# Array#choice always produce the same sequence **URL:** \ **Category:** Ruby\ **Created:** [February 5, 2009, 1:01pm UTC](https://www.ruby-forum.com/t/array-choice-always-produce-the-same-sequence/159529 "2009-02-05T13:01:52Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![crocco](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/fbc32d/32.png) [@crocco](https://www.ruby-forum.com/u/crocco)\ **Post date:** [February 5, 2009, 1:01pm UTC](https://www.ruby-forum.com/t/array-choice-always-produce-the-same-sequence/159529/1 "2009-02-05T13:01:52Z")
I’ve just stumbled upon a strange behaviour of Array#choice (using ruby 1.8.7- p72). As far I understand, an\_array.choice should be (almost) the same as an\_array[rand(an\_array.size)]. Thus, the two following pieces of code should be equivalent a = [1, 2, 3, 4] p 10.times.map{a.choice}.join p 10.times.map{a[rand(a.size)].join} However, this doesn’t seem to be the case. In particular, something like: ruby -e ‘a = [1, 2, 3, 4]; p 10.times.map{a.choice}.join’ always give the same result, while ---
**Author:** ![crocco](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/fbc32d/32.png) [@crocco](https://www.ruby-forum.com/u/crocco)\ **Post date:** [February 5, 2009, 1:11pm UTC](https://www.ruby-forum.com/t/array-choice-always-produce-the-same-sequence/159529/2 "2009-02-05T13:11:51Z")
Alle Thursday 05 February 2009, Stefano C. ha scritto: > ruby -e ‘a = [1, 2, 3, 4]; p 10.times.map{a.choice}.join’ > > always give the same result, while Sorry, hit the “Send” button too soon. Here’s the full version I’ve just stumbled upon a strange behaviour of Array#choice (using ruby 1.8.7- p72). As far I understand, an\_array.choice should be (almost) the same as an\_array[rand(an\_array.size)]. Thus, the two following pieces of code should be equivalent a = [1, 2, 3, 4] p 10.times.map{a.choice}.join p 10.times.map{a[rand(a.size)]}.join However, this doesn’t seem to be the case. In particular, something like: ruby -e ‘a = [1, 2, 3, 4]; p 10.times.map{a.choice}.join’ always give the same result, while ruby -e ‘a = [1, 2, 3, 4]; p 10.times.map{a[rand(a.size)]}.join’ gives a different result every time the command is executed (which, in my opinion, is the correct behaviour). It seems that to get the correct behaviour from choice, you need to call srand before using it. Does anyone know why this is necessary with choice but not with rand? Is it the intended behaviour or a bug? Thanks Stefano ---
**Author:** ![crocco](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/fbc32d/32.png) [@crocco](https://www.ruby-forum.com/u/crocco)\ **Post date:** [February 5, 2009, 1:26pm UTC](https://www.ruby-forum.com/t/array-choice-always-produce-the-same-sequence/159529/3 "2009-02-05T13:26:31Z")
Hi Stefano, it seems to work for me: irb(main):003:0\> a = [1, 2, 3, 4] =\> [1, 2, 3, 4] irb(main):004:0\> p 10.times.map{a.choice}.join “4414312344” =\> nil irb(main):006:0\> p 10.times.map{a[rand(a.size)]}.join “4342411331” =\> nil irb(main):007:0\> p 10.times.map{a.choice}.join “2333341212” =\> nil irb(main):008:0\> p 10.times.map{a.choice}.join “1312441122” =\> nil irb(main):009:0\> p 10.times.map{a.choice}.join “2412444223” =\> nil Z ruby -v ruby 1.8.7 (2008-08-11 patchlevel 72) [i486-linux] ---
**Author:** ![crocco](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/fbc32d/32.png) [@crocco](https://www.ruby-forum.com/u/crocco)\ **Post date:** [February 5, 2009, 6:12pm UTC](https://www.ruby-forum.com/t/array-choice-always-produce-the-same-sequence/159529/4 "2009-02-05T18:12:43Z")
On Thu, Feb 5, 2009 at 2:42 PM, Stefano C. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > (without arguments), I get the correct behaviour, that is a different number > every time the script is run. It looks like it was fixed in ruby1.9 (notice that #choice has been renamed to #sample): ruby1.9.1 -e “p [1,2,3,4].sample” 4 ruby1.9.1 -e “p [1,2,3,4].sample” 1 ruby1.9.1 -e “p [1,2,3,4].sample” 2 ruby1.9.1 -e “p [1,2,3,4].sample” 3 ruby1.9.1 -e “p [1,2,3,4].sample” 1 ruby1.9.1 -e “p [1,2,3,4].sample” File a bug or ask in ruby-core if it’s desired behavior. – Pozdrawiam Rados³aw Bu³at [http://radarek.jogger.pl](http://radarek.jogger.pl) - mój blog ---
**Author:** ![crocco](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/fbc32d/32.png) [@crocco](https://www.ruby-forum.com/u/crocco)\ **Post date:** [February 5, 2009, 2:43pm UTC](https://www.ruby-forum.com/t/array-choice-always-produce-the-same-sequence/159529/5 "2009-02-05T14:43:08Z")
Alle Thursday 05 February 2009, ruud grosmann ha scritto: > > srand > > before using it. Does anyone know why this is necessary with choice but > > not with rand? Is it the intended behaviour or a bug? > > > > Thanks > > > > Stefano > “4342411331” > Z ruby -v > ruby 1.8.7 (2008-08-11 patchlevel 72) [i486-linux] Maybe I wasn’t clear in explaining the issue. Within a single irb session (or ruby invocation in general) I, too, get different results at each invocation. The problem is that if I run the script twice, I get the same results for each call to choice (that is, the first call to choice gives the same result at each ruby invocation; the second call always gives the same result, which can be different from the first, and so on). A simpler example: File: test.rb p [1,2,3,4].choice ruby test.rb =\> 4 ruby test.rb =\> 4 ruby test.rb =\> 4 And so on. If at the beginning of the script I add a call to Kernel#srand (without arguments), I get the correct behaviour, that is a different number every time the script is run. Stefano