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Having 0 in that list is a mistake since the qsort algorithm takes that to mean that they are equal and hence it only needs to use one of those to compare with others.

A test that is sensitive to this is looking for "rising sequences". Scramble a sorted list. Make successive passes through the array, looking for the first, then second, then third, then fourth, etc elements. How many passes did it take? With your algorithm it will take fewer passes than with a well-shuffled array.

ObTrivia: Take a fresh deck of cards. Shuffle a few times. Do the above test by suit. 2 suits start off sorted front to back, 2 back to front. This test is able to detect that for a long time. (Certainly well after the "7 shuffles leaves a well-shuffled deck" factoid a lot of people hear. By some measures it is well shuffled. By this one it still isn't. :-)


In reply to RE (tilly) 2: Randomize an array by tilly
in thread Randomize an array by Zebu

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