Use list item index as index to another list in python

Question

I have two lists:

A = ['PA', 'AT', 'TR']
removlistv = ['TR', 'AI', 'IO', 'SO', 'CR', 'PH', 'RT']

I find the intersection of the two lists:

rm_nodes = set(A) & set(removelistv)

Which creates the set {'TR'}. I now want to find the index of that intersection (or intersections) in the list removelistv:

indices = [removelistv.index(x) for x in rm_nodes]

So far so good, indices contains the correct value. The problem starts when I want to use the index value (which in this case is [0] i.e. a list) to retrieve the matching item in a third list removelistst = ['TR0', 'AI1', 'IO1', 'SO0', 'CR1', 'PH0', 'RT1']. My goal is to remove the item 'TR0' from removelistst. Basically, I want to remove items from this list based on the output of the intersection of the two lists in the beginning.

I've tried the following:

numbers =[ int(x) for x in indices ]
removelistst[numbers]

Which returns the error:

TypeError: list indices must be integers or slices, not list

Can't you just do `removelistst.remove(value + str(index))`? — kennysliding, Nov 23 '20 at 16:08
Why don't you calculate [the difference directly](https://stackoverflow.com/questions/6486450/python-compute-list-difference)? — Thomas Weller, Nov 23 '20 at 16:14
Isn't `indices` a list of numbers? Why do you need to create the `numbers` list? — Barmar, Nov 23 '20 at 16:19
@Barmar I thought this was a matter of extracting the list item from `indices` to reference the item in removelistst, because `removelistst[indices]` didn't work for me. — jonna_983, Nov 23 '20 at 16:23
@Barmar, yeap I figured out later that it is the equivalent of `removelistst[indices]` — jonna_983, Nov 23 '20 at 16:26

score 4 · Accepted Answer · answered Nov 23 '20 at 16:12

4

Loop through indices and pop the specified element out of removelistst.

for index in sorted(indices, reverse=True):
    removelistst.pop(index)

I sort indices in reverse so that removing an element won't affect the indexes of later elements to be removed.

answered Nov 23 '20 at 16:12

Barmar

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Thanks, I tried it out without sorting to observe the problem. – jonna_983 Nov 23 '20 at 16:27

score 2 · Answer 2 · answered Nov 23 '20 at 16:14

2

There's lots of ways to do this, here's a list comprehension:

removelistst = [value for index, value in enumerate(removelistst) if index not in numbers]

answered Nov 23 '20 at 16:14

Bill Lynch

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Use list item index as index to another list in python

2 Answers2