CODEWITHSUNDEEP
pythonfor-looprange
BOSSrobot
BOSSrobot

Reputation: 27

Python range and for loops

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)?

Upvotes: 0

Views: 77

Answers (3)

Jason Yang
Jason Yang

Reputation: 13057

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)):

Upvotes: 1

bileleleuch
bileleleuch

Reputation: 161

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)

Upvotes: 2

Osman Mamun
Osman Mamun

Reputation: 2882

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)

Upvotes: 2

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