CODEWITHSUNDEEP
pythonarraysnumpy
Han Zhengzu
Han Zhengzu

Reputation: 3842

Select several part of 2-d array by the attribute of another array

Here is my question.

My target:

Select specific part of array a when the value of b[i,j] == True

My code here:

 select = a[np.array(np.where(b == True)).T]

But the result shows like some index are out of boundaries.

Does someone has any idea to achieve that?

Upvotes: 2

Views: 47

Answers (1)

Kasravnd
Kasravnd

Reputation: 107287

That's because you are transposing the index array. Also you don't need to convert the result of np.where() to numpy array, just pass it as index to first array.

Here is an example:

>>> b = np.random.choice([0, 1], size=(10,10))
>>> b
array([[0, 0, 0, 1, 0, 1, 1, 0, 0, 0],
       [0, 1, 1, 0, 0, 0, 0, 1, 1, 1],
       [1, 1, 1, 1, 1, 0, 0, 0, 1, 0],
       [1, 1, 0, 1, 0, 0, 1, 0, 0, 1],
       [0, 1, 0, 0, 1, 1, 1, 1, 1, 1],
       [1, 1, 1, 1, 1, 1, 0, 1, 0, 0],
       [0, 1, 1, 1, 0, 0, 0, 0, 0, 0],
       [0, 1, 0, 0, 1, 1, 1, 0, 0, 1],
       [1, 0, 1, 0, 0, 1, 0, 1, 0, 0],
       [1, 0, 0, 1, 0, 1, 1, 0, 0, 1]])
>>> a = np.random.choice(100, size=(10,10))
>>> 
>>> a
array([[47, 90, 94, 11, 17, 65, 95, 57, 36, 43],
       [65, 82, 37, 93, 65, 32,  8, 47, 16, 12],
       [66, 60, 40, 90, 34, 30, 40,  2, 36, 65],
       [ 8, 53, 69,  0, 61, 60, 94, 37, 77, 43],
       [59, 47, 21, 93, 58,  0, 92, 26, 17, 44],
       [98, 16, 33, 56, 39, 30, 14, 93, 93, 58],
       [96, 40, 35, 17, 21, 70, 26,  0, 21, 81],
       [47,  4, 20, 82, 19, 89, 50, 26, 38,  4],
       [60,  3, 72, 56, 78, 55, 60, 53,  3, 87],
       [80,  1, 65,  2, 92, 92, 97, 17, 55, 67]])

>>> a[np.where(b)]
array([11, 65, 95, 82, 37, 47, 16, 12, 66, 60, 40, 90, 34, 36,  8, 53,  0,
       94, 43, 47, 58,  0, 92, 26, 17, 44, 98, 16, 33, 56, 39, 30, 93, 40,
       35, 17,  4, 19, 89, 50,  4, 60, 72, 55, 53, 80,  2, 92, 97, 67])

Note that you are not have to use b==True as np.where condition, when you pass the boolean array it will choose the valid items which are determined by python's Truth Value Testing.

Upvotes: 1

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