Combining two numpy arrays to form an array with the largest value from each array

Question

I want to combine two numpy arrays to produce an array with the largest values from each array.

import numpy as np

a  = np.array([[ 0.,  0.,  0.5],
               [ 0.1,  0.5,  0.5],
               [ 0.1,  0.,  0.]])

b  = np.array([[ 0.,  0.,  0.0],
               [ 0.5,  0.1,  0.5],
               [ 0.5,  0.1,  0.]])

I would like to produce

array([[ 0.,  0.,  0.5],
       [ 0.5,  0.5,  0.5],
       [ 0.5,  0.1,  0.]])

I know you can do

a += b

which results in

array([[ 0. ,  0. ,  0.5],
       [ 0.6,  0.6,  1. ],
       [ 0.6,  0.1,  0. ]])

This is clearly not what I'm after. It seems like such an easy problem and I assume it most probably is.

Alex Riley · Accepted Answer · 2015-02-26 13:18:39Z

10

You can use np.maximum to compute the element-wise maximum of the two arrays:

>>> np.maximum(a, b)
array([[ 0. ,  0. ,  0.5],
       [ 0.5,  0.5,  0.5],
       [ 0.5,  0.1,  0. ]])

This works with any two arrays, as long as they're the same shape or one can be broadcast to the shape of the other.

To modify the array a in-place, you can redirect the output of np.maximum back to a:

np.maximum(a, b, out=a)

There is also np.minimum for calculating the element-wise minimum of two arrays.

edited Feb 26, 2015 at 13:18

answered Feb 26, 2015 at 12:28

Alex Riley

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Falko · Accepted Answer · 2015-02-26 12:28:07Z

6

You are looking for the element-wise maximum.

Example:

>>> np.maximum([2, 3, 4], [1, 5, 2])
array([2, 5, 4])

http://docs.scipy.org/doc/numpy/reference/generated/numpy.maximum.html

answered Feb 26, 2015 at 12:28

Falko

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Padraic Cunningham · Accepted Answer · 2015-02-26 12:41:54Z

3

inds =  b > a
a[inds] = b[inds]

This modifies the original array a which is what += is doing in your example which may or may not be what you want.

edited Feb 26, 2015 at 12:41

answered Feb 26, 2015 at 12:30

Padraic Cunningham

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