finding items that occur more than once in a list

[Raymond Hettinger]

On Mar 18, 2:57 am, Simon Forman <sajmik... at gmail.com> wrote:
> Is there a more efficient way to do this?
>
> def f(L):
>     '''Return a set of the items that occur more than once in L.'''
>     L = list(L)
>     for item in set(L):
>         L.remove(item)
>     return set(L)
>
> |>> f([0, 0, 1, 1, 2, 2, 3])
> set([0, 1, 2])


def f(L):
    seen = set()
    dups = set()
    for e in L:
        if e in seen:
            dups.add(e)
        else:
            seen.add(e)
    return dups



Raymond