Modify lists within lists using list comprehension

Question

So. I have a list containing lists

>>> f[:10]
[['-340', '495', '-153', '-910', '835', '-947'], ['-175', '41', '-421', '-714', '574', '-645'], ['-547', '712', '-352', '579', '951', '-786'], ['419', '-864', '-83', '650', '-399', '171'], ['-429', '-89', '-357', '-930', '296', '-29'], ['-734', '-702', '823', '-745', '-684', '-62'], ['-971', '762', '925', '-776', '-663', '-157'], ['162', '570', '628', '485', '-807', '-896']]

I want to change each value within each of the lists within the main list to an int. In other words, I want to end up with this

>>> f[:10]
[[-340, 495, -153, -910, 835, -947], [-175, 41, -421, -714, 574, -645], [-547, 712, -352, 579, 951, -786], [419, -864, -83, 650, -399, 171], [-429, -89, -357, -930, 296, -29], [-734, -702, 823, -745, -684, -62], [-971, 762, 925, -776, -663, -157], [162, 570, 628, 485, -807, -896]]

Now, I can easily do so with the following code:

for x in range(len(f)):
    for y in range(6):
        f[x][y] = int(f[x][y])

But I wanted to know if I could do this using list comprehension. I tried the following code

f = [int(x) for y in f for x in y]

but that results in one large list

>>> f[:10]
[-340, 495, -153, -910, 835, -947, -175, 41, -421, -714]

Anyone know how to accomplish the desired result using list comprehension?

That's because you are using a nested list comprehension. convert the items within a sublist, instead like `[[int(x) for x in sub] for sub in f]`. — Mazdak, Jan 20 '17 at 08:43

score 2 · Answer 1 · answered Jan 20 '17 at 08:44

With f as the starting list, use a nested comprehension:

>>> import pprint
>>> result = [[int(x) for x in l] for l in f]
>>> pprint.pprint(f)
[['-340', '495', '-153', '-910', '835', '-947'],
 ['-175', '41', '-421', '-714', '574', '-645'],
 ['-547', '712', '-352', '579', '951', '-786'],
 ['419', '-864', '-83', '650', '-399', '171'],
 ['-429', '-89', '-357', '-930', '296', '-29'],
 ['-734', '-702', '823', '-745', '-684', '-62'],
 ['-971', '762', '925', '-776', '-663', '-157'],
 ['162', '570', '628', '485', '-807', '-896']]
>>> pprint.pprint(result)
[[-340, 495, -153, -910, 835, -947],
 [-175, 41, -421, -714, 574, -645],
 [-547, 712, -352, 579, 951, -786],
 [419, -864, -83, 650, -399, 171],
 [-429, -89, -357, -930, 296, -29],
 [-734, -702, 823, -745, -684, -62],
 [-971, 762, 925, -776, -663, -157],
 [162, 570, 628, 485, -807, -896]]

Or list and map:

>>> result = [list(map(int, l)) for l in f]
>>> pprint.pprint(result)
[[-340, 495, -153, -910, 835, -947],
 [-175, 41, -421, -714, 574, -645],
 [-547, 712, -352, 579, 951, -786],
 [419, -864, -83, 650, -399, 171],
 [-429, -89, -357, -930, 296, -29],
 [-734, -702, 823, -745, -684, -62],
 [-971, 762, 925, -776, -663, -157],
 [162, 570, 628, 485, -807, -896]]

OP says they want to modify the existing list, not build a new one; could assign to `f[:]` of course — Chris_Rands, Jan 20 '17 at 08:53
@Chris_Rands - They're clearly okay with making a new one: `f = [int(x) for y in f for x in y]` in the question. — TigerhawkT3, Jan 20 '17 at 08:58
Maybe, but with their `for` loops they do modify the existing list. If they want a new list, it's an exact dupe of... http://stackoverflow.com/questions/18072759/python-nested-list-comprehension — Chris_Rands, Jan 20 '17 at 09:05

score 2 · Answer 2 · answered Jan 20 '17 at 08:48

2

You need a nested list comprehension for this. Try:

[[int(x) for y in f] for x in y]

answered Jan 20 '17 at 08:48

Anomitra

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Modify lists within lists using list comprehension

2 Answers2