Newbie question: Nested List Comprehensions

[Truman]

Looking at this code:

matrix = [
[1,2,3,4],
[5,6,7,8],
[9,10,11,12],
]
r = [[row[i] for row in matrix] for i in range(4)]
print®

Do not understand how it transposes rows into columns. For a start, does row[1] means [1] or [1,2,3,4]?
If it means 1 then does in each iteration row from "for row" refers to the row below the previous one?

[Larz60+]

Understanding a list comprehension may be easier for you if you vreak it into it's elements:

>>> for i in range(4):
...     for row in matrix:
...         print(f'i: {i}, row[i]: {row[i]}')
...
i: 0, row[i]: 1
i: 0, row[i]: 5
i: 0, row[i]: 9
i: 1, row[i]: 2
i: 1, row[i]: 6
i: 1, row[i]: 10
i: 2, row[i]: 3
i: 2, row[i]: 7
i: 2, row[i]: 11
i: 3, row[i]: 4
i: 3, row[i]: 8
i: 3, row[i]: 12
>>>

[Mekire]

Let's unwrap your list comp into standard for loops and add some print statements:

r = []
for i in range(4):
    col = []
    print("i = {}".format(i))
    for row in matrix:
        print("    row = {}".format(row))
        print("    row[{}] = {}\n".format(i, row[i]))
        col.append(row[i])
    r.append(col)

print(r)

Output:
i = 0
    row = [1, 2, 3, 4]
    row[0] = 1

    row = [5, 6, 7, 8]
    row[0] = 5

    row = [9, 10, 11, 12]
    row[0] = 9

i = 1
    row = [1, 2, 3, 4]
    row[1] = 2

    row = [5, 6, 7, 8]
    row[1] = 6

    row = [9, 10, 11, 12]
    row[1] = 10

i = 2
    row = [1, 2, 3, 4]
    row[2] = 3

    row = [5, 6, 7, 8]
    row[2] = 7

    row = [9, 10, 11, 12]
    row[2] = 11

i = 3
    row = [1, 2, 3, 4]
    row[3] = 4

    row = [5, 6, 7, 8]
    row[3] = 8

    row = [9, 10, 11, 12]
    row[3] = 12

i starts at 0 and then we loop throw all the rows and select the 0th element.  Then i becomes 1 and we select the 1th element.  Repeat until i is 4.

[Truman]

Thank you. What I would like to know now how computer knows to iterate through each row for every i. Why didn't it take only first row for i=0, second row for i=1 etc?

[stranac]

Because there are two loops.
The outer loop increments i, and for every value of i, the inner loop gets the i-th element of each row.

[Truman]

Get it, now I see that Larz60+ already presented those two loops in his message.