how to look for an element in nested list in parallel

Question

I have a list of elements A = ['a', 'b', 'c', 'd'] and a nested list B = [{'55','a'}, {'4','t'}, {'x','y','zx'}, {'b','d'}, {'66','c'}].
In addition, each element can appear only in one sub-list of B.

I want to find, for each element from A, the index of the sub-list from B that contains it ([0, 3, 4, 3]). Assume that all the elements from A appear somewhere in B.

Using a nested loop isn't practical for the size of my data.

Answer 1

First build a dictionary of the positions in B for each item, then use a list comprehension on A:

d = {k:i for i,s in enumerate(B) for k in s}
# {'55': 0, 'a': 0, 't': 1, '4': 1,
#  'zx': 2, 'y': 2, 'x': 2, 'd': 3,
#  'b': 3, 'c': 4, '66': 4}

out = [d[x] for x in A]

Or, if you are not sure that all elements of A are in B:

out = [d.get(x, -1) for x in A]

Output:

[0, 3, 4, 3]

Used input:

A = ['a', 'b', 'c', 'd']
B = [{'55','a'}, {'4','t'}, {'x','y','zx'}, {'b','d'}, {'66','c'}]