Question

I have two 3d numpy arrays a1 and a2 where len(a1) == len(a2) and a1[x, y, z] = id

I’m using this code to find out if there is changed data in any of the z layers

eq = a1 == a2
if eq.any():
    #find the indexes of the changed data

as the comment and title says I need to find the indexes of the changed data.
basically I have a list of objects that correspond the the positions in the array and I need to update those objects based on the id pulled form the array. I want to do this as fast as possible as this list can get REALLY large possibly more than 120,000 entries. but only a hundred or so of these entries are likely to change at any one time. as such I’d like to obtain a list of the changed indexes so I can call the object at that index and update it.

I do need to maintain the three components of the index

is there a way to do this without looping through the list? perhaps with numpy.nonzero()

Answer 1

Two more options:

np.argwhere(a1!=a2)
np.where(a1!=a2)

They both do the same thing but produce results in different formats (one suitable for indexing the arrays, the other more readable)