J: Number of sign changes between items of list

Question

Items of a are _1 or 1.

a =: 1 _1 _1 1 _1

There are 3 sign changes in a:

1, -1, -1, 1, -1
  Λ       Λ  Λ
 here   and here

How do I count them looplessly?

Answer 1

You pair them with 2 v\, check if they differ ~:/ and sum +/:

+/ 2 ~:/\ a
3

2 ~:/\ 1 1 1 _1 _1 _1 1 _1 1
0 0 1 0 0 1 1 1

+/2 ~:/\ 1 1 1 _1 _1 _1 1 _1 1
4

edit

Or you could line up the curtail }: and behead }. of a and compare them but that is notably less efficient.

+/(}: ~: }.) a