Something like
a = array([1,1,-1,-2,-3,4,5])
asign = np.sign(a)
signchange = ((np.roll(asign, 1) - asign) != 0).astype(int)
print signchange
array([0, 0, 1, 0, 0, 1, 0])
Now, numpy.roll does a circular shift, so if the last element has different sign than the first, the first element in the signchange array will be 1. If this is not desired, one can of course do a simple
signchange[0] = 0
Also, np.sign considers 0 to have it’s own sign, different from either positive or negative values. E.g. the “signchange” array for [-1,0,1] would be [0,1,1] even though the zero line was “crossed” only once. If this is undesired, one could insert the lines
sz = asign == 0
while sz.any():
asign[sz] = np.roll(asign, 1)[sz]
sz = asign == 0
between lines 2 and 3 in the first example.