Question

I have two arrays, and I have a complex condition like this: new_arr<0 and old_arr>0
I am using nonzero but I am getting an error. The code I have is this:

    indices = nonzero(new_arr<0 and old_arr>0)

I tried:

    indices = nonzero(new_arr<0) and nonzero(old_arr>0)

But it gave me incorrect results.

Is there any way around this? And is there a way to get the common indices from two nonzero statements. For example, if:

    indices1 = nonzero(new_arr<0)
    indices2 = nonzero(old_arr>0)

and these two indices would contain:

   indices1 = array([0, 1, 3])
   indices2 = array([2, 3, 4])

The correct result would be getting the common element from these two (in this case it would be the element 3). Something like this:

    result = common(indices1, indices2)

Answer 1

Try indices = nonzero((new_arr < 0) & (old_arr > 0)):

In [5]: import numpy as np

In [6]: old_arr = np.array([ 0,-1, 0,-1, 1, 1, 0, 1])

In [7]: new_arr = np.array([ 1, 1,-1,-1,-1,-1, 1, 1])

In [8]: np.nonzero((new_arr < 0) & (old_arr > 0))
Out[8]: (array([4, 5]),)