[Homework] Bad syntax? Getting a crash. Cycle While + If + Matrix

Hello. I have a homework that the objective is to select the best path we could take, having a given matrix (matrix with heights) called 'Heights'.

This is a example (green is the path we take)

We start at (1,1) and need to go to (i,j) [in this case (4,4)]. And the preferred way to go is SouthEast > East > South. If we have the same value in the southeast block and the south block, we go to the southeast block.

Also we need to check first what is the difference of heights from the block we are to the block we want to go

So in the case, we go to the SouthEast block since is the one we "move" less.

In the end it ask to us, to make a matrix with the results. With the example I gave you, when we call the function we should get:

M =  1  1  10
     2  2  11
     2  3  15
     3  3  18
     3  4  19
     4  4  21

_________________________________________________________________________________ _________________________________________________________________________________

Now the code I make:

function res = get_path(Heights)

      i = 1;
      j = 1;	
      M = [1,1,Heights(1,1)];

      while i <= size(Heights,1) && j <= size(Heights,2)
        if abs(Heights(i,j) - Heights(i+1,j+1)) <= abs(Heights(i,j) - Heights(i,j+1)) && abs(Heights(i,j) - Heights(i+1,j+1)) <= abs(Heights(i,j) - Heights(i+1,j))
          x = i+1;
          y = j+1;
          h = Heights(i+1,j+1);

        elseif abs(Heights(i,j) - Heights(i+1,j+1)) <= abs(Heights(i,j) - Heights(i,j+1)) && abs(Heights(i,j) - Heights(i+1,j+1)) > abs(Heights(i,j) - Heights(i+1,j))
          x = i+1;
          y = j;
          h = Heights(i+1,j);

        elseif abs(Heights(i,j) - Heights(i+1,j+1)) > abs(Heights(i,j) - Heights(i,j+1)) && abs(Heights(i,j) - Heights(i,j+1)) <= abs(Heights(i,j) - Heights(i+1,j))
          x = i;
          y = j+1;
          h = Heights(i,j+1);

        else abs(Heights(i,j) - Heights(i+1,j+1)) > abs(Heights(i,j) - Heights(i,j+1)) && abs(Heights(i,j) - Heights(i,j+1)) > abs(Heights(i,j) - Heights(i+1,j))
          x = i+1;
          y = j;
          h = Heights(i+1,j);
        end

        M = [M; x,y,h]; 

      end

      res = M

end

I think the code is right, but when I go to the command line, and write: get_path(Heights) it change line and then I cant write anything.

Answer 1

[ag]

Hi Jaquim,

The issue that you are facing is because the while loop runs for infinite times since the variables "i" and "j" never get updated and therefore never reach the boundary conditions of the while loop.

The below code snippet demonstrates how to resolve this:

function res = get_path(Heights)
    while i < size(Heights,1) && j < size(Heights,2) % ensure i + 1 and j + 1 never exceed the size of the matrix 
        % logic and the if else if ladder
        i = x; j = y;
        M = [M; x,y,h]; 
    end
    res = M
end

Hope this helps!