ZeroCrossing on a graph

I have to create a zerocrossing script which says when a graph passes from positive values to negative values (only this case) and then it plots a graph including all these points of zero crossing. Can someone help me with a code?

[Guillaume]

So... what is your question?

[Nicholas Saladini]

I'm trying to create a code but i can't do it.

[dpb]

Consider what happens to diff(y) and look at

doc sign

for ideas...

[Nicholas Saladini]

if true
  % code
plot(sig_AP, 'y')
hold on
for i = 1:length(sig_AP)-1
  if ( sig_AP(i)>0 && sig_AP(i+1)<0 )
      sig_AP(i+1) = 0-sig_AP(i+1);
      x = sig_AP(i);
      y = sig_AP(i+1);
      if ( x > y )
          ZC(i) = sig_AP(i+1);
      else
          ZC(i) = sig_AP(i);
      end
  else
      ZC(i) = 0;
  end
end
ZC = ZC'
plot (ZC, 'k')
end
I try something like this but it doesn't work... i'm not very able with matlab..