# dimension error with * operator

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Douglas Brenner on 28 Sep 2018
Commented: OCDER on 28 Sep 2018
This line of code: asynch = C1(:)*N(:)*C(:)'/(delta-2) throws an error: Error using * Inner matrix dimensions must agree. That would seem to be right C1 is a 1X100 double and N is a 100x100 double. So I changed the code to T = transpose(C1); asynch = C1(:)*N(:)*T(:)'/(delta-2). This makes T a 100X1 double and the code should run but I get the same error. What's wrong. Thanks
OCDER on 28 Sep 2018
synch = C1(:)*C1(:)'/(delta-2)
^
This works because of the transpose operator.
C1(:) = Nx1 vector
C1(:)' = 1xN vector
so C1(:)*C1(:)' works as a (Mx1) * (1xM) = MxM vector (inner dimension agrees)
In your case, you have a matrix N that is 100x100.
N = 100x100 matrix
N(:) = 10000x1 vector
C(:)*N(:) will NOT WORK because it's a (Mx1) * (10000x1) = ????

OCDER on 28 Sep 2018
Although N is a 100x100 matrix, when you do N(:), you make it into a 10000x1 vector. Get rid of the "(:)" when you want to proper matrix math.

Pratik Bajaria on 28 Sep 2018
Hello,
Try .* operator. It is the normal matrix multiplication, the one you are trying is doing a Hadamard multiplication.
Regards, Pratik

Douglas Brenner on 28 Sep 2018
The paper from which I got the code uses * and that works in this line: synch = C1(:)*C1(:)'/(delta-2). Not only do I not get an error but I get what I think is the right answer. For asynch = C1(:)*N(:)*C(:)'/(delta-2), I think the problem is that N is 2D and C1 is one D but I got that code from the paper too.

R2014a

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