Find all index of list containing a value Python

Question

I have 2 lists

lst = [1,2,3,4,5,1,2,1,2,3]
lst2 = [1,2,3]

I am trying to see the index elements in lst are in lst2. Currently I am doing;

ind = []
for x in lst2:
    if x in lst:
        ind.append(lst.index(x))

I realize the problem is that lst.index(x) only returns the first occurrence of the element so x=1 will always return lst.index(1) = 0.

Is there a way to return all the indexes that contain the element?

These kind of operation look like they want to be implemented with sets. What the actual purpose of finding the index? — Klaus D., Jun 16 '17 at 16:29
I have predicted data and observed data, stored in DataFrames. I need those indexes to find when the dates are the same in both indexes so I can compare several different filed. — Ahmed, Jun 16 '17 at 16:39

Vlad Sydorenko · Accepted Answer · 2017-06-16T21:18:21.637

5

You might use something like

ind = [[i for i, value in enumerate(lst) if value == x] for x in lst2]

I don't know a purpose of this operation, but it might be more useful to use dict comprehension:

ind = {x: [i for i, value in enumerate(lst) if value == x] for x in lst2}

because in this case you will have output like that: {1: [0, 5, 7], 2: [1, 6, 8], 3: [2, 9]}, which might be easier to use.

edited Jun 16 '17 at 21:18

answered Jun 16 '17 at 16:30

Vlad Sydorenko

score 1 · Answer 2 · answered Jun 16 '17 at 16:28

1

This does what you asked for:

ind = [i for i, x in enumerate(lst) for y in lst2 if x == y]

answered Jun 16 '17 at 16:28

zipa

What does the documentation say? Search for "Python method enumerate". – Prune Jun 16 '17 at 16:47

2 Answers2