My if statement with multiple conditions gives wrong values

I have an if-statement in a double for-loop, where I want to get some certain values ('triangles') out, like this:

for j = jsweep(sweep,1):jsweep(sweep,3):jsweep(sweep,2);
            for i = isweep(sweep,1):isweep(sweep,3):isweep(sweep,2); 
                if onsrc(i,j) == false;
                    tt_locmin = init_tt; 
                    if (((1 < i) && (i < nx)) && ((1 < j) && (j < ny))) 
                        triangles = [1,2,3,4,5,6,7,8];
                    elseif ((i == 1) && (j == 1)) 
                        triangles = [3,4];

                        elseif ((i == nx) && (j == 1)) 
                          traingles = [1,2];
                      elseif ((i == 1) && (j == ny)) 
                          triangles = [5,6];
                      elseif ((i == nx) && (j == ny)) 
                          triangles = [7,8];
                      elseif ((j < ny) && (i == 1))
                          triangles = [3,4,5,6]; 
                      elseif ((j < ny) && (i == nx)) 
                          triangles = [1,2,7,8];
                      elseif ((i < nx) && (j == 1)) 
                          triangles = [1,2,3,4];
                      elseif ((j == ny) && (i < nx))
                          triangles = [5,6,7,8];
                      end
          end
  end

But I can't seem to get the right values of 'triangles' out. When the loop reaches i = nx, j = 1, it still gives me the values triangles = [1, 2, 3, 4], where it should give me triangles = [1, 2]. What is wrong?

[Walter Roberson]

You are overwriting triangles in every iteration of the loop, so the end result is going to be whatever was computed on the last iteration.

How is nx being calculated? I hypothesize that you might have calculated it using floating point values instead of as an integer: if so then you could be suffering from round-off error.

Answer 1

[Roger Stafford]

Could it be because you have misspelled 'triangles' as 'traingles' at that point?

[KL]

I didn't see this coming! Good catch Roger!

[Thea_Q]

Just realized this as well, but it did not fixed the problem. I pre-allocated the triangles, which solved the main problem. But nice catch ;)

Answer 2

[KL]

Store the output of every iteration of triangles in a cell array. For example,

 traingles = cell(desiredSize); %pre-allocate properly

then in the for loop,

 triangles{i,j} = [1,2] or [1,2,3,4]

For your problem, I'd suspect triangles value is unchanged from its previous iteration (from when (i < nx) && (j == 1)). Possibly the outer if loop condition | onsrc(i,j) == false| is not satisfied and hence you don't go inside the internal if-elseif statements.

This is why pre-allocating and storing the result everytime would help figure out the reason why it happens.

one additional tip: try not to use i and j as iterating variables in matlab because they are the default imaginary units here. It may not cause trouble now but better to avoid them as a best practice.

https://www.mathworks.com/help/matlab/ref/i.html

[Thea_Q]

Thanks, this solved it!