Thread Subject:

matrix-vector vs. vector-matrix multiplication

I=eye(3);
x=[2;-1;7];

A=I*x (possible)
A=x*I (impossible)

does this means that multiplication of matrix with vector is possible but vic-vice-versa not?

Both in matlab ans maths?

"Sofia Häggberg" <hd.int.assNOSPAM@gmail.com> wrote in message <i3rkm0$95t$1@fred.mathworks.com>...
> I=eye(3);
> x=[2;-1;7];
>
> A=I*x (possible)
> A=x*I (impossible)
>
> does this means that multiplication of matrix with vector is possible but vic-vice-versa not?
>
> Both in matlab ans maths?

Well, it means the multiplication of a 3x1 vector with a 3x3 matrix is not possible. But this is not a MATLAB limitation. Try:

A=x.'*I;

"Sofia Häggberg" <hd.int.assNOSPAM@gmail.com> wrote in message <i3rkm0$95t$1@fred.mathworks.com>...
> I=eye(3);
> x=[2;-1;7];
>
> A=I*x (possible)
> A=x*I (impossible)
>
> does this means that multiplication of matrix with vector is possible but vic-vice-versa not?
>
> Both in matlab ans maths?

well... it's all there in...

     doc mtimes;

us

"Sofia Häggberg" <hd.int.assNOSPAM@gmail.com> wrote in message <i3rkm0$95t$1@fred.mathworks.com>...
> I=eye(3);
> x=[2;-1;7];
>
> A=I*x (possible)
> A=x*I (impossible)
>
> does this means that multiplication of matrix with vector is possible but vic-vice-versa not?
>
> Both in matlab ans maths?

You'd have to transpose x.

The dimensions of a matrix are [m (columns) n (rows)]
Size [m1 n1]*[m2 n2] requires that n1,m2 are equal and the result will be size [m1 n2].

I'd recommend picking up a linear algebra text book or wiki page.