multiply two vectors - I want a scalar but I get a vector?

If what you essentially want is the sum of the products, then all you need is sum(a*a)

Two problems, multiplication in the wrong order, and the wrong multiply function.

> print(t(a)%*%a)
     [,1]
[1,]   14

Equivalently:

> a=matrix(c(1,2,3),ncol=3)
> print (a %*% t(a))
     [,1]
[1,]   14

Here a is a matrix of 1 row, three columns.

See ?"%*%" and ?"*"

You can simply do this,

> a <-c(1,2,3)
> b <-t(a)

> b %*% a

Here, %*% acts as a matrix product.

Related