Python range and for loops

Question

I was toying around with Python and I ran the following:

for i,j in (range(-1, 2, 2),range(-1, 2, 2)):
    print(i,j)

I expected this to print

>> -1, -1
>> 1, 1

However what came out was

>> -1, 1
>> -1, 1

(1) Why is this the output and what is going on behind the scenes

(2) If I wanted to get all combinations of -1 and 1 (which are(-1,-1), (-1,1), (1,-1), and (1,1)) I know that I can use a nested for loop

for i in range(-1,2,2):
    for j in range(-1,2,2):
        print(i,j)

However is there a way to do this in one line with one for loop call (what I was trying to do with the original code)?

You have two question here, can you please edit it down to one? Thanks — user12867493
– user12867493, Commented Jun 19, 2020 at 0:49
Sorry about the two questions in what was supposed to be one question. I thought they were so related that they could be thought of as parts of a question. Someone already answered the question so I won't change it now but I will keep that in mind for the future! Thanks! — BOSSrobot
– BOSSrobot, Commented Jun 19, 2020 at 1:00

Osman Mamun · Accepted Answer · 2020-06-19 00:55:41Z

2

For your first question, in the first iteration loop it takes range(-1, 2, 2) which expands to [-1, 1] and then assigns i=-1 and j=1. Similarly in the second cycle it does the same for the second range(-1, 2, 2). You're lucky that you have two elements in that range. Otherwise it would raise error, e.g., range(-1, 4, 2) would raise error as it has 3 elements. You need to use i, j, k for that loop.

For your second question use itertools.product.

for i in product(range(-1, 2, 2), range(-1, 2, 2)): 
    print(i)
    #(-1, -1)
    #(-1, 1)
    #(1, -1)
    #(1, 1)

answered Jun 19, 2020 at 0:55

Osman Mamun

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Comments

bileleleuch · Accepted Answer · 2020-06-19 01:05:22Z

2

First question : You are iterating over i and j over a tuple (range(-1, 2, 2), range(-1, 2, 2)). try this code and you'll understand.

>>> for i in (range(-1, 2, 2),range(-1, 2, 3)):
...     print(i)
... 
range(-1, 2, 2)
range(-1, 2, 3)

answered Jun 19, 2020 at 1:05

bileleleuch

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Jason Yang · Accepted Answer · 2020-06-19 01:11:53Z

1

You can think range(-1, 2, 2) is (-1, 1), then

for i, j in (range(-1, 2, 2), range(-1, 2, 2)):

is similar

for i, j in ((-1, 1), (-1, 1)):

You should do it like this

for i, j in zip(range(-1, 2, 2), range(-1, 2, 2)):
    print(i, j)

To all combinations of -1 and 1, use this is more flexiable

for i in range(-1,2,2):
    for j in range(-1,2,2):
        print(i,j)

or this

from itertools import product

for i, j in product(range(-1, 2, 2), range(-1, 2, 2)):

answered Jun 19, 2020 at 1:11

Jason Yang

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