How to multiply a variable for matrix?

Hello I'm trying to do this kind of operation:

A = 20:100:20000

B = [2 3 0 0 0 0; 3 4 -5 0 0 0; 0 7 8 -3 0 0; 0 0 3 2 -3 0; 0 0 0 2 1 1; 0 0 0 0 6 4]

C = A.*B

and it gives me the following error:

??? Error using ==> times Matrix dimensions must agree.

How can I fix it?

Answer 1

[Shane Overington]

Hi Gabriel,

The issue here (as the returned error is indicating) is that the matrices you are trying to multiply are not the same size.

Matrix A is 1 by 200 and Matrix B is 6 by 6.

Are these the matrices you want to multiply, or should they be structured differently?

If you want to do an element-wise multiplication (i.e. C = A.*B) then A and B both need to be 1 by 200 matrices (or both 6 by 6), so that the request can be evaluated as a multiplication of each of the locations in the respective matrices.

For example:

A = [2 3 4 5 6 7];

B = [3 5 6 7 8 9];

C = A.*B;

Result is:

C = [6 15 24 35 48 63]

Regards, Shane

[Gabriel Oliviero]

Actually what I'm trying to do is exactly multiply each element of A by the 6x6 matrix in order to plot a AxB graph.

[Stephen23]

What size do you expect the output to be, when you multiply a 1x200 matrix with a 6x6 matrix? Which of these do you want?:

https://en.wikipedia.org/wiki/Matrix_multiplication

Answer 2

[Walter Roberson]

If you want the entire matrix B to be multiplied by each element of A in turn, then

C = bsxfun(@times, reshape(A,1,1,[]), B);

This would produce a 6 x 6 x 200 array

[Gabriel Oliviero]

Actually what I'm trying to do is exactly multiply each element of A by the 6x6 matrix in order to plot a AxB graph.

[Walter Roberson]

What are you hoping your output graph would look like?

C = permute( bsxfun(@times, reshape(A,1,1,[]), B), [3 1 2]);

The result of that would be 200 x 6 x 6

For any given value of A, how many output lines do you want, and do you want them across the rows of the 6 x 6 or do you want them down the columns of the 6 x 6?

[Gabriel Oliviero]

My graph is the variation of frequencies (represented by A) versus the result of operations with 6x6 matrix from mass spring damper system. I want to relate the displacement of that system with the variation of frequencies.

[Walter Roberson]

That does not tell me anything about what you want the graph to look like. Are you graphing surfaces? 3D lines? 36 2D lines across the width? 6 2D lines for each A value? 6 3D lines for each A value? A 6 x 6 contour plot for each A value?