select first occurance of minimum index from numpy array

Question

I am trying to find out the index of the minimum value in each row and I am using below code.

#code
import numpy as np
C = np.array([[1,2,4],[2,2,5],[4,3,3]])
ind = np.where(C == C.min(axis=1).reshape(len(C),1))
ind

#output
(array([0, 1, 1, 2, 2], dtype=int64), array([0, 0, 1, 1, 2], dtype=int64))

but the problem it is returning all indices of minimum values in each row. but I want only the first occurrence of minimum values. like

(array([0, 1, 2], dtype=int64), array([0, 0, 1], dtype=int64))

Use argmin : C.argmin(1) and np.arange(C.shape[0]) for the row indices. — Divakar
– Divakar, Commented Apr 18, 2017 at 20:35
Divakar Thanks, please add it in answer I will mark it as the correct answer. — Rudresha Parameshappa
– Rudresha Parameshappa, Commented Apr 18, 2017 at 20:38

Divakar · Accepted Answer · 2017-04-18 20:41:11Z

If you want to use comparison against the minimum value, we need to use np.min and keep the dimensions with keepdims set as True to give us a boolean array/mask. To select the first occurance, we can use argmax along each row of the mask and thus have our desired output.

Thus, the implementation to get the corresponding column indices would be -

(C==C.min(1, keepdims=True)).argmax(1)

Sample step-by-step run -

In [114]: C   # Input array
Out[114]: 
array([[1, 2, 4],
       [2, 2, 5],
       [4, 3, 3]])

In [115]: C==C.min(1, keepdims=1) # boolean array of min values
Out[115]: 
array([[ True, False, False],
       [ True,  True, False],
       [False,  True,  True]], dtype=bool)

In [116]: (C==C.min(1, keepdims=True)).argmax(1) # argmax to get first occurances
Out[116]: array([0, 0, 1])

The first output of row indices would simply be a range array -

np.arange(C.shape[0])

To achieve the same column indices of first occurance of minimum values, a direct way would be to use np.argmin -

C.argmin(axis=1)

Collectives™ on Stack Overflow

select first occurance of minimum index from numpy array

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related