Error using * Matrix dimensions must agree. please help.

Question

jj kena 2014-10-28

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编辑： jj kena 2014-10-29

 N_BF=10;
 N_s=700;
  A1=zeros(N_BF,N_BF);
  Wu=zeros(N_BF,N_BF);
  Ww=zeros(N_BF,N_BF);
  b1=zeros(N_BF,1);
 % compute interval
 t2=cputime;
 for i=1:N_s
 x1=-1300+rand; x2=0+rand; x3=0+rand;
 f=[x2/x3; -x1*x2/x3; x1];
 BF=[ x1^2;  x1*x2;  x1*x3;  x2^2  ;x2*x3 ; x3^2 ;
      x1/x3;  x1*x2/x3 ; x2^2/x3 ; x2^4/x3^2];
 DBF= [ 2*x1    0         0
        x2      x1        0
        x3      0         x1
        0       2*x2      0
        0       x3        x2 
        0       0         2*x3
        1/x3    0         -x1/x3^2
        x2/x2   x1/x3     -x1*x2/x3^2
        0       2*x2/x3   -x2^2/x3^2
        0       4*x2^3    -2*x2^4/x3^3];
    Aa=[x1;x2;x3]'*[x1;x2;x3];
    A1=A1+BF*f'*DBF';
    b1=b1+BF*Aa;
    g1=[0;1;0]';
    g2=[0;-1;0]';
    for j= 1:N_BF
       Wu(:,:,j)=Wu(:,:,j)+BF*DBF(j,:)*g2*g2'*DBF';
       Ww(:,:,j)=Ww(:,:,j)+BF*DBF(j,:)*g1*g1'*DBF';
    end
 end
getting the error at Wu(:,:,j)=Wu(:,:,j)+BF*DBF(j,:)*g2*g2'*DBF';

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Answer 1

Abhiram Bhanuprakash 2014-10-28

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编辑：Abhiram Bhanuprakash 2014-10-28

在 MATLAB Online 中打开

Hi JJ Kena,

I think the error is because one row of DBF is 1X3, and g2 is also 1X3. So, when it tries to multiply both of them, you get the dimension mismatch error.

Paranthesizing the g2*g2' term would resolve the issue.

Thus, change the two lines of code to:

Wu(:,:,j)=Wu(:,:,j)+BF*DBF(j,:)*(g2*g2')*DBF';
Ww(:,:,j)=Ww(:,:,j)+BF*DBF(j,:)*(g1*g1')*DBF';

and you would be able to run it.

Also, if you observe, MATLAB shows an orange symbol in the Editor window for the two lines, and if you hover over it, you can see a message which says 'Paranthesize the multiplication to ensure the result is Hermitian'.

Hope this resolves your issue. MATLAB rocks!

Cheers!

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jj kena 2014-10-28

编辑：jj kena 2014-10-29

Abhiram Bhanuprakash, thanks alot, did that n saw another problem too, its at

Wu=zeros(N_BF,N_BF); Ww=zeros(N_BF,N_BF); these two shlould be, Wu=zeros(N_BF,N_BF,N_BF); Ww=zeros(N_BF,N_BF,N_BF);

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Answer 2

yonatan gerufi 2014-10-28

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https://ww2.mathworks.cn/matlabcentral/answers/160387-error-using-matrix-dimensions-must-agree-please-help#answer_156852

在 MATLAB Online 中打开

well, your line:

Wu(:,:,j)=Wu(:,:,j)+BF*DBF(j,:)*g2*g2'*DBF';

is totally messed up with dimension.

think what dimentions each parameter should be, and see the difference with debug mode.

saying all that, pay attention that if you want element-by-element multiplication you should use "."

e.g. A.*B

good luck.

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Answer 3

jj kena 2014-10-29

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yonatan gerufi ,from BF*DBF(j,:)*g2*g2'*DBF', BF is 10x1, BDF(j,:) is 1x3, g2 is 3x1, g2' is 1x3, and DBF' is 3x10, if i think you can multiply those matrices without a problem to give u a 10x10 matrice.