Adding a 1d numpy array to a specifc row of a 2d numpy array

Question

I have an array

a = np.ones((5, 5))

that looks like this

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

and then another array

b = np.array([0, 0, 0, 1, 0])

and I am struggling to find a piece of code in Numpy documentation that would add array b to the specific row in array a

for example I would like to add array b only to the 3rd row of array a so that

c = [1, 1, 1, 1, 1]
    [1, 1, 1, 1, 1]
    [1, 1, 1, 2, 1]
    [1, 1, 1, 1, 1]
    [1, 1, 1, 1, 1]

fountainhead · Accepted Answer · 2019-03-08 18:24:29Z

2

Use this:

a[2] += b

Output:

print (a)
[[1. 1. 1. 1. 1.]
 [1. 1. 1. 1. 1.]
 [1. 1. 1. 2. 1.]
 [1. 1. 1. 1. 1.]
 [1. 1. 1. 1. 1.]]

If you don't want to modify a, and want the result in a different array c, it's best done as a two step process:

c = np.copy(a)
c[2] += b

answered Mar 8, 2019 at 18:01

fountainhead

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