Python nested list comprehension (accessing nested elements)

Question

I'm having trouble understanding the syntax here.

matrix_a = [[1, 2], [3, 4], [5, 6]]
matrix_b = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

[a for a, b in matrix_a]

output: [1, 3, 5]

[a for b, a in matrix_a]

output: [2, 4, 6]

I understand a little about how list-comprehensions work, but I don't understand the syntax when accessing certain elements within a nested list.

I just can't wrap my head around this syntax. How is this syntax working? What does the comma represent? What does a for a mean? Can you explain whats going on under the hood? And finally how would you do this with matrix_b

The question listed as a duplicate is about nested list comprehensions, this question is not about that. — thebjorn, Nov 08 '17 at 09:28

score 3 · Accepted Answer · answered Nov 08 '17 at 09:19

3

If you convert it to a for loop it might be easier to see..?

res = []
for item in matrix_a:
    a, b = item   # a = item[0]; b = item[1]
    res.append(a)

you're basically unpacking the individual items in the list and picking one of them.

answered Nov 08 '17 at 09:19

thebjorn

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How would this look as a list-comprehension for matrix_b? – Christopher Reece Nov 08 '17 at 09:26
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the case of matrix_b is the same, but you will unpack three vars `[a for a,b,c in matrix_b]` – Éttore Leandro Tognoli Nov 08 '17 at 09:26
1

Ok, I see it now. I was reading it as two separate statements `a for a,` and `b in matrix_a`. Wow thanks for clarifying that :D You guys are awesome. – Christopher Reece Nov 08 '17 at 09:29

score 0 · Answer 2 · answered Nov 08 '17 at 09:27

0

I think you have mistaken in writing output here [a for a, b in matrix_a] returns [1 2 4] which is logical , returning first element of each nested list item

see the screenshot

answered Nov 08 '17 at 09:27

Ranjeet Singh

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10

You could have copied the text instead of using an image. Also, this doesn't seem like an _answer_ ..? – thebjorn Nov 08 '17 at 09:33

score 0 · Answer 3 · answered Nov 08 '17 at 09:27

just understand in this way:

[a for b, a in matrix_a]  #as
[a for [a, b] in matrix_a] #or
[a for (a, b) in matrix_a]
#matrix_a has elements as list of length 2 each
#so your list comprehenssion says, give me a output list -- having first element'a' from each element of matrix_a(whose each element represented as [a,b] type, with comma, becomes tuple (a,b)),
# then from [b,a] give me a, that is 2nd element
# and it will fail if you apply same on matrix_b, cause each element of matrix_b is not of type [a,b] i:e of length 2
# you will get "ValueError: too many values to unpack"

Let me know if anything is not clear. Thanks.

Python nested list comprehension (accessing nested elements)

3 Answers3