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pythonnumpymatrix
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Reputation: 3377

python: Multiply two 1d matrices in numpy

a = np.asarray([1,2,3])

b = np.asarray([2,3,4,5])

a.shape

(3,)

b.shape

(4,)

I want a 3 by 4 matrix that's the product of a and b

1
2    *    2 3 4 5
3

np.dot(a, b.transpose())

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: objects are not aligned

dot product is only equivalent to matrix multiplication when the array is 2d, so np.dot doesn't work.

Upvotes: 3

Views: 2230

Answers (4)

Rishabh Gupta
Rishabh Gupta

Reputation: 73

the following code does as demanded basically you did a mistake while reshaping the arrays , you should reshape them so as to ensure matrix multiplication property is satisfied.

    a = np.asarray([1,2,3])
    b = np.asarray([2,3,4,5])
    a.reshape(3,1)

Upvotes: 0

user2357112
user2357112

Reputation: 281152

This is np.outer(a, b):

In [2]: np.outer([1, 2, 3], [2, 3, 4, 5])
Out[2]: 
array([[ 2,  3,  4,  5],
       [ 4,  6,  8, 10],
       [ 6,  9, 12, 15]])

Upvotes: 7

hpaulj
hpaulj

Reputation: 231385

No need to use the matrix subtype. Regular array can be expanded to 2d (and transposed if need).

In [2]: a=np.array([1,2,3]) 
In [3]: b=np.array([2,3,4,5])

In [4]: a[:,None]
Out[4]: 
array([[1],
       [2],
       [3]])

In [5]: a[:,None]*b   # outer product via broadcasting
Out[5]: 
array([[ 2,  3,  4,  5],
       [ 4,  6,  8, 10],
       [ 6,  9, 12, 15]])

Other ways of making that column array

In [6]: np.array([[1,2,3]]).T
Out[6]: 
array([[1],
       [2],
       [3]])

In [7]: np.array([[1],[2],[3]])
Out[7]: 
array([[1],
       [2],
       [3]])

In [9]: np.atleast_2d([1,2,3]).T
Out[9]: 
array([[1],
       [2],
       [3]])

Upvotes: 4

Matthew Gray
Matthew Gray

Reputation: 1676

Instead of asarray, use asmatrix.

import numpy as np
a = np.asmatrix([1,2,3])
b = np.asmatrix([2,3,4,5])
a.shape #(1, 3)
b.shape #(1, 4)
np.dot(a.transpose(),b)

Results in:

matrix([[ 2,  3,  4,  5],
        [ 4,  6,  8, 10],
        [ 6,  9, 12, 15]])

Upvotes: 0

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