MATLAB: Simple if/else logic in MATLAB

Question

Hi there, I've been working on a piece of code for calculating voltage and electric field inside coaxial cables of different sizes.

While doing my computation, I needed to do an if/else statement and was boggled as to why these two versions of the statement did not work interchangeably. For reference, both these statements are executed in a nested for loop (i, j are declared – there is no syntax error – just a logic one).

Option 1:

if ((i < innerstartx) || (i > innerendx) || (j < innerstarty) || (j > innerendy))
    V(i,j) = (V(i+1,j) + V(i-1,j) + V(i,j+1) + V(1,j-1))/4;
else
end

Option 2:

if (i>=innerstartx && i<=innerendx && j>=innerstarty && j<=innerendy)
else
    V(i,j)=0.25*(V(i+1,j)+V(i-1,j)+V(i,j+1)+V(i,j-1));
end

Currently, option 2 works, but option 1 doesn't. I checked over the logic a few times and I am pretty sure it is sound (simply De Morgan's inversion).

I was just curious though, as I'd like to be able to avoid this sort of problem should it arise in the future.

Thanks for your help.

sas

Answer 1

As far as I can see the logic is ok. You can test it manually by inserting a check in the code:

if ~isequal( ...
   not(i < innerstartx || i > innerendx || ...
       j < innerstarty || j > innerendy), ...
      (i >= innerstartx && i <= innerendx && ...
       j >= innerstarty && j <= innerendy))
  error('Fail!');
end

But these lines differ:

    V(i,j) = (V(i+1,j) + V(i-1,j) + V(i,j+1) + V(1,j-1)) / 4;
    V(i,j) = (V(i+1,j) + V(i-1,j) + V(i,j+1) + V(i,j-1)) * 0.25;
                                                 ^

The last V term uses "1" in the option 1 version, but "i" in the option 2.