How to make a matrix of arrays in numpy?

Question

I want to make a 2x2 matrix

T = [[A, B],
     [C, D]]

where each element A,B,C,D is an array (of same size, of course). Is this possible?

I would like to be able to multiply these matrix, for example multiplying two matrix T1 and T2 should give me

T1*T2 = [[A1*A2, B1*B2],
         [C1*C2, D1*D2]]

which is still a matrix of arrays of the same size. Is there such a multiplication function?

And also, if I multiply T with a normal scalar matrix t = [[a,b],[c,d]] where a,b,c,d are scalar numbers, the the multiplication should give me

t*T = [[a*A, b*B],
       [c*C, d*D]]

How can I do this? An example or a link to related material would be great.

root · Accepted Answer · 2013-03-20 07:07:06Z

2

Doesn't your first question just work as you would expect?

In [1]: import numpy as np

In [2]: arr = np.arange(8).reshape(2, 2, 2)

In [3]: arr
Out[3]: 
array([[[0, 1],
        [2, 3]],

       [[4, 5],
        [6, 7]]])

In [4]: arr*arr
Out[4]: 
array([[[ 0,  1],
        [ 4,  9]],

       [[16, 25],
        [36, 49]]])

As for your second question, just reshape it to a 3 dimensional array:

In [5]: arr2 = np.arange(4).reshape(2, 2)

In [6]: arr2
Out[6]: 
array([[0, 1],
       [2, 3]])

In [7]: arr2 = arr2.reshape(2, 2, 1)

In [8]: arr2
Out[8]: 
array([[[0],
        [1]],

       [[2],
        [3]]])

In [9]: arr*arr2
Out[9]: 
array([[[ 0,  0],
        [ 2,  3]],

       [[ 8, 10],
        [18, 21]]])

edited Mar 20, 2013 at 7:07

answered Mar 20, 2013 at 6:40

root

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1 Comment

LWZ Over a year ago

Thanks! This is awesome. I think my mistake to the first part is that I should use array function to construct the matrix, not the matrix function. Second part works perfectly, exactly what I want.

Shankar · Accepted Answer · 2013-03-20 06:56:22Z

0

>>> from numpy import array, matrix, multiply
>>> A = array([1, 2, 3])
>>> 
>>> B = array([2, 3, 4])
>>> 
>>> C = array([4, 5, 6])
>>> 
>>> D = array([7, 8, 9])
>>> 
>>> T1 = matrix([A, B, C, D])
>>> T1
matrix([[1, 2, 3],
        [2, 3, 4],
        [4, 5, 6],
        [7, 8, 9]])
>>> T2 = T1 * 100
>>> T2
matrix([[100, 200, 300],
        [200, 300, 400],
        [400, 500, 600],
        [700, 800, 900]])
>>> 
>>> multiply(T1, T2)
matrix([[ 100,  400,  900],
        [ 400,  900, 1600],
        [1600, 2500, 3600],
        [4900, 6400, 8100]])
>>>

answered Mar 20, 2013 at 6:56

Shankar

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