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copied!<p>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.</p> <p>[].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.</p> <p>JS Analysis (with graphs!): <a href="http://www.robweir.com/blog/2010/02/microsoft-random-browser-ballot.html" rel="noreferrer">http://www.robweir.com/blog/2010/02/microsoft-random-browser-ballot.html</a></p> <p>Ruby is no different! It has the same problem. :)</p> <pre><code>#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 </code></pre> <p>=></p> <pre><code>[[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]] </code></pre> <p>Each element should occur in each position about 20000 times. [].sort_by(rand) gives much better results.</p> <pre><code>#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 ... </code></pre> <p>=> </p> <pre><code>[[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]] </code></pre> <p>Similarly for [].shuffle (which is probably fastest)</p> <pre><code>[[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]] </code></pre>

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