Extracting first n columns of a numpy matrix

Question

I have an array like this:

  array([[-0.57098887, -0.4274751 , -0.38459931, -0.58593526],
         [-0.22279713, -0.51723555,  0.82462029,  0.05319973],
         [ 0.67492385, -0.69294472, -0.2531966 ,  0.01403201],
         [ 0.41086611,  0.26374238,  0.32859738, -0.80848795]])

Now I want to extract the following:

   [-0.57098887, -0.4274751]
   [-0.22279713, -0.51723555]
   [ 0.67492385, -0.69294472]
   [ 0.41086611,  0.26374238]

So basically just first 2 columns..

Numpy documentation: slicing. Always check the documentation first. — Joel Cornett
– Joel Cornett, Commented May 16, 2012 at 19:12
@JoelCornett: Thanks.. so slicing is the term.. it gets pretty hard if you know the concept but not the term.. :) many thanks :) — frazman
– frazman, Commented May 16, 2012 at 19:13

NPE · Accepted Answer · 2012-05-16 19:13:40Z

94

If a is your array:

In [11]: a[:,:2]
Out[11]: 
array([[-0.57098887, -0.4274751 ],
       [-0.22279713, -0.51723555],
       [ 0.67492385, -0.69294472],
       [ 0.41086611,  0.26374238]])

answered May 16, 2012 at 19:13

NPE

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Contango · Accepted Answer · 2021-02-08 12:18:30Z

32

I know this is quite an old question -

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

Let's say, you want to extract the first 2 rows and first 3 columns

A_NEW = A[0:2, 0:3]
A_NEW = [[1, 2, 3],
         [4, 5, 6]]

Understanding the syntax

A_NEW = A[start_index_row : stop_index_row, 
          start_index_column : stop_index_column)]

If one wants row 2 and column 2 and 3

A_NEW = A[1:2, 1:3]

Reference the numpy indexing and slicing article - Indexing & Slicing

edited Feb 8, 2021 at 12:18

Contango

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answered May 14, 2015 at 18:20

o12d10

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