matrix multiplication

Hello,

I m wrinting a code for itertaive closest point and i got this error

??? Error using ==> mtimes
Inner matrix dimensions must agree.

RR=V * U';

V and U are two matrix in diffent size what do you think ?

Answer 1

[Wayne King]

It's just telling you that the two matrices are not conformable for multiplication. The number of columns in V has to match the number of rows in U'

RR = randn(10,10)*(randn(2,10))';  % is ok

RR = randn(10,10)*(randn(10,12))';  % is not

[Fatma Gargouri]

ok so it must that the number of rows = colomns !!

and what is the diffrence between * and .* ?

[Wayne King]

.* is element by element multiplication so,

x = randn(10,1);

y = randn(10,1);

z = x.*y; % this is ok -- multiply x and y element by element

but

x*y % errors

Answer 2

[Rick Rosson]

Please try the following:

   [M,N] = size(V);
   [P,Q] = size(U');

   if N==P
      fprintf('ok\n\n');
   else
      fprintf('error\n\n');
   end

Answer 3

[Rick Rosson]

Using * by itself will perform matrix multiplication, whereas using .* will perform element-by-element multiplication.

• For matrix multiplication, e.g. A*B, the number of columns in A must equal the number of rows in B.
• For element-by-element multiplication, e.g. A.*B, the size and shape of both A and B must be exactly the same. In other words, if A is M x N, then B must also be M x N.
• For more information, please see: Arithmetic Operators
• You can use the size function to compute the size and shape of any given array.
• You can use the assert function to test for the truth of a logical expression. For example:

   assert(size(A,2) == size(B,1), ...
        'The inner dimensions of A and B do not agree.');

HTH.

Rick

[Fatma Gargouri]

thank you Rick

this is very helpful

I should then review my code and verify the sizes of matrix