the first time there is zero before and after a particular index

Hi All, I have a matrix which looks like this:

1 1 1 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 Let's say that I want to find out at what indices there is the first zero happening before and after index 20. How do we do this? So, in my example here, my points of interest will be: 7 (for before) and 37 (for after).

Thanks

Answer 1

[the cyclist]

Here is one straightforward way:

x = [1 1 1 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0];
anchor = 20;

positionIndex = 1:numel(x);

lastPriorZero = max(find(x==0 & positionIndex<anchor))
firstLaterZero = min(find(x==0 & positionIndex>anchor))

Answer 2

[Image Analyst]

If you want to find the 0's just outside the 1's, and you have the Image Processing Toolbox, you can do this:

x = [1 1 1 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0]
dilatedx = imdilate(x, [1,1,1]) % Grow 1 regions outward
indexes = find(dilatedx ~= x)  % Find locations of mismatches.

and you get

indexes =
   4     7    37

as you should.