CODEWITHSUNDEEP
javacollections
xitnesscomplex
xitnesscomplex

Reputation: 37

Does Collections.shuffle guarantee the result is not sorted?

Does Collections.shuffle guarantee the resulting list is not sorted in ascending or descending order?

Suppose I have a list of Strings letters containing "a", "b", "c", "d".

After Collections.shuffle(letters) will letters ever be equal to a-b-c-d or d-c-b-a?

If letters can end up ordered, is there a way to prevent Collections.shuffle from leaving the list sorted in ascending or descending order?

Upvotes: 0

Views: 163

Answers (1)

Jeffrey Bosboom
Jeffrey Bosboom

Reputation: 13663

Of course it doesn't, because then it wouldn't be a random shuffle. Just because those two permutations look ordered to us humans doesn't mean they should be avoided when selecting a random permutation.

If you do want to avoid ascending and descending order, you can shuffle in a loop checking whether the list is ordered.

while (isSorted(list) || isSorted(list, Comparator.reverseOrder())
    Collections.shuffle(list);

with an appropriate implementation of isSorted. (Note that this will be an infinite loop for a 0 or 1 element list!)

Upvotes: 6

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