The Art of R Programming

Recycling

When applying an operation to two vectors that requires them to be the same length, R automatically recycles, or repeats, the shorter one, until it is long enough to match the longer one. Here is an example:

> c(1,2,4) + c(6,0,9,20,22)
[1]  7  2 13 21 24
Warning message:
longer object length
  is not a multiple of shorter object length in: c(1, 2, 4) + c(6,
  0, 9, 20, 22)

The shorter vector was recycled, so the operation was taken to be as follows:

> c(1,2,4,1,2) + c(6,0,9,20,22)

Here’s a more subtle example:

> x
  [,1] [,2]
[1,]    1    4
[2,]    2    5
[3,]    3    6
> x+c(1,2)
  [,1] [,2]
[1,]    2    6
[2,]    4    6
[3,]    4    8

Again, keep in mind that matrices are actually long vectors. Here, x, as a 3-by-2 matrix, is also a six-element vector, which in R is stored column ...