Ruby - return an array in random order

Question

What is the easiest way to return an array in random order in Ruby? Anything that is nice and short that can be used in an IRB session like

[1,2,3,4,5].random()
# or 
random_sort([1,2,3,4,5])

Quill

Answer 1

If you don't have [].shuffle, [].sort_by{rand} works as pointed out by sepp2k. .sort_by temporarily replaces each element by something for the purpose of sorting, in this case, a random number.

[].sort{rand-0.5} however, won't properly shuffle. Some languages (e.g. some Javascript implementations) don't properly shuffle arrays if you do a random sort on the array, with sometimes rather public consequences.

JS Analysis (with graphs!): http://www.robweir.com/blog/2010/02/microsoft-random-browser-ballot.html

Ruby is no different! It has the same problem. :)

#sort a bunch of small arrays by rand-0.5
a=[]
100000.times{a <<  [0,1,2,3,4].sort{rand-0.5}}

#count how many times each number occurs in each position
b=[]
a.each do |x|
    x.each_index do |i|
        b[i] ||=[]
        b[i][x[i]] ||= 0
        b[i][x[i]] += 1
    end
end
p b

=>

[[22336, 18872, 14814, 21645, 22333],
 [17827, 25005, 20418, 18932, 17818],
 [19665, 15726, 29575, 15522, 19512],
 [18075, 18785, 20283, 24931, 17926],
 [22097, 21612, 14910, 18970, 22411]]

Each element should occur in each position about 20000 times. [].sort_by(rand) gives much better results.

#sort with elements first mapped to random numbers
a=[]
100000.times{a <<  [0,1,2,3,4].sort_by{rand}}

#count how many times each number occurs in each position
...

=>

[[19913, 20074, 20148, 19974, 19891],
 [19975, 19918, 20024, 20030, 20053],
 [20028, 20061, 19914, 20088, 19909],
 [20099, 19882, 19871, 19965, 20183],
 [19985, 20065, 20043, 19943, 19964]]

Similarly for [].shuffle (which is probably fastest)

[[20011, 19881, 20222, 19961, 19925],
 [19966, 20199, 20015, 19880, 19940],
 [20062, 19894, 20065, 19965, 20014],
 [19970, 20064, 19851, 20043, 20072],
 [19991, 19962, 19847, 20151, 20049]]

Ironware

Answer 2

array.shuffle

Stylolite

It seems to be in Array in 1.8.7 but not 1.8.6 or earlier. [1,2,3,4,5].shuffle works fine for me in 1.8.7. – Mako

In 1.8.6 you can get shuffle from backports or just use .sort_by {rand} (if you can live with the fact that it's O(n log n)). – Heliotaxis

Answer 3

What about this?

Helper methods for Enumerable, Array, Hash, and String that let you pick a random item or shuffle the order of items.

http://raa.ruby-lang.org/project/rand/

Preece