Efficiently detect sign-changes in python

Question

I want to do exactly what this guy did:

Python - count sign changes

However I need to optimize it to run super fast. In brief I want to take a time series an

Answer 1

I see people using diff a lot in their solutions, but xor seems to be much faster and the result is the same for bools (a good pointer to that might also be the fact that using diff gives a deprecated warning.... :) ) Here is an example:

positive = a2 > 0
np.where(np.bitwise_xor(positive[1:], positive[:-1]))[0]

Time it measures it to be around one and a half faster to diff for me:)

If you do not care about edge cases it might be better to use

positive = np.signbit(a2)

but positive = a2 >0 seems faster (and cleaner) than signbit AND checking for 0s (e.g. positive = np.bitwise_or(np.signbit(a2),np.logical_not(a2)) is slower...)