index of N highest elements from a list of numpy array [duplicate]

Question

I need a function that returns the index (row and column) of the N highest element from a matrix/list of numpy array.

Suppose that I have the following matrix:

a = [[4,2,3], 
     [5,0,3]]

I would like to get a list of the indexes (row,column) of the N highest elements. For example, if I want the index of the 4 highest element, the function should return

[(1,0), (0,0), (0,2), (1,2)]

I have tried implementing this as following, but it returns a list of (value, row number), which is not want I need

for i, sub_list in enumerate(a):
    max_list.append((max(sub_list), i))

I think you have an error in your output. (1, 2) should be included instead of (0, 1), since 3 > 2 — user3483203
– user3483203, Commented Feb 14, 2020 at 14:29
Does this answer your question? How do I get indices of N maximum values in a NumPy array? There are solutions for 2D arrays as well. — Georgy
– Georgy, Commented Feb 14, 2020 at 15:02

user3483203 · Accepted Answer · 2020-02-14 14:43:01Z

4

I would flatten, argsort, and unravel_index

f = a.ravel().argsort()
[*zip(*np.unravel_index(f[-n:], a.shape))]

[(0, 2), (1, 2), (0, 0), (1, 0)]

As yatu points out, if you have a larger array, and a small-ish n, you can replace argsort with np.argpartition(a.ravel(), -n).

edited Feb 14, 2020 at 14:43

answered Feb 14, 2020 at 14:28

user3483203

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