CODEWITHSUNDEEP
pythonarraysnumpy
helloswift123
helloswift123

Reputation: 163

How to modify the value of numpy array based on another array?

I have a numpy array. I want to modify one array index by the chosen elements of another array. For example:

import numpy as np
t1 = np.ones((10,3))
t2 = np.arange(10)
t1[np.where(t2>5)][:,2] = 10
print(t1)

What I want t1 is:

array([[1., 1., 1.],
       [1., 1., 1.],
       [1., 1., 1.],
       [1., 1., 1.],
       [1., 1., 1.],
       [1., 1., 1.],
       [1., 1., 10.],
       [1., 1., 10.],
       [1., 1., 10.],
       [1., 1., 10.]])

But the output of t1 is:

array([[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., 1., 1.],
       [1., 1., 1.]])

What the problem there?

Upvotes: 3

Views: 5698

Answers (3)

Daniel F
Daniel F

Reputation: 14399

The most pythonic way to do this is probably

t1[:, 2] = np.where(t2 > 5,       # where t2 > 5
                    10,           # put 10
                    t1[:, 2])     # into the third column of t1

Besides added clarity, for very large objects this will have a time benefit, as there is no creation of an intermediate indexing array np.where(t2 > 5) and no resulting intermediate callbacks to that python object - everything is done in-place with c-compiled code.

Upvotes: 2

Austin
Austin

Reputation: 26037

You can do:

t1[np.where(t2>5), 2] = 10

Syntax: array[<row>, <col>]

Upvotes: 1

oppressionslayer
oppressionslayer

Reputation: 7224

It's backwards, it should be:

t1[:,2][np.where(t2>5)] = 10

output:

array([[ 1.,  1.,  1.],
       [ 1.,  1.,  1.],
       [ 1.,  1.,  1.],
       [ 1.,  1.,  1.],
       [ 1.,  1.,  1.],
       [ 1.,  1.,  1.],
       [ 1.,  1., 10.],
       [ 1.,  1., 10.],
       [ 1.,  1., 10.],
       [ 1.,  1., 10.]])

Upvotes: 3

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