Find the index corresponding to the max value of a numpy array

Question

I have a numpy.array of data called f, I know the max value in it is f_max=max(f) but I would like to know the index in the array corresponding to the maximum value.

I tried:

count = 0
while (f[count]!=fmax)
    conto ++

but I receive an error:

SyntaxError: invalid syntax

Could anyone help me?

On the syntax error: you need a : at the end of the while statement, and use count += 1, but the answer below is the right way to do it. — Alex Z
– Alex Z, Commented May 24, 2014 at 17:32
@Caos note that I have edited your question and included the numpy tag based on the information below that you are using a numpy array. If this information is incorrect then please say so. — Ffisegydd
– Ffisegydd, Commented May 24, 2014 at 17:38

acrider · Accepted Answer · 2014-05-24 18:13:01Z

4

If you're already using numpy, you can do this with argmax().

import numpy as np
a = np.array([1, 5, 2, 6, 3])

index = a.argmax()

answered May 24, 2014 at 18:13

acrider

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Comments

Sukrit Kalra · Accepted Answer · 2014-05-24 17:30:29Z

1

The easiest way would be to find the max and then, look for its index.

>>> a = [1, 5, 2, 3, 4]
>>> val = max(a)
>>> a.index(val)
1

You could also use enumerate to get a list of indices and the values corresponding to them and choose the max among them.

>>> list(enumerate(a))
[(0, 1), (1, 5), (2, 2), (3, 3), (4, 4)]
>>> index, _ = max(enumerate(a), key = lambda x: x[1])
>>> index
1

answered May 24, 2014 at 17:30

Sukrit Kalra

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5 Comments

Chaos Over a year ago

Doing this I receive: 'numpy.ndarray' object has no attribute 'index'

Sukrit Kalra Over a year ago

@Caos: You should have mentioned you have a Numpy Array.

Sukrit Kalra Over a year ago

@Caos: You should use np.argmax in that case.

Chaos Over a year ago

Sorry I am new to python ad I thought I was using a list

Charles Over a year ago

This solution only works to show the first instance of a max, is there a more robust solution?

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