Pop rows of a numpy array with delete()

January 27, 2022

With the following code, I iterate over an array in reverse order and print and pop the last item. In the end I expect to see an empty array, but it is not empty.

a = array([5,1,2,4,9,2]).reshape(-1, 1)
for j in a[::-1]:
    print(j.item())
    np.delete(a, len(a)-1, 0)
print("a=", a)

Output is

2
9
4
2
1
5
a= [[5]
 [1]
 [2]
 [4]
 [9]
 [2]]

How can I fix that? With the syntax of delete(array, pos, axis), I expect to delete the rows from last to start.

>Solution :

numpy.delete does not work in place:

>>> np.delete(a, len(a)-1, 0)
array([[5],
       [1],
       [2],
       [4],
       [9]])

I am not sure why you would want to iterate over an array this way but you would need:

a = np.array([5,1,2,4,9,2]).reshape(-1, 1)
for j in a[::-1]:
    print(j.item())
    a = np.delete(a, len(a)-1, 0)  ## assign back to a
print("a=", a)

output: