Return to previous step

Hello,

I have a code that includes some steps. I would like to know how to make the process return to the first step if a condition is true. In the loop if k=length of x0, the process needs to back to step 1. Also I need to know how many interactions the code did in the end.

 function [x_opt,f_opt,k] = gc_completo(fob,g_fob,x0,tol_grad);
tic

 c0 = feval(g_fob,x0); 
 k = 0;    

 if norm(c0) < tol_grad
    x_opt = x0;    
    f_opt = feval(fob,x_opt); 

 else 
    d= -c0; %step 1
    alfa0 = equal_interval_line_search(x0,d,fob,0.5,1e-6);
    x1= x0+ alfa0*d;
    c1 = feval(g_fob,x1); %evaluate gradient at x1

    while norm(c1) > tol_grad
        beta = (norm(c1)/norm(c0))^2;
        d= -c1+beta*d;
        alfa1 = equal_interval_line_search(x1,d,fob,0.5,1e-6);
        x2= x1+alfa1*d;
        c0=c1;
        c1= feval(g_fob,x2);
        x1=x2;

      k=k+1;
        end
    x_opt = x1; %optimum point
    f_opt = feval(fob,x_opt); %cost funtion value
end

 toc

Answer 1

[Walter Roberson]

It is not clear what an "interaction" is that you want to count. I will guess it has to do with the number of searches.

    search_count = 0;
    redo_step1 = true;

    while redo_step1
      redo_step1 = false;
      d= -c0; %step 1
      alfa0 = equal_interval_line_search(x0,d,fob,0.5,1e-6);
      search_count = search_count + 1;
      x1 = x0+ alfa0*d;
      c1 = feval(g_fob,x1); %evaluate gradient at x1
      k = 0;

      while norm(c1) > tol_grad
        beta = (norm(c1)/norm(c0))^2;
        d= -c1+beta*d;
        alfa1 = equal_interval_line_search(x1,d,fob,0.5,1e-6);
        search_count = search_count + 1;
        x2 = x1 + alfa1*d;
        c0=c1;
        c1= feval(g_fob,x2);
        x1=x2;
        k = k+1;
        if k == length(x0)
          redo_step1 = true;
        end
      end
    end

[Matheus]

I tried it out, but the code is running a long time and does not give me an answer. I did not understand the line "while redo_step1".

[Image Analyst]

You forgot to put in a failsafe. Never use a while loop without one or you could get an infinite loop, which is what might be happening. For example if it's your expectation that the loop should never execute more than a million times, do this:

maxIterations = 1000000;
loopCounter = 0;
while redo_step1 && loopCounter < maxIterations
    % Do stuff
    loopCounter = loopCounter + 1;
end
if loopCounter >= maxIterations
    warningMessage = sprintf('Max iterations of %d reached and loop exited early', maxIterations);
    uiwait(warndlg(warningMessage));
end

[Walter Roberson]

You said that you wanted to redo step 1 in some cases. The framework I presented shows a way of handling that

1. set a flag to true indicating that step1 needs to be done
2. enter into a loop that will not exit until that flag is false
3. assume that re-doing is the exception that will be specifically detected, so set the flag to be false inside the loop. This way the "while" loop will exit next time around unless something specifically says that it needs to be redone
4. do the calculation. Inside the calculation, if the conditions are right, set the flag indicating you need to redo.

I would suggest, though, that probably instead of

        if k == length(x0)
          redo_step1 = true;
        end

you want

        if k == length(x0)
          redo_step1 = true;
          break
        end

otherwise your inner loop is not exited when k reaches the boundary.