How to extract elements from a list using indices in Python?

Question

If you have a list in python, and want to extract elements at indices 1, 2 and 5 into a new list, how would you do this?

This is how I did it, but I'm not very satisfied:

>>> a
[10, 11, 12, 13, 14, 15]
>>> [x[1] for x in enumerate(a) if x[0] in [1,2,5]]
[11, 12, 15]

Is there a better way?

More in general, given a tuple of indices, how would you use this tuple to extract the corresponding elements from a list, even with duplication (e.g. tuple (1,1,2,1,5) produces [11,11,12,11,15]).

Is there a garantee that the tuple only contains valid indexes? — Charles Beattie, Apr 12 '10 at 11:41

score 89 · Accepted Answer · answered Apr 12 '10 at 11:37

89

Perhaps use this:

[a[i] for i in (1,2,5)]
# [11, 12, 15]

answered Apr 12 '10 at 11:37

unutbu

842,883
184
1,785
1,677

btw this is also much faster. fetching K items from a list of size N is of complexity O(K) here and O(N*K) in OP's solution. – yairchu Apr 12 '10 at 11:41
2

ok... thank you. This was damn trivial, and it's clear that I'm too tired. – Stefano Borini Apr 12 '10 at 11:43
1

No problem. Any day I can answer a Stefano Borini question is a (rare, but) good day :) – unutbu Apr 12 '10 at 11:52
Personally I've never seen Python written that way, but I have to agree. That is pretty genius, and readable! – Xavier Ho Apr 21 '10 at 14:16

score 10 · Answer 2 · answered Apr 12 '10 at 11:45

10

I think you're looking for this:

elements = [10, 11, 12, 13, 14, 15]
indices = (1,1,2,1,5)

result_list = [elements[i] for i in indices]

answered Apr 12 '10 at 11:45

lugte098

2,271
5
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score 6 · Answer 3 · answered Apr 12 '10 at 11:41

6

Try

numbers = range(10, 16)
indices = (1, 1, 2, 1, 5)

result = [numbers[i] for i in indices]

answered Apr 12 '10 at 11:41

Juho Vepsäläinen

26,573
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score 5 · Answer 4 · answered Aug 19 '15 at 04:30

Use Numpy direct array indexing, as in MATLAB, Julia, ...

a = [10, 11, 12, 13, 14, 15];
s = [1, 2, 5] ;

import numpy as np
list(np.array(a)[s])
# [11, 12, 15]

Better yet, just stay with Numpy arrays

a = np.array([10, 11, 12, 13, 14, 15])
a[s]
#array([11, 12, 15])

score 3 · Answer 5 · answered Apr 12 '10 at 11:43

3

Bounds checked:

 [a[index] for index in (1,2,5,20) if 0 <= index < len(a)]
 # [11, 12, 15]

answered Apr 12 '10 at 11:43

Charles Beattie

5,739
1
29
32

How to extract elements from a list using indices in Python?

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