In Python, How to Index a List with Another List

In Python, how do I index a list with another list?

T = [L[i] for i in Idx]

How to find index of element of one list in another list which includes the same elements?

If you know that all the values in B are unique, one way would be to create a dictionary that maps the values in B to their indices.

b_dict = {}
for i, b in enumerate(B):
    b_dict[b] = i

Then, loop over A and get all the values from b_dict.

a_indices = [b_dict[a] for a in A]

With your lists, we get

A: [1, 2, 3, 4, 5, 6]
B: [3, 4, 1, 6, 2, 5]
a_indices: [2, 4, 0, 1, 5, 3]

This solution is O(n) as opposed to the others, which are O(n^2), and will therefore be much faster on large lists.

Generating a list using another list and an index list

This will call __contains__ on every call for idx but should be reasonable for small(ish) lists.

list2 = [list1[i] if i in idx else 0 for i in range(len(list1))]

or

list2 = [e if i in idx else 0 for i, e in enumerate(list1)]

Also, do not write code like this. It is much less readable than your example. Furthermore, numpy may give you the kind of syntax you desire without sacrificing readability or speed.

---

import numpy as np 

...

arr1 = np.array(list1)

arr2 = np.zeros_like(list1)
arr2[idx] = arr1[idx]

Use list item index as index to another list in python

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.

Create a list from another list indexes

This snippet should do what you are asking for (in a pythonic/explicit way):

list_1 = [11, 10, 7, 0, 3, 5, 8, 1, 4, 6, 9, 2]

# keep the indexes of every element
indexes = {v: idx for idx, v in enumerate(list_1)}
# return the index of every next element - if there isn't, return the index for 0
list_2 = [indexes.get(v+1, indexes[0]) for v in list_1]

How to slice a list using indexes in another list

Using itertools.pairwise:

>>> from itertools import pairwise
>>> for i, j in pairwise([0, 888, 1776, 2664, 3551, 4438]):
...     print(i, j)
...
0 888
888 1776
1776 2664
2664 3551
3551 4438

What's next? I think you can solve it yourself.

Using index from a list to get another value/element in another list

Perhaps use a list_comprehension:

print([j_set[item] for i in on_going for item in range(len(e_list)) if i in e_list[item]])
#[2, 3]

---

Related Topics