getting the index of min values with Numpy Python

Question

The function below separates each value into chunks separated by indexes index with the values in L_list. So it outputs the minimum value between indexes 3-5 which is -5 and the index of the value. Both the numpy_argmin_reduceat(a, b) and the Drawdown function do as planned however the index output of the numpy_argmin_reduceat(a, b) is faulty it The minimum values of Drawdown do not match with the indexes of the outputs of numpy_argmin_reduceat(a, b).How would I be able to solve this? Arrays:

import numpy as np
# indexes          0, 1, 2,3,4, 5, 6,7, 8, 9,10, 11, 12
L_list = np.array([10,20,30,0,0,-5,11,2,33, 4, 5, 68, 7])
index =  np.array([3,5,7,11])

Functions:

#getting the minimum values
Drawdown = np.minimum.reduceat(L_list,index+1)

#Getting the min Index 
def numpy_argmin_reduceat(a, b):
    n = a.max() + 1  # limit-offset
    id_arr = np.zeros(a.size,dtype=int)
    id_arr[b] = 1
    shift = n*id_arr.cumsum()
    sortidx = (a+shift).argsort()
    grp_shifted_argmin = b
    idx =sortidx[grp_shifted_argmin] - b
    min_idx = idx +index
    return min_idx


min_idx =numpy_argmin_reduceat(L_list,index+1)
#printing function
DR_val_index = np.array([np.around(Drawdown,1), min_idx])
DR_result = np.apply_along_axis(lambda x: print(f'Min Values: {x[0]} at index: {x[1]}'), 0, DR_val_index)

Output

Min Values: -5 at index: 4
Min Values: 2 at index: 6
Min Values: 4 at index: 8
Min Values: 7 at index: 11

Expected Output:

Min Values: -5 at index: 5
Min Values: 2 at index: 7
Min Values: 4 at index: 9
Min Values: 7 at index: 12

jakevdp · Accepted Answer · 2021-05-25 20:54:21Z

0

If you change the line

id_arr[b[1:]] = 1

to

id_arr[b] = 1

I think the function will behave as you hope.

answered May 25, 2021 at 20:54

jakevdp

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2 Comments

georgehere Over a year ago

Hi I have edited the question could you take a look at it again. I am having another issue , with the alteration of the indexes

Mad Physicist Over a year ago

Don't alter questions, since that invalidates the work people put into answering them: ask new ones. On a side note, what's the source of the argmin function you have? It looks familiar :)

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