I find indexing in python for numpy array weird

Question

I'm quite new to python and I'm working on some data manipulation. I thought the indexing in numpy would be [row][column], but it's not what I see when I execute in python. Below is a simple example of how python is behaving. I don't understand why I get the same results for the last two commands:

import numpy as num

test_arr = num.array([[1, 2, 3],[4, 5, 6], [7, 8, 9]],dtype=num.float32)

test_arr[0][:]
array([1., 2., 3.], dtype=float32)

test_arr[:][0]
array([1., 2., 3.], dtype=float32)

I would expect

test_arr[0][:]
array([1., 2., 3.], dtype=float32)

test_arr[:][0]
array([1., 4., 7.], dtype=float32)

Can someone explain why python behaves as it does and how to get the 0th index of all rows?

Even with Python lists [:] only creates a copy; with arrays it's a view. So it's doing nothing in your examples. Each [] is a separate indexing step. Take time to read numpy basics, such as this indexing page: docs.scipy.org/doc/numpy/reference/arrays.indexing.html — hpaulj
– hpaulj, Commented Apr 2, 2020 at 15:52

Aly Hosny · Accepted Answer · 2020-04-02 14:08:09Z

1

to get a column in numpy array use [:,n] where n is your column number

test_arr[:,0]
array([1., 4., 7.], dtype=float32)

answered Apr 2, 2020 at 14:08

Aly Hosny

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Mohamed Shabeer kp · Accepted Answer · 2020-04-02 14:11:42Z

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In  : import numpy as num

In  : test_arr = num.array([[1, 2, 3],[4, 5, 6], [7, 8, 9]],dtype=num.float32)

In  : test_arr[0,:]
Out : [1. 2. 3.]

In  : test_arr[:,0]
Out : [1. 4. 7.]

answered Apr 2, 2020 at 14:11

Mohamed Shabeer kp

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