Python列表作为列表索引

Question

Suppose we have two lists

list_A = [1,3,4,54,3,5,6,2,6,77,73,39]
list_B = [0,3,2,8]

I want to access elements of list_A that have the values in list_B as their indices (without using loops).

After implementing it, the result should be as follows for the above case:

[1, 54, 4, 6]

Is there any easy method to do this without bothering with for loops (calling it explicitly in the code) ?

Answer 1

Everything will use a loop internally* , but it doesn't have to be as complicated as you might think. You can try a list comprehension:

[list_A[i] for i in list_B]

Alternatively, you could use operator.itemgetter :

list(operator.itemgetter(*list_B)(list_A))

---

>>> import operator
>>> list_A = [1,3,4,54,3,5,6,2,6,77,73,39]
>>> list_B = [0,3,2,8]
>>> [list_A[i] for i in list_B]
[1, 54, 4, 6]
>>> list(operator.itemgetter(*list_B)(list_A))
[1, 54, 4, 6]

---

* OK! Maybe you don't need a loop with recursion, but I think it's definately overkill for something like this.

Answer 2

Recursion answer

No iteration/loops here. Just recursion.

>>> list_A = [1,3,4,54,3,5,6,2,6,77,73,39]
>>> list_B = [0,3,2,8]
>>> def foo(src, indexer):
...     if not indexer:
...         return []
...     else:
...         left, right = indexer[0], indexer[1:]
...         return [src[left]] + foo(src, right)
... 
>>> foo(list_A, list_B)
[1, 54, 4, 6]

Answer 3

map answer

>>> list_A = [1,3,4,54,3,5,6,2,6,77,73,39]
>>> list_B = [0,3,2,8]
>>> map(lambda i: list_A[i], list_B)
[1, 54, 4, 6]

Disclaimer

I don't think this technically meets the requirement that there be no for-loop .

Answer 4

If you're using numpy, you can do the following:

list_A[list_B]  // yields [1, 54, 4, 6]

Edit: As Nick T pointed out, don't forget to convert to arrays first!

Answer 5

Numpy can do this with its indexing in a fairly straightforward manner. Convert the list to an array then you're gold. You can pass any sort of iterable as indices.

>>> import numpy
>>> list_A = [1,3,4,54,3,5,6,2,6,77,73,39]
>>> list_B = [0,3,2,8]
>>> numpy.array(list_A)[list_B]
array([ 1, 54,  4,  6])