Comments on: Interview Questions: Shuffling an Array

By: gman

gman — Fri, 20 Nov 2009 13:52:04 +0000

I was asked this question in an interview. I gave the seemingly correct but subtly wrong solution. Good to know the correct solution.

By: praveen

praveen — Tue, 16 Jun 2009 23:57:51 +0000

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will smith
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interview questions-interview questions

By: Hanna

Hanna — Tue, 26 Aug 2008 19:08:39 +0000

Hi,
Great effort after reading this I really good very nice point how to shuffle array keep it up
and post more article also check Pre Developer

Thanks

By: Andy

Andy — Sat, 12 Apr 2008 14:14:10 +0000

What if the source and destination are picked using a random number generator N times? Given that the generator is unifrom, aren’t all permutations possible?

By: phil

phil — Fri, 11 Apr 2008 08:35:19 +0000

rue, see here: http://eigenclass.org/hiki.rb?sort_by+rand+is+biased

By: Jesse

Jesse — Fri, 11 Apr 2008 01:39:02 +0000

rue,

That’s not really true, though. In fact, that “solution” violates multiple requirements of the problems.

One, it’s not in-place. Two, whether or not the distribution is uniform depends on the sorting algorithm that sort_by uses.

By: rue

rue — Thu, 10 Apr 2008 21:06:49 +0000

Excellent note about the algorithm.

However, since pragmatism trumps all, the “right” way to shuffle an Array in Ruby is this:

class Array
def shuffle!
replace(array.sort_by { rand })
end
end

As a bonus, it is a language/core lib bug and not your fault if there is a problem with the distribution ;)

By: Jesse

Jesse — Thu, 10 Apr 2008 17:00:27 +0000

cc,

You’re right about the first point, but not the second. Both self.size and size work because we’re inside the Array instance.

By: cc

cc — Thu, 10 Apr 2008 10:15:53 +0000

Note: – I could be wrong here but I think you’re code example#1 has a typo, you define variable r and don’t use it, and use variable k and don’t define it

and shouldn’t it be n= self.size in example#1 as well as example#2?

By: phil

phil — Thu, 10 Apr 2008 09:38:08 +0000

For yet another Ruby implementation of Array#shuffle see Random numbers in Ruby, http://snippets.dzone.com/posts/show/4697

Nice post!