get the maximum element in an array using "=="

Question

I have an array x :

numpy.random.seed(1)
#input
x = numpy.arange(0.,20.,1)
x = x.reshape(5,4)
print(x)

[[ 0.  1.  2.  3.]
 [ 4.  5.  6.  7.]
 [ 8.  9. 10. 11.]
 [12. 13. 14. 15.]
 [16. 17. 18. 19.]]

I want to access the maximum element in this array. In my assignment, the answer has this line to access the maximum one in x :

print(x[x==x.max()])
[19.]

I search documentation but found only one way of accessing the maximum element using argmax. I don't find the way of using "==" in the documentation, so I don't understand how this works. Can anyone explain why this works and show where it is in the documentation?

Kien Nguyen · Accepted Answer · 2021-01-15 15:38:20Z

2

This is called Boolean array indexing.

With x == x.max(), the below boolean array is generated:

[[ False  False  False  False]
 [ False  False  False  False]
 [ False  False  False  False]
 [ False  False  False  False]
 [ False  False  False  True]]

Then with x[x==x.max()], the above array is used as a mask to filter the elements of x that correspond to the True.

Reference:

https://numpy.org/devdocs/reference/arrays.indexing.html#boolean-array-indexing

edited Jan 15, 2021 at 15:38

answered Jan 15, 2021 at 13:47

Kien Nguyen

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M Z · Accepted Answer · 2021-01-15 13:40:05Z

2

This is using a boolean mask array. Here's the documentation.

What you're doing is you're producing a boolean array, then using that as a mask to index into your original array:

# x == x.max()
[[ False  False  False  False]
 [ False  False  False  False]
 [ False  False  False  False]
 [ False  False  False  False]
 [ False  False  False  True]]

answered Jan 15, 2021 at 13:40

M Z

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1 Comment

Armadillan Over a year ago

@user14416085 Here is another relevant piece of documentation: numpy.org/doc/stable/reference/… Your original code is basically saying "Create an array with all the elements in the original array that are equal to the maximum element in the original array".

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