Question

I have two vectors and I want a matrix which elements are the sum of each element of vector 1 and each element of vector 2.

For example, the first element in the first row of this matrix is the sum of the first element of vector 1 and the first element of vector 2; the second element of the first row is the sum of the first element of vector 1 and the second element of vector 2 and so on.

For example, with these two vectors

u <- c(1,2,3)
v <- c(4,5,6)

The desired result would be:

#       [,1] [,2] [,3]
# [1,]    5    6    7
# [2,]    6    7    8
# [3,]    7    8    9

---

What I have tried:

A <- matrix( c(1:6), 3, 3 )

for(i in 1:3)
{
   for(j in 1:3)
   {
      A[j][i] <- u[i]+v[j]
   }
}

But I get some warnings:

Warning messages:
1: In A[j][i] <- u[i] + v[j] :
  number of items to replace is not a multiple of replacement length
2: In A[j][i] <- u[i] + v[j] :
  number of items to replace is not a multiple of replacement length
3: In A[j][i] <- u[i] + v[j] :
  number of items to replace is not a multiple of replacement length
4: In A[j][i] <- u[i] + v[j] :
  number of items to replace is not a multiple of replacement length
5: In A[j][i] <- u[i] + v[j] :
  number of items to replace is not a multiple of replacement length
6: In A[j][i] <- u[i] + v[j] :
  number of items to replace is not a multiple of replacement length

Can anybody help me?

Answer 1

This is how you would do it (note the matrix subset is not two brackets, but comma separated):

u <- c(1,2,3)
v <- c(4,5,6)

A <- matrix( c(1:6), 3, 3 )

for(i in 1:3)
{
   for(j in 1:3)
   {
      A[i,j] <- u[i]+v[j]
   }
}

But this is not the way someone who knows R would approach it. In general there are better ways to do things in R than nested for-loops. Another way is:

A <- outer(u,v,`+`)