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Re^2: A bad shuffle

by tlm (Prior)
on Mar 20, 2005 at 23:37 UTC ( #441074=note: print w/replies, xml ) Need Help??

in reply to Re: A bad shuffle
in thread A bad shuffle

Thank you for the information. Perhaps I should have made clearer in that my encounter with this algorithm, which motivated the whole write up, was in research-oriented/scientific code in which it was being used to sample permutations of an array uniformly at random. My contention is that this is a misuse of this algorithm, because it does not sample the space of permutations uniformly.

But in light of what you write about the algorithm's standing and pedigree, the title of my meditation is an awful one. Maybe the whole node should be retracted for the sake of not confusing others. Let me know what you think.

Update: I found a version of Fisher-Yates online (linked from here):

#include <stdlib.h> void shuffle(int *array, size_t n) { if (n > 1) { size_t i; for (i = 0; i < n - 1; i++) { /**/ size_t j = i + rand()/(RAND_MAX / (n - i) + 1); /**/ int t = array[j]; array[j] = array[i]; array[i] = t; } } }
This is equivalent to the algorithm used by my random_perm, not the one used by naive_shuffle. To get the latter algorithm, the lines indicated with /**/ above would have to be changed to:
for (i = 0; i < n; i++) { size_t j = rand()/(RAND_MAX / n);
the lowliest monk

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Re^3: A bad shuffle
by Zaxo (Archbishop) on Mar 20, 2005 at 23:55 UTC

    No, don't withdraw it. I think the thing to do is try and figure out the difference between a "fair shuffle" and a uniformly distributed selection over permutations.

    Are the two the same if there are identical cards in the deck? What is the problem that motivated this?

    Sometimes confusion, like greed, is good.

    After Compline,

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