vector multiplication .* vs *

I have these two vectors,

u= [ 1 2 -1 2 1]

v=[ -1 0 2 0 1]

why does v*u give me an error, whilst u*v' give me a -2, which is the result of a scalar multiplication?

u and v have the same size, so shouldn't v*u go through?

Thanks in advance.

[Stephen23]

@Omar A Jiminez: knowing the difference between matrix operations and element-wise operations is critical for being able to use MATLAB properly: not just multiplication, but transpose, division, and power too. You need to read the MATLAB documentation:

https://www.mathworks.com/help/matlab/matlab_prog/array-vs-matrix-operations.html

Answer 1

[Walter Roberson]

v*u is algebraic matrix multiplication, for which the rule is that for an (M x N) * (P x Q) operation, that N must equal P and the output size is M x Q -- so (M x N) * (N x Q) giving M x Q. You have (1 x 5) * (1 x 5) which violates that rule. When you take u * v' then you have (1 x 5) * (5 x 1) giving 1 x 1.

v.*u would go element by element, result(K) = v(K) * u(K) which would be 1 x 5 result.

Answer 2

[Shomari Edwards]

24.5 + 64/3.52 + 8.3 · 12.53√

3 76.4−28/15

[Walter Roberson]

It is not clear what you are trying to convey here?

Answer 3

[John E Keevil]

The simple answer to the question is:-

u= [ 1 2 -1 2 1] and v=[ -1 0 2 0 1] are both row vectors. You cannot matrix multiply them because the number of rows and columns are not compatible for matrix multiplication. To multiply u*v by matrix multiplication requires the number of rows of u to equal the number of columns of v, which it does not since u has one row and v has 5 columns. To correct that, you have to transpose v, then it has one column. v' is the transpose of v. This why u*v' works.