From: Joao Silva <rubyforum@...>
Date: 2008-05-16T05:52:46+09:00
Subject: Re: Randomizing an Array?

Florian Frank wrote:
> Daniel Waite wrote:
>> Florian Frank wrote:
>>> Don't do this, better use (1..10).sort_by { rand }. Your version is
>>> equivalent to (1..10).sort { 1 } and *always* creates the same
>>> permutation for this array.
>> 
>> It's random for me. Both work.
> 
> No, really, it isn't. It may "look random", but try sorting (1..10) many
> times and ponder the coincidence.

That's because sort{ foo } expects foo to be in -1,0,1.

rand() is always [0, 1).

rand(2)-1 is (-1,1) however and thus will work just fine. :)

One could do sort{rand <=> rand}, but that's ~9x slower by my testing.

irb(main):163:0> (1..10).to_a.sort{rand }
=> [10, 6, 1, 7, 3, 8, 5, 9, 4, 2]
irb(main):164:0> (1..10).to_a.sort{rand }
=> [10, 6, 1, 7, 3, 8, 5, 9, 4, 2]
irb(main):165:0> (1..11).to_a.sort{rand }
=> [11, 6, 1, 7, 3, 8, 5, 9, 4, 10, 2]
irb(main):166:0> (1..12).to_a.sort{rand }
=> [12, 7, 1, 8, 3, 9, 5, 10, 2, 11, 6, 4]

vs

irb(main):186:0> (1..12).to_a.sort{rand(2) -1}
=> [1, 2, 6, 11, 9, 4, 8, 7, 5, 10, 3, 12]
irb(main):187:0> (1..12).to_a.sort{rand(2) -1}
=> [1, 9, 8, 3, 5, 6, 11, 4, 2, 10, 7, 12]


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