Question

In python I have numpy.ndarray called a and a list of indices called b. I want to get a list of all the values of a which are not in -10..10 places around the indices of b.
This is my current code, which takes a lot of time to run due to allocations of data (a is very big):

    aa=a
    # Remove all ranges backwards
    for bb in b[::-1]:
        aa=np.delete(aa, range(bb-10,bb+10))

Is there a way to do it more efficiently? Preferably with few memory allocations.

Answer 1

np.delete will take an array of indicies of any size. You can simply populate your entire array of indicies and perform the delete once, therefore only deallocating and reallocating once. (not tested. possible typos.)

bb = np.empty((b.size, 21), dtype=int)
for i,v in enumerate(b):
    bb[i] = v+np.arange(-10,11)

np.delete(a, bb.flat)  # looks like .flat is optional

Note, if your ranges overlap, you’ll get a difference between this and your algorithm: where yours will remove more items than those originally 10 indices away.