Given a vector, this MATLAB function returns the indices of the zero crossings, or sign changes. There's not much to it.

 Z = ZEROCROSS(V)

 Return the indices of changes in sign between components of the
 vector.

 >> A = randn([10 1])

 A =

  -0.49840598306643
   1.04975509964655
  -1.67055867973620
  -2.01437026154355
   0.98661592496732
  -0.06048256273708
   1.19294080740269
   2.68558025885591
   0.85373360483580
   1.00554850567375

 >> zerocross(A)

 ans =

     2
     3
     5
     6
     7