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pythonnumpymaxindices
Peter Smit
Peter Smit

Reputation: 28696

How to get the index of a maximum element in a NumPy array along one axis

I have a 2 dimensional NumPy array. I know how to get the maximum values over axes:

>>> a = array([[1,2,3],[4,3,1]])
>>> amax(a,axis=0)
array([4, 3, 3])

How can I get the indices of the maximum elements? I would like as output array([1,1,0]) instead.

Upvotes: 161

Views: 343184

Answers (6)

Hadi Mir
Hadi Mir

Reputation: 5123

There is argmin() and argmax() provided by numpy that returns the index of the min and max of a numpy array respectively.

Say e.g for 1-D array you'll do something like this

import numpy as np

a = np.array([50,1,0,2])

print(a.argmax()) # returns 0
print(a.argmin()) # returns 2

And similarly for multi-dimensional array

import numpy as np

a = np.array([[0,2,3],[4,30,1]])

print(a.argmax()) # returns 4
print(a.argmin()) # returns 0

Note that these will only return the index of the first occurrence.

Upvotes: 6

Loich
Loich

Reputation: 918

Allow me to give an up-to-date answer. We use the function argmax() with the parameter axis that can be defined as follows:

  • axis=0, the index of the max element per column will be returned.
  • axis=1 the index of the max element per row will be returned.

The following code will help you to better understand.

import numpy as np 
a = np.array([[1,2,3],[4,3,1]])
#This will print the index of the max value for each column.
print(a.argmax(axis=0)) # output: [1 1 0]
#This will print the index of the max value for each row.
print(a.argmax(axis=1)) # output: [2 0]

Upvotes: 1

Sevak Avakians
Sevak Avakians

Reputation: 938

argmax() will only return the first occurrence for each row. http://docs.scipy.org/doc/numpy/reference/generated/numpy.argmax.html

If you ever need to do this for a shaped array, this works better than unravel:

import numpy as np
a = np.array([[1,2,3], [4,3,1]])  # Can be of any shape
indices = np.where(a == a.max())

You can also change your conditions:

indices = np.where(a >= 1.5)

The above gives you results in the form that you asked for. Alternatively, you can convert to a list of x,y coordinates by:

x_y_coords =  zip(indices[0], indices[1])

Upvotes: 46

blaz
blaz

Reputation: 1443

>>> import numpy as np
>>> a = np.array([[1,2,3],[4,3,1]])
>>> i,j = np.unravel_index(a.argmax(), a.shape)
>>> a[i,j]
4

Upvotes: 127

ahmed
ahmed

Reputation: 55

v = alli.max()
index = alli.argmax()
x, y = index/8, index%8

Upvotes: 3

eumiro
eumiro

Reputation: 212825

>>> a.argmax(axis=0)

array([1, 1, 0])

Upvotes: 189

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