Python code arrays based to index using condition?

Question

I have two python ndarrays arr1 and arr2 as follows:

    import numpy as np
    arr1 = 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])

arr2 = 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 arr1 where first value of corresponding tuple in arr2 is >0 and (b) the index of all those values in arr1

The first array has length 28 and the second 26. Should't they have the same length? — Diego Palacios
– Diego Palacios, Commented May 28, 2021 at 8:30

tdenimal · Accepted Answer · 2021-05-28 08:27:33Z

1

A simple solution by just storing the arr1 indices that matches the a) conditions.



#Define the  output list
arr1_index = []

#Loop on arr1,arr2
for i,(a1,a2) in enumerate(zip(arr1,arr2)):
    #(a) all the values in arr1 where first value of corresponding tuple in arr2 is >0 
    #(b) the index of all those values in arr1
    # Just need to store indices from arr1 that match the a) conditions
    if a2[0]>0:
        arr1_index += [i]
   
print(arr1_index)
print(arr1[arr1_index])

answered May 28, 2021 at 8:27

tdenimal

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tdenimal Over a year ago

it should work on arrays of different size, just keep in mind that the longest array will be truncated to the smallest one size

Diego Palacios · Accepted Answer · 2021-05-28 08:29:11Z

1

To get the values you can just use indexing with boolean arrays:

a = arr1[arr2[:,0] > 0]

To get the indices, use the nonzero function:

b = np.nonzero(arr2[:,0] > 0)[0]

However, your this won't work in your example arrays, as they don't have the same length. arr1 has length 28 and arr2 length 26.

edited May 28, 2021 at 8:29

answered May 28, 2021 at 8:18

Diego Palacios

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