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) ?
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.
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]
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!
arr_A = np.array(list_A)>>> 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]
I don't think this technically meets the requirement that there be no for-loop.
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])
for-loop. The obvious way to do it is with a list comprehension. Does[A[x] for x in B]use afor-loop? Maybe. Doesmap(lambda i: list_A[i], list_B)? Nofor-loopin sight, but there is iteration inmap. How about numpy solutions? No iteration visible, but ...