Detecting change in sign of values of a column

If I a data file with 3 columns the first being the x cordinates second being the y cordinates and the third being the z.

Data(:,2); %produces all the y values which are +-0.1 either side of y=0

So If I have

Data(:,2)=[0.03;0.05;0.08;-0.01;-0.05;-0.06;0.02;0.03;        -0.05;-0.01]
                         %^sign change    %^sign change   ^^sign change
                         %so must pass    %so must pass
                         %y=0             %y=0

How would I run a loop that goes through Data(:,2) and finds where the y values have changed sign and stores that position. So in my example it would detect the first change in sign from 0.08 to -0.01 (4th position of -0.01) the second change in sign at-0.06 and 0.02 (7th position of 0.02) the third sign change at 0.03 -0.05 (9th position of -0.05) which shows they have passed through y=0 and store those positions as a new column

NewC=[4;7;9];
               

I hope that makes sense and any help would be appreciated .

Answer 1

[Adam]

NewC = find( diff( sign( Data(:,2) ) ) ) + 1;