# Pythonic way to get some rows of a matrix

I was thinking about a code that I wrote a few years ago in Python, at some point it had to get just some elements, by index, of a list of lists.

I remember I did something like this:

``````def getRows(m, row_indices):
tmp = []
for i in row_indices:
tmp.append(m[i])
return tmp
``````

Now that I've learnt a little bit more since then, I'd use a list comprehension like this:

``````[m[i] for i in row_indices]
``````

But I'm still wondering if there's an even more pythonic way to do it. Any ideas?

I would like to know also alternatives with numpy o any other array libraries.

Asked by: Connie801 | Posted: 06-12-2021

It's worth looking at NumPy for its slicing syntax. Scroll down in the linked page until you get to "Indexing, Slicing and Iterating".

Answered by: John317 | Posted: 07-01-2022

It's the clean an obvious way. So, I'd say it doesn't get more Pythonic than that.

Answered by: Lenny392 | Posted: 07-01-2022

As Curt said, it seems that Numpy is a good tool for this. Here's an example,

``````from numpy import *

a = arange(16).reshape((4,4))
b = a[:, [1,2]]
c = a[[1,2], :]

print a
print b
print c
``````

gives

``````[[ 0  1  2  3]
[ 4  5  6  7]
[ 8  9 10 11]
[12 13 14 15]]
[[ 1  2]
[ 5  6]
[ 9 10]
[13 14]]
[[ 4  5  6  7]
[ 8  9 10 11]]
``````

Answered by: Byron204 | Posted: 07-01-2022

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