Python array extraction

Question

I am extracting the first two rows of an array A. In x1, I first used A[:,] and then [0:2,:]. In x2, I immediately used [0:2,:]. They are basically the same as shown below. However, I saw someone used A[:,][0:2,:] but I really don't know the reason for having [:,] first. This seems redundant to me. Is there any specific reason that we use A[:,][0:2,:] rather than A[0:2,:]? Sorry if this question looks so easy.

import numpy as np

A =np.array([[1,2,3,4],[5,6,7,8],[9,10,11,12]])

x1=A[:,][0:2,:]

x2=A[0:2,:]

x1
Out[1]: 
array([[1, 2, 3, 4],
       [5, 6, 7, 8]])

x2
Out[2]: 
array([[1, 2, 3, 4],
       [5, 6, 7, 8]])

Well, whoever you saw using A[:,][0:2,:] probably shouldn't be doing that as it is indeed redundant. — rth
– rth, Commented Jun 16, 2015 at 17:17

Anand S Kumar · Accepted Answer · 2015-06-16 17:18:38Z

1

If you do the following code , you will see the reason for the same behaviour -

import numpy as np
A =np.array([[1,2,3,4],[5,6,7,8],[9,10,11,12]])
x1=A[:,]
print(x1)
>> array([[1,2,3,4],[5,6,7,8],[9,10,11,12]])

This is because in numpy , , is used as the slicing character , if you do `A[:,1] , it would give the results only from 2nd column (index = 1).

If you want reference

answered Jun 16, 2015 at 17:18

Anand S Kumar

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2 Comments

Stat Over a year ago

So having [:,] in A[:,][0:2,:] is redundant. Right?

Anand S Kumar Over a year ago

Yes , I believe so, looks like an unneccessary overhead to me

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