Getting errors and incorrect matrix form.

Hello I am relatively new to the matlab program and am struggling with this assignment.

Recall that if A is an m n matrix and B is a p q matrix, then the product C = AB is defined if and only if n = p, in which case C is an m q matrix. (a) Write a function M-file that takes as input two matrices A and B, and as output produces the product by rows of the two matrices. For instance, if A is 3 4 and B is 4 5, the product AB is given by the matrix C = [A(1,:)*B; A(2,:)*B; A(3,:)*B] The function file should work for any dimension of A and B and it should perform a check to see if the dimensions match (Hint: use a for loop to define the rows of C).

I have been able to clear my function of errors so that it returns an answer, however its not coming back as a matrix of the correct dimensions. I get the answer returned row by row it appears. My function is written as follows:

I have typed the following elements into matrices A and B in the command window with the ans following.

I expected a 2x2 matrix with elements [14,14;42,42].

Thanks for the help!

[Star Strider]

Your first version of this post, with code I could actually copy and run, was preferable. I edited it with the [{}Code] button so the code showed clearly, something you can easily do.

Please do not use screencaps of code! Nobody wants to type it in, and that’s what we have to do in that situation.

The essence (with my edit), copied from the previous correct version:

A=[1,1,1;3,3,3];
B=[2,2;5,5;7,7];
[m,n]=size(A);%Determines dimensions of A
[p,q]=size(B);%Determines dimensions of B
if (p==n) %Check the dimensions of the matrix to determine if its possible
    C=zeros(m,q) %Creates a matrix, C, to be filled, initially zeros.
    for i=1:m;
        C(i,:)=A(i,:)*B %Computes every row of A times matrix B
    end
else disp('dimensions do not match') %Dimesions do not match message
    C=[];
end

[Hadassah Fromowitz]

thank you! the C(i,:) part was what we were forgetting... (it was C= in the post)

Answer 1

[Star Strider]

You forgot to index ‘C’ !

C(i,:)=A(i,:)*B %Computes every row of A times matrix B

That works.

(It’s best not to use i and j as variables, because MATLAB uses them as its imaginary operators. Not a problem here, but it can cause confusion if you’re dealing with complex numbers.)

[Sundevil62301]

WOW fast response!!! That was what I needed. I had attempted variations of this and didn't quite get it.

Much appreciated!

[Star Strider]

My pleasure!

And greetings from Albuquerque!