find x intercepts numerically

I'm trying to find a quick way of returning the index of all the places where an [x,y] data set crosses the x axis.

I've tried: zci = @(v) find(v(:).*circshift(v(:), [-1 0]) <= 0)

But it isn't returning all the zeros and also always returns the last value in my dataset as a zero even though it isn't. I'm testing with a third order polynomial and can only get my code to work if it have only one root.

Thanks!

Answer 1

[Ameer Hamza]

Try this

f = @(x) find(diff(sign(x)));

Example

>> x = [1 2 -1 -2 -1 2 3 1 -2 -3 -2 1];
>> f(x)
ans =
     2     5     8    11

[Elizabeth Ramsey]

Exactly what I needed, thanks so much!