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Re: Select value from array w/o replacement (or destroying the array)

by fenLisesi (Priest)
on Dec 03, 2007 at 09:33 UTC ( #654527=note: print w/ replies, xml ) Need Help??

in reply to Select value from array w/o replacement (or destroying the array)

I think tye's solution is the best. If for some reason you would prefer one of the others, the answer would depend on how big n is compared to the size of the array. If it is small, say, smaller than 10%, then just pick a random number in the range 0..$#a and pick another one if that had been picked before. Such collisions should be rare for small n / @a; If it is larger, then shuffle first, but do not shuffle the array itself, especially if the array elements themselves tend to be big (which doesn't seem to be the case here). Instead, create a separate index array and shuffle that -- something like @indices = shuffle 0..$#a. Below, I tried to add comments to tye's code. I may have messed up, so wait until wiser monks have had a chance to peruse it.

use strict; use warnings; ## How many elements to pick use constant N => 10; ## The original array that must not be altered my @letters = ('a'..'z'); ##-----------------------------------------------------+ ## Generate code that will pick an element from ## @letters at each iteration without repeating ##-----------------------------------------------------+ my $picker_of_next_rand = gen_picker( \@letters ); ## Now execute (iterate) that code N times for (1..N) { print $picker_of_next_rand->(); } print "\n"; ## look nice print @letters, "\n"; ## original untouched exit( 0 ); ##-----------------------------------------------------+ ## This sub generates and returns code that will pick ## a random element from @$aref at each iteration ## without repeating ##-----------------------------------------------------+ sub gen_picker { my ($aref) = @_; my @indices = 0 .. $#$aref; ## closes on @indices return sub { if (! @indices) { die "No more items left to pick from"; } ##---------------------------------------------+ ## pick an int in the range (0 .. $#$aref), ## the range of the array's indices ##---------------------------------------------+ my $random_index = int rand @indices; ## $pick is the element at the random_index my $pick = $aref->[ $indices[ $random_index ] ]; ##---------------------------------------------+ ## We are about to pop the index array. Save ## the element at the pop-end of the index array ## by copying it into the position of the $pick ## (clobber index of $pick, won't be used again) ##---------------------------------------------+ $indices[ $random_index ] = $indices[ -1 ]; ## Now it is safe to pop the index array pop @indices; return $pick; }; }

which prints something like the following:

vkdbfymxie abcdefghijklmnopqrstuvwxyz

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Re^2: Select value from array w/o replacement (or destroying the array)
by BioNrd (Monk) on Dec 04, 2007 at 16:30 UTC
    I agree that this solution is going to suit my needs. The code works really well, but I don't understand a part of it.
    my $picker_of_next_rand = gen_picker( \@letters ); print $picker_of_next_rand->();
    I under stand that is is a reference/dereference type thing.

    What I don't understand is how to use the variable $pick in a meaningful (non-global) way. i.e. $var_to_use_later = $list[$pick]; I have had trouble with reference/dereference structure before, and I have tried to use the camal book. It is not sinking in, and combined with a bit of code I am not familar is a headache for me.

    I appreciate all that has been done to this point for me, it is just that last little push in the right direction I need.


    Even a blind squirrel finds a nut sometimes.

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