If statement in a loop

Question

sedef kocakaplan 2017-5-9

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/339508-if-statement-in-a-loop

评论： Jan 2017-5-10

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

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Jan 2017-5-9

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?

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Answer 1

Jan 2017-5-9

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/339508-if-statement-in-a-loop#answer_266308

编辑：Jan 2017-5-9

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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.

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sedef kocakaplan 2017-5-9

Dear Jan,

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

Jan 2017-5-10

@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.

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