Fix Python – Rank items in an array using Python/NumPy, without sorting array twice

Question

Asked By – joshayers

I have an array of numbers and I’d like to create another array that represents the rank of each item in the first array. I’m using Python and NumPy.

For example:

array = [4,2,7,1]
ranks = [2,1,3,0]

Here’s the best method I’ve come up with:

array = numpy.array([4,2,7,1])
temp = array.argsort()
ranks = numpy.arange(len(array))[temp.argsort()]

Are there any better/faster methods that avoid sorting the array twice?

Now we will see solution for issue: Rank items in an array using Python/NumPy, without sorting array twice


Answer

Use advanced indexing on the left-hand side in the last step:

array = numpy.array([4,2,7,1])
temp = array.argsort()
ranks = numpy.empty_like(temp)
ranks[temp] = numpy.arange(len(array))

This avoids sorting twice by inverting the permutation in the last step.

This question is answered By – Sven Marnach

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