如何对一系列矩阵求和？

Question

so say I had a matrix

> A<-matrix(c(5,1,3,5,6,6,7,8,5),nrow=3,ncol=3)
> A
     [,1] [,2] [,3]
[1,]    5    5    7
[2,]    1    6    8
[3,]    3    6    5

and I wanted to take the sum of i from 1 to 3 multiplied by each corresponding column.

so like

for (i in 1:3){
i*A[,i]
something 
}

then there is more code after this that I cant seem to figure out however simple it may seem, goal is to sum 1* first column + 2* second column + 3* 3rd column using some sort of iterative loop with i.

Answer 1

If you are looking for a for loop version, maybe you can try the code below

s <- 0
for (i in 1:3) {
  s <- s + i*A[,i,drop = FALSE]
}

such that

> s
     [,1]
[1,]   36
[2,]   37
[3,]   30

---

As mentioned by @eastclintw00d in the comment, a simple way is using matrix multiplication

> A%*% 1:3
     [,1]
[1,]   36
[2,]   37
[3,]   30