Array#choice always produce the same sequence


#1

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


#2

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


#3

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]


#4

On Thu, Feb 5, 2009 at 2:42 PM, Stefano C. 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 - mój blog


#5

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