Conditional indexing with python ndarray

Question

I have two python ndarrays p and q as follows:

import numpy as np
p = np.array([1.        , 1.        , 0.1862802 , 0.19957115, 0.18623812,
       0.1802321 , 0.17464815, 0.16460853, 0.1487719 , 0.12968006,
       0.10464501, 0.07183418, 0.00124706, 0.27353592, 0.81713212,
       0.23720725, 0.21802175, 0.21959138, 0.22401754, 0.22662527,
       0.22777369, 0.23269387, 0.23293132, 0.23374038, 0.24089565,
       0.19958937, 0.23910928, 0.24252447])

q = np.array([[ 1.        ,  1.        ],
       [-0.357316  ,  0.1862802 ],
       [-0.34402505,  0.19957115],
       [-0.35735808,  0.18623812],
       [-0.36336411,  0.1802321 ],
       [-0.36894805,  0.17464815],
       [-0.37898767,  0.16460853],
       [-0.3948243 ,  0.1487719 ],
       [-0.41391615,  0.12968006],
       [-0.4389512 ,  0.10464501],
       [-0.47176202,  0.07183418],
       [-0.54234915,  0.00124706],
       [ 0.27353592,  0.81713212],
       [-0.30638895,  0.23720725],
       [-0.32557445,  0.21802175],
       [-0.32400482,  0.21959138],
       [-0.31957866,  0.22401754],
       [-0.31697093,  0.22662527],
       [-0.31582252,  0.22777369],
       [-0.31090234,  0.23269387],
       [-0.31066488,  0.23293132],
       [-0.30985582,  0.23374038],
       [-0.30270055,  0.24089565],
       [-0.34400684,  0.19958937],
       [-0.30448692,  0.23910928],
       [-0.30107173,  0.24252447]])

I want to get (a) all the values in p where first value of corresponding tuple in q is >0 and (b) the index of all those values in p

Quang Hoang · Accepted Answer · 2021-03-02 21:36:18Z

1

Use np.where:

idx = np.where(q[:,0]>0)

print(idx, p[idx])

Output:

(array([ 0, 12]),) [1.         0.00124706]

answered Mar 2, 2021 at 21:36

Quang Hoang

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Gerry Over a year ago

Thanks @QuangHoang. However, not clear why idx in idx = np.where(q[:,0]>0) is a tuple of ndarray and not just instance of ndarray

GIOVANNI QUINONES VALDEZ · Accepted Answer · 2021-03-02 22:29:28Z

1

Obtaining the values in p whose corresponding tuple has the first value > 0:

idx = np.where(q[:,0]>0)
p[idx] 
##output [1.   0.00124706]

Finding the indices of these values in p

for val in p[idx]:
        print(val, "index", np.where(p == val))
##output
1.0 index array([0, 1]) # 1.0 appears twice in p
0.00124706 index array([12])

answered Mar 2, 2021 at 22:29

GIOVANNI QUINONES VALDEZ

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