Filtering numpy array of numpy arrays

Question

I've got a np array of arrays and I want to remove all elements that match a condition. I want to avoid for loops to try to make it faster.

The np array is of shape [N,2]. I want to remove all sub-arrays where the first element is equal to zero.

[[1,2],
 [0,5], # <--- Remove
 [5,1],
 [0,3], # <--- Remove
 [1,1],
 [0,0], # <--- Remove
 [5,0],
 .....
 [5,5]]

I hope there is a solution with just one line of code. I just can't figure this line out. I hope somebody can help me.

Thank you.

So you have a NumPy array containing NumPy arrays? Or do you have a single 2-dimensional array? — miradulo
– miradulo, Commented Aug 24, 2018 at 16:53
Is this one array with shape (N,2) and integer dtype, or an array of shape (N,) with object dtype? — hpaulj
– hpaulj, Commented Aug 24, 2018 at 17:05

Sheldore · Accepted Answer · 2018-08-24 16:59:21Z

7

Provided arr is your input array, do the following:

arr = arr[arr[:,0] !=0]

Here arr[:,0] correspond to the first element/column or each subarray. Then you check arr[:,0] !=0 which returns you those indices where the first element is not equal to zero. Then these indices are automatically passed to arr which gives you what you want.

answered Aug 24, 2018 at 16:59

Sheldore

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