MATLAB: Making bigger matrix with smaller matrixes elements.

bigger matrix from smaller

I'm assembling a bigger matrix with elements of other smaller matrixes. in my method I build a connecting vector for example C=[1 5 6 7 8 9] when in each element like for example C(2)=5, 2 is a local dimension in smaller matrix and 5 is a global dimension in main matrix which is assembled.

If I want to use easy code writing I should use this kind of coding:

    %for i=1:size(C,2)
    for j=1:size(C,2)
    B(C(i),C(j))=A(i,j)

but I'm trying to use Matlab special features in matrix calculations to put A with local dimension in B with global dimension. because Matlab features are fast and need less code writing.

Note: A and B are n*n and m*m matrixes.

Best Answer

 B(C,C)=A;

Related Solutions

MATLAB: What is the mistake,

I'll suggest your mistake is one of definition.

What do you expect when you multiply the elements of the first column with the elements of the first row? In your example, there are 3 products, so that multiplication must result in a vector of length 3.

Oh, do you want to add the elements too? In that case, this is just a matrix multiply.

Taking the columns of matrix a times the rows of matrix b, we get simply:

c = b*a;

Learn to use MATLAB!

Do you wish to compute it using loops for homework? While I don't do your homework, you have made a credible effort.

a=[1 2 3;4 5 6;7 8 9];
b=[1 2 3;4 5 6;7 8 9];
sa = size(a,2);
sb = size(b,1);
c= zeros(sa,sb);
for i=1:sa
  for j=1:sb
    c(i,j)=dot(a(:,i),b(j,:));
  end
end

Note my use of size instead of length. length is dangerous here. (Read the help for length to understand why.) As well, I used a dot product. Finally, see that I preallocated c with zeros. This is important.

Your teacher won't even let you use dot?

a=[1 2 3;4 5 6;7 8 9];
b=[1 2 3;4 5 6;7 8 9];
sa = size(a,2);
sb = size(b,1);
sk = size(a,1);
c= zeros(sa,sb);
for i=1:sa
  for j=1:sb
    for k = 1:sk
      c(i,j)= c(i,j) + a(k,i)*b(j,k);
    end
  end
end

MATLAB: How to use 2d matrix to index into 3d matrix

Use linear indexing

 N = size(C,1)*size(C,2); 
 idx  = [1:N]+(ID(1:N)-1)*N;
 B = reshape(C(idx), size(C,1), size(C,2));

Best Answer

Related Solutions

MATLAB: What is the mistake,

MATLAB: How to use 2d matrix to index into 3d matrix

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