How list comprehension works for two for loop?

January 5, 2022

This is two for loop code, where the inner loop depends upon the first loop. When I tried to create list compression it gives the wrong result. Please suggest what is missing.

for i in range(5):
    for j in range(i):
        print(j, end=', ')
print()

print([j for j in range(i) for i in range(5)])

Output:

0, 0, 1, 0, 1, 2, 0, 1, 2, 3,
[0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 3, 3, 3, 3, 3]

Thanks!!!

>Solution :

Where you read the for loops from top to bottom, you read the generators from left to right in the list comprehension.

#              outer/first       inner/second
>>> print([j for i in range(5) for j in range(i)])
[0, 0, 1, 0, 1, 2, 0, 1, 2, 3]

The example you use in your question only works because i is still defined from the previous for loops when you run range(i). If you tried to run that in a fresh interpreter, you would get a NameError on i.