finding and changing values in matrix that satisfies 2 conditions

Hi there, I am new in matlab and try to solve this problem for couple of hours, I have vector which contains various triggers and a lot of zeros:

A=[0;0;0;0;0;0;0;0;2;0;0;1;0;0;0;0;0;0;0;0;1;0;0;0;1;0;0;0;0;0;0;0;4;0;1;0;0;0;0;0;0;0;0;5;1];

what I need to do is to find the second value in matrix A which is bigger than 0 and change it to 100. In this case it is the 12th number, value "1", let's say it is target_1. Then I need to change every second number bigger then 0 and change it to 100, but we have to start counting from target_1.

So the final matrix will be like this:

B=[0;0;0;0;0;0;0;0;2;0;0;100;0;0;0;0;0;0;0;0;1;0;0;0;100;0;0;0;0;0;0;0;4;0;100;0;0;0;0;0;0;0;0;5;100];

Thank you very much ahead for your time and help.

Answer 1

[Adam]

idx = find( A > 0 );
idx = idx(2:2:end);
B = A
B(idx) = 100;

[Rene Sebena]

Thanks, this helped me a lot!

Answer 2

[Alexandra Harkai]

idx = find(A); % find indices of nonzero elements in A
B = A; % initially set B to be same as A
B(idx(2:2:end)) = 100;

[Rene Sebena]

Thank you very much, this is working!