Python code for counting number of zero crossings in an array

Question

I am looking to count the number of times the values in an array change in polarity (EDIT: Number of times the values in an array cross zero).

Suppose I have an array:<

Answer 1

Seems like, you want to group numbers by their sign. This could be done using built-in method groupby:

In [2]: l = [80.6,  120.8,  -115.6,  -76.1,  131.3,  105.1,  138.4,  -81.3, -95.3,  89.2,  -154.1,  121.4,  -85.1,  96.8,  68.2]

In [3]: from itertools import groupby

In [5]: list(groupby(l, lambda x: x < 0))
Out[5]: 
[(False, ),
 (True, ),
 (False, ),
 (True, ),
 (False, ),
 (True, ),
 (False, ),
 (True, ),
 (False, )]

Then you should use function len which returns the number of groups:

In [7]: len(list(groupby(l, lambda x: x < 0)))
Out[7]: 9

Obviously, there will be at least one group (for a non-empty list), but if you want to count the number of points, where a sequence changes its polarity, you could just subtract one group. Do not forget about the empty-list case.

You should also take care about zero elements: shouldn't they be extracted into another group? If so, you could just change the key argument (lambda function) of groupby function.