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function construction - How do you check if there are any equal arguments(even sublist) in a list?


I would like to set up a function which has to return True if at least two arguments of a given List are equal.


So if I give {1,4,6,2} to the function it has to return False (since none of his arguments are equal) and the same would happen if I gave {{1,2,3},{2,3,4}}, while if I gave {1,2,3,1,4} or {{1,2,3},{2,0,0},{1,2,3},{2,1,2}} it has to return True.


I know this is a very simple problem and i think you can easily tell me a convenient way to achieve that.




Answer



If there are repeated elements in the list, then calling Union[] on it will shorten it so that this element only appears once, so a simple implementation would be to test these lengths:


 test[list_] := Length[Union[list]] != Length[list]

If you wanted to know which elements where repeated, you this could be accomplished by using Gather[] to collect identical elements, and picking out which groups have more then one element.


 repeats[list_] := Select[Gather[list], Length[#] > 1 &][[1 ;;, 1]]

Note, I'm using Union rather then DeleteDuplicates[] since (as Mr. Wizard corrected me) it is faster. I can't say why except that DeleteDuplicates[] retains the order of elements which may require slightly more bookkeeping. And in this case we don't care about the book keeping. Naturally if you really needed something really speedy, a better solution exists which doesn't search through the entire list, but stops if just a single duplicate is found, Mr. Wizards Answer is just such an function, since Signature exits early if duplicates exist, though it becomes slower if no duplicates are present, it's a trade off.


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