MATLAB Answers

problem with Matrix Multiplication

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shawin
shawin on 13 Jan 2017
Answered: Are Mjaavatten on 13 Jan 2017
I have a matrix D which is 10x2, and a center Cent is 2x2 and a w diagonal matrix which is 2x2 , i have an equation:
Result = 1/W^2*(sqrt(w*(X-Y)*SIGMA^(-1)*w*(X-Y)'));
the code which i used is:
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
D= [0.1 0.2 ; 0.3 0.4
0.03 0.7; 0.1 0.7
0.08 0.06; 0.04 0.9
0.11 0.13; 0.12 0.32
0.21 0.71; 1.01 0.33
];
Sigma= cov(D);
Cnt=[-1.982,1.278;0.431,-0.278]
k=2;
w=[0.6 0; 0 0.4];
Wp= 0.6^2 +0.4^2;
N=length(D);
dist=zeros(k,1);
for l=1:N
for i=1:k
dist(i) = 1/Wp*(sqrt( w*(D(l,:)-Cnt(i,:)) * inv(Sigma) * W*(D(l,:)-Cnt(i,:))'));
end
end
iam receiving error : Error using * Inner matrix dimensions must agree.
any help please

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Answers (1)

Are Mjaavatten
Are Mjaavatten on 13 Jan 2017
Note that in Matlab, vectors are matrices of column or row shape and. A 2 by 2 matrix can only be multiplied by a column vector, not by a row vector.
x = (D(l,:)-Cnt(i,:)) is a 1 by 2 matrix ( row vector)
w is a 2 by 2 matrix, so w*x will result in an error
Maybe what you want to do is to replace the content of the innermost loop by:
x = (D(l,:)-Cnt(i,:));
dist(i) = 1/Wp*(sqrt( x*W * inv(Sigma) * W'*x'));

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