Can someone please explain this Python code?

Question

Whilst I know what this piece of code does i have no clue how it achieves it. Can someone please explain it in the dumbest way?

vec = [[1,2,3], [4,5,6], [7,8,9]]
[num for elem in vec for num in elem]
[1, 2, 3, 4, 5, 6, 7, 8, 9]

The first for part of the code returns this:

[num for elem in vec]
[0, 0, 0]

Are they indexes for each nested lists first entry?

Thanks!

are you on jupyter notebook? if `num` is not previously defined, as for your case I suppose, `[num for elem in vec]` must raise Error — Lante Dellarovere, Apr 21 '19 at 11:30

ruohola · Answer 1 · 2019-04-22T14:43:11.020

It's just a list comprehension with two loops. It's roughly equal to this:

ls = [] 
for elem in vec:
    for num in elem:
        ls.append(num)

The first for part of the code returns this:
[num for elem in vec]
[0, 0, 0]

It doesn't make sense to only look at that part of the code. That's like only looking at the last 2 lines of my answer's code example and trying to understand what they're doing without looking at the other lines.

Can someone please explain this Python code?

1 Answers1