The second to the last line of code is confusing to me

Question

I am quite new to python and programming in general so I am still trying to understand the details in practice. This is a problem I found online so I can practice nested loops more. If my question is missing anything or you do not understand my question, please let me know. I would like to get better at asking good questions as well.

list =[[1, 2], [3,4]]
m = 1
print(list)
for i in range(0, 2):
    m *= 10
    for j in range(0, 2):
        list[i][j] *= m  # This part right here.
print(list)

This is what prints on the terminal:

[[1, 2], [3, 4]]
[[10, 20], [300, 400]]

I was trying to go through this block of code step by step to make sure I understand it but this part is stumping me. I understand that the whole function of this nested for loop is to multiply the items in the 1st list with 10 and the 2nd list with 100. I also know what the *= m part is, the part that's confusing to me is the code right before that on the same line.

So far I tried to just copy this specific part in google and see if anything came up. I could not find anything that would make sense. I also tried to just run this whole line and see what printed (list[i][j] *= m)(I changed the variable to numbers obviously). That only came up with a type error... There are no type variables left in list[2]. I was trying to isolate it to see what just this part does but it apparently doesn't work like that. I guess i need to think outside the box a little more maybe.

Answer 1

If we take i to be 1 and j to be 1, list[i][j] would be 4. list[i] = [3,4] so what you're doing is finding index 1 of the list [3, 4]

Answer 2

list is a list containing nested lists.

list[0] is the list [1, 2]. list[1] is the list [3, 4].

When you use two indexes like list[i][j], it first gets the nested list list[i], then accesses the [j] element of that. So when i == 0 and j == 1, this accesses the list element containing 2.

*= m then multiplies that list element. So when i == 0 and m == 10, it multiplies the values in the first sublist by 10. Then when i == 1 and m == 100 it multiplies the values in the second sublist by 100.

Answer 3

list = [[1,2], [3,4]] is an array of arrays. To get a single element, you have to subscript list twice, which is exactly what list[i][j] is.

list[0] returns [1,2], the 0-th element of the list, which is a sublist

list[0][0] returns 1, the 0-th element of the 0-th sublist

list[0][1] returns 2, the 1st element of the 0-th sublist

.

list[1] returns [3,4], the 1st element of the list, which is a sublist

list[1][0] returns 3, the 0-th element of the 1st sublist

list[1][1] returns 4, the 1st element of the 1st sublist

.

Also, the reason why list[2] doesn't return anything is because list only has two elements, which have the index 0 and 1, so trying to get the element at index 2 will not work