value" any more to the left.
- And you don't have to shift the stuff to the left
- any more to the right.
- So then, we are done,
- and I *think* this could work
- unless I made some silly mistakes.
- So let's try to see if I could get this—
- if this actually works as a sorting algorithm.
- Let me save it, insertion_sort,
- and let me run it.
- Alright, so I didn't have any, at least, syntax mistakes.
- Syntax just means the actual characters I used—
- I didn't forget to put a colon here or greater than sign—
- it was actually able to process this,
- interpret this right over here.
- But let's see if it actually works.
- So let me define "a" list.
- Let's say [7,1,3,5,9,2]
- and let me put another 3 in there.
- So that is "a",
- Then let me see—this is the moment of truth.
- insertion_sort(a), let's see what happens.
- So remember, we're sorting it in place,
- this function doesn't return anything.
- But it should take whatever list that was,
- and had changed up all the elements
- so that now they are in order.
- So this is the moment of truth.
- Let's see what "a" looks like.
- There you go! It is sorted.
- So I don't think I made any major mistakes.
- So there you go.
- You have a version of insertion sort.
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