Argmax of numpy array returning non-flat indices

Question

I’m trying to get the indices of the maximum element in a Numpy array.
This can be done using numpy.argmax. My problem is, that I would like to find the biggest element in the whole array and get the indices of that.

numpy.argmax can be either applied along one axis, which is not what I want, or on the flattened array, which is kind of what I want.

My problem is that using numpy.argmax with axis=None returns the flat index when I want the multi-dimensional index.

I could use divmod to get a non-flat index but this feels ugly. Is there any better way of doing this?

Asked By: Andreas Mueller

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Source

Answer 1

np.where(a==a.max())

returns coordinates of the maximum element(s), but has to parse the array twice.

>>> a = np.array(((3,4,5),(0,1,2)))
>>> np.where(a==a.max())
(array([0]), array([2]))

This, comparing to argmax, returns coordinates of all elements equal to the maximum. argmax returns just one of them (np.ones(5).argmax() returns 0).

Answered By: eumiro

Answer 2

You could use numpy.unravel_index() on the result of numpy.argmax():

>>> a = numpy.random.random((10, 10))
>>> numpy.unravel_index(a.argmax(), a.shape)
(6, 7)
>>> a[6, 7] == a.max()
True

Answered By: Sven Marnach

Answer 3

To get the non-flat index of all occurrences of the maximum value, you can modify eumiro’s answer slightly by using argwhere instead of where:

np.argwhere(a==a.max())

>>> a = np.array([[1,2,4],[4,3,4]])
>>> np.argwhere(a==a.max())
array([[0, 2],
       [1, 0],
       [1, 2]])

Answered By: Richard Ludlow

Argmax of numpy array returning non-flat indices

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