If statement in a loop

Hi everyone,

I am trying to run the code below. The problem is co is not working in a loop. I just obtained the last raw for the computation. What should I do to work the code with a looping co?(F_l is a force vector)

co = 1;
for c = 1:na-2
  co=co+1  
for ni = 3:3:length((reale))

      if ni==3;
          F_l(ni) = 2*pi*((DeltaR/na)/6);

      elseif ni==6;

          F_l(ni) =2*pi*( ((DeltaR/na)/3)+ (2/na+2/na)*DeltaR/6);

      elseif ni==length((reale));  

          F_l(ni)=2*pi*((na-1)/na+2)*DeltaR/6;

      else       
          F_l(ni) =2*pi*( ((DeltaR*((co-1)/na+2*co/na))/6)+ ((DeltaR*((co+1)/na+2*co/na))/6)) ;   

end
end
end

[Jan]

The question is not clear. Of course you can use co inside a loop and inside the if branches. Please explain what "co is not working" exactly means. What does "the last row for the computation" mean?

Answer 1

[Jan]

With some guessing:

co = 1;
F_l = zeros(length(reale), na - 2);  % Pre-allocate!!!
for c = 1:na-2
  co = co + 1;  
  for ni = 3:3:length(reale)
    if ni == 3
        F_l(ni, c) = 2*pi*((DeltaR/na)/6);
    elseif ni == 6
        F_l(ni, c) = 2*pi*(((DeltaR/na)/3) + (2/na+2/na)*DeltaR/6);
    elseif ni == length(reale)
        F_l(ni, c) = 2*pi*((na-1)/na+2)*DeltaR/6;
    else       
        F_l(ni, c) = 2*pi*(((DeltaR*((co-1)/na+2*co/na))/6) + ...
                     ((DeltaR*((co+1)/na+2*co/na))/6));
    end
  end
end

Is this the wanted result? Should F_l contain the zeros for ni=1,2, etc.? Then the code can be simplified:

co  = 1;
nr  = length(reale);
F_l = zeros(nr, na - 2);  % Pre-allocate!!!

F_l(3,  1:na-2) = 2*pi*((DeltaR/na)/6);
F_l(6,  1:na-2) = 2*pi*(((DeltaR/na)/3) + (2/na+2/na)*DeltaR/6);
F_l(nr, 1:na-2) = 2*pi*((na-1)/na+2)*DeltaR/6;

for c = 1:na-2
  co = co + 1;  
  F_l(9:3:length(reale)-3, c) = 2*pi*(((DeltaR*((co-1)/na+2*co/na))/6) + ...
                                ((DeltaR*((co+1)/na+2*co/na))/6));
end

And even this can be written without a loop.

[sedef kocakaplan]

Hi Jan,

Thank you for your answer. Sorry for not giving detail explanation. F_l is a vector. (F_l = zeros(length(reale),1). I want F_l (at the else part), change in values as co increases. Yet matlab calculates the all the F_l values only with co=co(end).

[Jan]

In your original code you overwrite the elements of F_l in each iteration of the for c loop. What do you want to obtain instead? Currently all we see is the failing assignment of the elements of this array. But what do you want instead? "change in values as co increases" is clear, but at which index should which value appear?

[sedef kocakaplan]

For co 2,3,4... F_l(ni=9,12,15...) should be written.

Also pre-definition of F_l is,

 F_l = zeros(length(reale),1);

[sedef kocakaplan]

Dear Jan,

I re-write the code in a different way. Thanks for your comments.

[Jan]

@sedef kocakaplan: This is not clear: "For co 2,3,4... F_l(ni=9,12,15...) should be written." These are 2 loop counters: one for co and one for ni. But you get one vector as output only.