How to find the index of matching elements in a list using Python?

Question

I have two lists:

l1 = [1, 46, 8, 9, 4, 76, 797, 342, 3, 5, 67, 42, 87]
l2 = [42, 34, 5, 78, 8, 9, 4]

I want to find out the index of l1 that have matches in l2.

Can anyone help me out?

print([i for i, v in enumerate(l1) if v in set(l2)])

Matthias
– Matthias

2020-04-13 11:00:07 +00:00
Commented Apr 13, 2020 at 11:00 — Matthias
– Matthias, Commented Apr 13, 2020 at 11:00

RMPR · Accepted Answer · 2020-04-13 12:26:39Z

0

Naive method:

for i in range(len(l1)):
    try:
        if l1[i] in l2:
            print(i)
    except ValueError:
        pass

You can also use a oneliner, which is more efficient because the lookup in a set is done in constant time, credits to @Matthias for pointing that out:

[i for i, v in enumerate(l1) if v in set(l2)]

edited Apr 13, 2020 at 12:26

answered Apr 13, 2020 at 10:57

RMPR

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1 Comment

Matthias Over a year ago

if you use a set for l2 the complexity is O(n).

0Interest · Accepted Answer · 2020-04-13 12:30:14Z

0

The problem with enumerate and set is that it does not keep the order, here's a code that does this:

print([l1.index(k) for k in l2 if k in l1])

answered Apr 13, 2020 at 12:30

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