First common element from two lists

Question

x = [8,2,3,4,5]
y = [6,3,7,2,1]

How to find out the first common element in two lists (in this case, \"2\") in a concise and elegant way? Any list

Answer 1

Use set - this is the generic solution for arbitrary number of lists:

def first_common(*lsts):
    common = reduce(lambda c, l: c & set(l), lsts[1:], set(lsts[0]))
    if not common:
        return None
    firsts = [min(lst.index(el) for el in common) for lst in lsts]
    index_in_list = min(firsts)
    trgt_lst_index = firsts.index(index_in_list)
    return lsts[trgt_lst_index][index_in_list]

An afterthought - not an effective solution, this one reduces redundant overhead

def first_common(*lsts):
    common = reduce(lambda c, l: c & set(l), lsts[1:], set(lsts[0]))
    if not common:
        return None
    for lsts_slice in itertools.izip_longest(*lsts):
        slice_intersection = common.intersection(lsts_slice)
        if slice_intersection:
            return slice_intersection.pop()