CODEWITHSUNDEEP
pythonarraysnumpy
rookie
rookie

Reputation: 11

multiply two numpy arrays to make a matrix

I got some output of my program, say

a = [0.5 0.73 -0.48] (I don't see commas there, which I feel a bit weird)

b = [0.1 0.2 0.3 0.4 0.5] (same as above)

I checked each by using type() and they were numpy.ndarray, I then use len() to verify their size and got 3 and 5 respectively. I also did shape to find that a is (3,) and b is (5,)

Now, I wonder how could I multiply these two to get a matrix of size 3 by 5?

I have tried so many ways and none has worked.

Upvotes: 1

Views: 191

Answers (2)

Julius
Julius

Reputation: 91

Try the outer product: https://numpy.org/doc/stable/reference/generated/numpy.outer.html

import numpy as np

a = np.array([0.5, 0.73, -0.48])
b = np.array([0.1, 0.2, 0.3, 0.4, 0.5])

result = np.outer(a, b)
print(result)

Output:

[[ 0.05 ,  0.1  ,  0.15 ,  0.2  ,  0.25 ],
[ 0.073,  0.146,  0.219,  0.292,  0.365],
[-0.048, -0.096, -0.144, -0.192, -0.24 ]]

Upvotes: 2

Tim Roberts
Tim Roberts

Reputation: 54867

You don't see commas because those are numpy vectors. Numpy displays its arrays without commas.

The "@" operator is matrix multiply. The key is that you need to turn these into 2D arrays.

import numpy as np
a = np.array([0.5, 0.73, -0.48])
b = np.array([0.1, 0.2, 0.3, 0.4, 0.5])
result = a.reshape(3,1)@b.reshape(1,5)
print(result)

output:

[[ 0.05   0.1    0.15   0.2    0.25 ]
 [ 0.073  0.146  0.219  0.292  0.365]
 [-0.048 -0.096 -0.144 -0.192 -0.24 ]]

Upvotes: 1

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