Combining 3 for loops into 1 in Matlab

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Aftab Ahmed Khan 2016-2-25

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https://ww2.mathworks.cn/matlabcentral/answers/270002-combining-3-for-loops-into-1-in-matlab

评论： Star Strider 2016-2-26

I have this section of code which is working fine but i don't like the way i have implemented those 3 separate for loops. Can anyone suggest to me that how can i merge them together to make it even more efficient and compact ? Thank you

    clear variables;
    close all;
    clc;
    ilambda=5;
    mu=2;
    n=2;
    jmax=n+1;
    P= sym('P',[jmax,jmax]);
    for j1 = 1:jmax
            for j2 = 2:jmax-1
                [c1,c2,c3,c4,c5]=coefficients(ilambda,mu,j1,j2);    
                if j1<jmax
                    E(j1, j2) = c1*P(j1, j2) - c2 * P(j1+1, j2) - c3 * P(j1, j2+1) - c4 * P(j1, j2-1);
                else
                    E(j1, j2) = c1 * P(j1, j2) - c3 * P(j1, j2+1) - c4 * P(j1, j2-1);
                end
            end
    end
        j2=1;
        for j1=1:jmax;
            [c1,c2,c3,c4,c5]=coefficients(ilambda,mu,j1,j2);    
            if (j1<jmax)
                E(j1, j2) = c1*P(j1, j2) - c2 * P(j1+1, j2) - c3 * P(j1, j2+1);
            else
                E(j1, j2) = c1*P(j1, j2) - c3 * P(j1, j2+1);
            end
        end
        j2=jmax;
        for j1=1:jmax;
            [c1,c2,c3,c4,c5]=coefficients(ilambda,mu,j1,j2); 
            if (j1==1)
                E(j1, j2) = c1*P(j1, j2) - c2 * P(j1+1, j2) - c4 * P(j1, j2-1);
            elseif (j1==jmax)
                c1=((j1-1)*mu)+((j2-1)*mu);
                E(j1, j2) = c1 * P(j1, j2) - c4 * P(j1, j2-1) - c5 * P(j1-1, j2);
            else
                E(j1, j2) = c1*P(j1, j2) - c2 * P(j1+1, j2)- c4 * P(j1, j2-1)- c5 * P(j1-1, j2);
            end
        end

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Aftab Ahmed Khan 2016-2-26

编辑：Aftab Ahmed Khan 2016-2-26

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Hi, I think your guess is right. The way i am approaching this problem is like this but i can't map those coefficients in a right order. I want to map them like the table given below.

clear variables;
close all;
clc;
jmax=3;
mu=2;
ilambda=5;
      C=zeros(jmax^2);
          for j1=1:jmax
              for j2=1:jmax
                  switch j2
                      case 1                                                                  
                              c1=ilambda+((j1-1)*mu)+((j2-1)*mu);
                              c2=(((j1-1)+1)*mu);
                              c3=(((j2-1)+1)*mu);
                              c4=ilambda;
                              c5=ilambda;
                          if (j1<jmax)
                              C(j1,j2)=c1;
                              C(j1+1,j2)=c2;
                              C(j1,j2+1)=c3;
                          else
                              C(j1,j2)=c1;
                              C(j1,j2+1)=c3;
                          end
                      case jmax                                                               
                              c1=ilambda+((j1-1)*mu)+((j2-1)*mu);
                              c2=(((j1-1)+1)*mu);
                              c3=(((j2-1)+1)*mu);
                              c4=ilambda;
                              c5=ilambda; 
                          if (j1==1)
                              C(j1,j2)=c1;
                              C(j1+1,j2)=c2;
                              C(j1,j2-1)=c4;
                          elseif (j1==jmax)
                              c1=((j1-1)*mu)+((j2-1)*mu);
                              C(j1,j2)=c1;
                              C(j1,j2-1)=c4;
                              C(j1-1,j2)=c5;
                          else
                              C(j1,j2)=c1;
                              C(j1+1,j2)=c2;
                              C(j1,j2-1)=c4;
                              C(j1-1,j2)=c5;
                          end
                      otherwise
                              c1=ilambda+((j1-1)*mu)+((j2-1)*mu);
                              c2=(((j1-1)+1)*mu);
                              c3=(((j2-1)+1)*mu);
                              c4=ilambda;
                              c5=ilambda;
                          if j1<jmax
                              C(j1,j2)=c1;
                              C(j1+1,j2)=c2;
                              C(j1,j2+1)=c3;
                              C(j1,j2-1)=c4;
                          else
                              C(j1,j2)=c1;
                              C(j1,j2+1)=c3;
                              C(j1,j2-1)=c4;
                          end
                  end
              end
          end

Star Strider 2016-2-26

The matrix you created looks as though it will work. It may seem awkward code, but if it maps your coefficients correctly, and is faster and more efficient than using the Symbolic Math Toolbox, that’s likely the best you can hope for. It solves the problem of getting the coefficients in the correct order, because you’re mapping them directly to your matrix. Conditional statements are difficult to make into efficient, vectorised code.

If it works, go with it!

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Combining 3 for loops into 1 in Matlab

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Combining 3 for loops into 1 in Matlab

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