Add an array's values to a row in a numpy matrix

Question

I want to add the values in a numpy array to the values in a specific row of a numpy matrix.

Given:

A = [[0, 0], [0, 0]]

b = [1, 1]

I want to add b to the values in the first row in A. The expected output is:

[[1, 1], [0, 0]]

I tried using the "+" operator, but got an error:

>>> import numpy
>>> a = numpy.zeros(shape=(2,2))
>>> a
array([[ 0.,  0.],
       [ 0.,  0.]])
>>> b = numpy.ones(shape=(1,2))
>>> b
array([[ 1.,  1.]])

>>> a[0, :] += b

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: non-broadcastable output operand with shape (2,) doesn't match the broadcast shape (1,2)

What is the best way to do this?

Ashwini Chaudhary · Accepted Answer · 2015-08-24 21:49:25Z

2

There's a difference between b = [1, 1] and b = [[1, 1]]. a[0, :] += b failed for you because broadcasting is not possible in this case.

If b can contain variable number of rows then we can take a slice of a using b's length and add b to it.

>>> a = numpy.zeros(shape=(2,2))
>>> b = numpy.ones(shape=(1,2))
>>> a[:len(b)] += b
>>> a
array([[ 1.,  1.],
       [ 0.,  0.]])

Or if b contains only one row then:

>>> a = numpy.zeros(shape=(2,2))
>>> a[0] += b[0]
>>> a
array([[ 1.,  1.],
       [ 0.,  0.]])

edited Aug 24, 2015 at 21:49

answered Aug 24, 2015 at 21:44

Ashwini Chaudhary

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Comments

Nader Hisham · Accepted Answer · 2015-08-25 07:42:26Z

0

a = np.zeros((2 , 2))
b = np.ones((1 ,2))
np.concatenate([b , np.array([a[0]])])

answered Aug 25, 2015 at 7:42

Nader Hisham

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