When does loop end?

Hi, sorry for making it a little long but I wanted to ask a clear specific question.

I'm working on this flow dynamics problem and I'm getting different results using my approach than the expected results.

I spent the last few days trouble shooting the problem and I believe the issues is coming from a For Loop nested with IF statements.

Here's a sketch from my 2-D grid where I'm writing code to try to determine boundary conditions for flow between the grids.

My issues has been with the diagonal cells, namely: A,C,G & I.

The IF statements share some conditions with the B,D,F& H as you can see in my code below

Here's my code:

Imax = 15; 
Jmax = 15;

   for i=1:Imax*Jmax
        if i<=Imax || mod(i,Imax)==1      % No-flow boundary conditions
            lamdaWA(i)=0;
            lamdaTA(i)=0;
        else
            if P(i,t)>=P(i-Imax,t)
                lamdaWA(i)=lamdaW(i,t); 
                lamdaTA(i)=lamdaT(i,t);
            else
                lamdaWA(i)=lamdaW(i-Imax-1,t);
                lamdaTA(i)=lamdaT(i-Imax-1,t);
            end
        end
        if i<=Imax                     
            lamdaWB(i)=0;
            lamdaTB(i)=0;
        else
            if P(i,t)>=P(i-Imax,t)
                lamdaWB(i)=lamdaW(i,t);
                lamdaTB(i)=lamdaT(i,t);
            else
                lamdaWB(i)=lamdaW(i-Imax,t);
                lamdaTB(i)=lamdaT(i-Imax,t);
            end
        end
        if i<=Imax || mod(i,Imax)==0
            lamdaWC(i)=0;
            lamdaTC(i)=0;
        else
            if P(i,t)>=P(i-Imax,t)
                lamdaWC(i)=lamdaW(i,t);
                lamdaTC(i)=lamdaT(i,t);
            else
                lamdaWC(i)=lamdaW(i-Imax+1,t);
                lamdaTC(i)=lamdaT(i-Imax+1,t);
            end
        end        
        if mod(i,Imax)==1
            lamdaWD(i)=0;            
            lamdaTD(i)=0;
        else
            if P(i,t)>=P(i-1,t)
                lamdaWD(i)=lamdaW(i,t);
                lamdaTD(i)=lamdaT(i,t);
            else
                lamdaWD(i)=lamdaW(i-1,t);
                lamdaTD(i)=lamdaT(i-1,t);
            end
        end
        if mod(i,Imax)==0
            lamdaWF(i)=0;
            lamdaTF(i)=0;
        else
            if P(i,t)>=P(i+1,t)
                lamdaWF(i)=lamdaW(i,t);
                lamdaTF(i)=lamdaT(i,t);
            else
                lamdaWF(i)=lamdaW(i+1,t);
                lamdaTF(i)=lamdaT(i+1,t);
            end
        end
        if i>(Imax*Jmax)-Imax || mod(i,Imax)==1
            lamdaWG(i)=0;
            lamdaTG(i)=0;
        else
            if P(i,t)>=P(i+Imax,t)
                lamdaWG(i)=lamdaW(i,t);
                lamdaTG(i)=lamdaT(i,t);
            else
                lamdaWG(i)=lamdaW(i+Imax-1,t);
                lamdaTG(i)=lamdaT(i+Imax-1,t);
            end
        end
        if i>(Imax*Jmax)-Imax
            lamdaWH(i)=0;
            lamdaTH(i)=0;
        else
            if P(i,t)>=P(i+Imax,t)
                lamdaWH(i)=lamdaW(i,t);
                lamdaTH(i)=lamdaT(i,t);
            else
                lamdaWH(i)=lamdaW(i+Imax,t);
                lamdaTH(i)=lamdaT(i+Imax,t);
            end
        end
        if i>(Imax*Jmax)-Imax || mod(i,Imax)==0
            lamdaWI(i)=0;
            lamdaTI(i)=0;
        else
            if P(i,t)>=P(i+Imax,t)
                lamdaWI(i)=lamdaW(i,t);
                lamdaTI(i)=lamdaT(i,t);
            else
                lamdaWI(i)=lamdaW(i+Imax+1,t);
                lamdaTI(i)=lamdaT(i+Imax+1,t);
            end
        end        
    end

My specific question is: When does the For loop jump to the next item? That means from i=1 to 1=2?

For example, when i = 5, does the code read the first 3 IF statements for A,B,C or does it just read the first one, jump to the end of the FOR loop and repeat for i=6?

Answer 1

[Dana]

Unless you have a continue statement somewhere (which you don't), everything enclosed between the initial for statement and the end statement that encloses the loop is run before advancing to the next index i. What makes you think MATLAB would stop after reading only the first 3 if statements?

[Mizo Amreya]

I'm sorry, I might not have been clear in my question. I'm thinking MATLAB might be skipping some IF statements. For a specific index i, does MATLAB read all IF statements before jumping to the next index? Or once an IF statement is satisfied, it moves to the next index?

[Dana]

As I said, unless you tell it to do otherwise, MATLAB reads everything inside the loop on each iteration, exactly as if that code wasn't contained in a loop at all. It is definitely not skipping any IF statements.

So you have 8 root-level IF statements inside that loop, each of which also contains a nested IF statement in the else block. All 8 of the root-level IF statements will be executed on every iteration of this loop. The 8 nested IF statements may or may not be executed on a given iteration, depending on whether the corresponding parent IF statement evaluates to a false.

If you're getting some unexpected result, I can assure you it's not because MATLAB is skipping anything.

[Mizo Amreya]

Okay, thanks for explaining. I will dig into the problem further.