Extract data from numpy array to create another one

Question

Assume I have the following numpy array :

a = np.array([[4, 5, 8],
              [7, 2, 9],
              [1, 5, 3]])

and I want to extract points from the array 'a' to have this array :

b = array([[4, 8],
           [1, 3]])

How can I do this ?

PS : In my real case I have 13*13 matrix and I want to create a 3*3 matrix from the first one

13x13 -> 3x3: what are the criteria to select the elements for the output? — FObersteiner
– FObersteiner, Commented Feb 1, 2020 at 17:18
I have a function and I want to extract the points that the user want, let's say the user execute the function like this: my_function([1, 3, 5]), then I create the cartesian power from these points, namely: {(1,1), (1,3), ..., (5, 5)} and I extract all these points from my 13*13 matrix to create a 3*3 matrix — Yacine Rouizi
– Yacine Rouizi, Commented Feb 1, 2020 at 17:34

Ethan · Accepted Answer · 2020-02-01 17:31:28Z

2

You can use np.ix_() for this to create a map of which values you want by location.

>>> a = np.arange(1,10).reshape(3,3)
>>> a
array([[1, 2, 3],
       [4, 5, 6],
       [7, 8, 9]])

>>> b=np.ix_([0,2],[0, 2])
>>> a[b]
array([[1, 3],
       [7, 9]])

answered Feb 1, 2020 at 17:31

Ethan

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