extract specific points from a matrix of planar coordinates

I have a 600x2 matrix with a series of planar points: x values on first column and y values on second one. How can I extract only the x values for which the corresponding y is null? Thank you!

[John Chilleri]

What type of variable is your matrix (i.e. cell)? Or if it's a normal matrix, how do you define null?

Answer 1

[KSSV]

Let A be your 600*2 matrix.

x = A(:,1) ;
y = A(:,2) ;
iwant = x(y==0) ;   % respective _x_ for which y=0

[Tommaso Di Noto]

Thank you! The passages are right; the only problem is that instead of 0 I have values such as 2e-04 or -4e-05. So if I write "==0" I get an empty vector. How can I solve this?

Answer 2

[Tommaso Di Noto]

I solved it; I just had to write:

iwant=x(y<0.001 & y>-0.001);

Thanks again!