Re: Randomly select values from array

I'm guessing that "selecting for of them" is a typo for "selecting four of them". If not, please post a correction. Note that there may not be a way to remove all the numbers from the array. What if all the elements are negative, or greater than 15 say?

Here is a sketch of just one way; there are many others, some of them more efficient by various metrics. First, sort the array randomly. Then see whether the first four elements sum to 15. If so, remove them. Keep going until you have an array with four or fewer elements or until you get bored. This code fragment performs the selection, testing and removal. Wrapping it in code to loop and, optionally, terminating the procedure is left as an exercise because I don't want to do all your work for you.

@data = sort {rand < rand} @data;
$data[0]+$data[1]+$data[2]+$data[3] == 15 and push @stored, [splice @d
+ata, 0, 4];
[download]

Paul

Comment on Re: Randomly select values from array Download Code

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Re^2: Randomly select values from array by BrowserUk (Patriarch) on May 16, 2010 at 17:36 UTC
To emphasis how bad that sort-based "shuffle" is, consider that over a million shuffles of four values, 1/3rd of the possibilities are never chosen. And the rest are chosen so disproportionately that 'fairness' doesn't enter the equation: `++$h{ join'',sort{ rand() < rand() } 'a'..'d' } for 1 .. 1e6;; printf "$_ : %.2f%%\n", $h{$_} / 1e4 for sort keys %h;; abcd : 12.48% abdc : 12.47% bacd : 12.48% badc : 12.49% cabd : 3.15% cadb : 3.12% cbad : 3.14% cbda : 3.13% cdab : 6.26% cdba : 6.26% dabc : 3.13% dacb : 3.15% dbac : 3.13% dbca : 3.11% dcab : 6.26% dcba : 6.25%` [download] Contrast that with `use List::Util qw[ shuffle ];; ++$q{ join'',shuffle 'a'..'d' } for 1 .. 1e6;; printf "$_ : %.2f%%\n", $q{$_} / 1e4 for sort keys %q;; abcd : 4.18% abdc : 4.12% acbd : 4.14% acdb : 4.20% adbc : 4.17% adcb : 4.18% bacd : 4.15% badc : 4.19% bcad : 4.17% bcda : 4.16% bdac : 4.17% bdca : 4.15% cabd : 4.14% cadb : 4.17% cbad : 4.19% cbda : 4.18% cdab : 4.19% cdba : 4.20% dabc : 4.17% dacb : 4.15% dbac : 4.16% dbca : 4.22% dcab : 4.14% dcba : 4.13%` [download] And the latter is more efficient to boot. Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. "Science is about questioning the status quo. Questioning authority". In the absence of evidence, opinion is indistinguishable from prejudice. RIP an inspiration; A true Folk's Guy	[reply] [d/l] [select]
Re^3: Randomly select values from array by ikegami (Patriarch) on May 16, 2010 at 17:51 UTC
heh, use an older Perl (or `use sort '_quicksort';`), and you get `abcd : 100%`! Talk about not random.	[reply] [d/l] [select]
Re^3: Randomly select values from array by blakew (Monk) on May 16, 2010 at 18:51 UTC
A touch slow, but it works now: `use Algorithm::Permute qw(permute); @a = 'a' .. 'd'; $afact = 1; $afact *= $_ for 2 .. @a; permute { $h{ join '', sort { rand() < rand() } @a } += 1 / $afact; } @a for 1 .. 1e5; # a million takes a couple minutes printf "$_ : %.2f%%\n", $h{ $_ } / 1e3 for sort keys %h;` [download]	[reply] [d/l]
Re^4: Randomly select values from array by BrowserUk (Patriarch) on May 16, 2010 at 19:11 UTC
But why combine an O( N! ) algorithm with an O(N log N) algorithm, when Fischer-Yates is O(N)? List::Util`::shuffle()` does a million 4 way shuffles in less than 1 second.	[reply] [d/l]
Re^5: Randomly select values from array by blakew (Monk) on May 16, 2010 at 19:37 UTC
Re^3: Randomly select values from array by Xilman (Hermit) on May 17, 2010 at 08:04 UTC
"To emphasis how bad that sort-based "shuffle" is, consider that over a million shuffles of four values, 1/3rd of the possibilities are never chosen. And the rest are chosen so disproportionately that 'fairness' doesn't enter the equation:" I must try to remember that some people have difficulty recognizing meiosis (not that I'm including BrowserUk in that list). My original post included the words: "some of them more efficient by various metrics". Oh well. Paul	[reply]
Re^4: Randomly select values from array by BrowserUk (Patriarch) on May 17, 2010 at 08:52 UTC
Oh. Please do include me in that. I had absolutely no idea what meiosis meant before you mentioned it. I'm still sure how it applies here. "some of them more efficient In the first instance, it wasn't about efficiency. Just the fact that it didn't work.	[reply]
Re^2: Randomly select values from array by ikegami (Patriarch) on May 16, 2010 at 17:06 UTC
`@data = sort {rand < rand} @data;` [download] is not guaranteed to work, much less return something random. Use List::Util's `shuffle` instead.	[reply] [d/l] [select]


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