rep(x) function replicates the values x.

rep(x, ...)
rep.int(x, times)
rep_len(x, length.out)

x: numeric vector, factor, list etc.
...: arguments including times (default = 1), length.out, each (each elements how many times)

> rep(8,4)
[1] 8 8 8 8
> rep(NA,6)
[1] NA NA NA NA NA NA
> rep(1:5)

[1] 1 2 3 4 5

Repeat 1 to 5 two times:

> x <- rep(1:5,2)
> x

[1] 1 2 3 4 5 1 2 3 4 5

Convert to a 5 × 2 matrix:

> dim(x) <- c(5,2)
> x

[,1] [,2] [,3] [,4] [,5]
[1,] 1 3 5 2 4
[2,] 2 4 1 3 5

Each element replicates two times:

> x <- rep(1:5,each=2)
> x

[1] 1 1 2 2 3 3 4 4 5 5

Convert to a 5 × 2 matrix:

> dim(x) <- c(5,2)
> x

[,1] [,2] [,3] [,4] [,5]
[1,] 1 2 3 4 5
[2,] 1 2 3 4 5

The repeat times of each elements can be different:

> rep(1:5,c(2,1,3,2,0))
[1] 1 1 2 3 3 3 4 4
> rep(c(3,5,12),c(4,2,5))
 [1]  3  3  3  3  5  5 12 12 12 12 12

length.out or len determines the desired length of the return vector.

> rep(1:5,length.out=8)
[1] 1 2 3 4 5 1 2 3
> rep(1:5,each=2, len=13)
[1] 1 1 2 2 3 3 4 4 5 5 1 1 2

Quick and simple version of length specified repeat:

> rep_len(1:3,10)
[1] 1 2 3 1 2 3 1 2 3 1

> rep.int(1:5,2)

[1] 1 2 3 4 5 1 2 3 4 5

Repeat a factor:

> x <- factor(LETTERS[1:6])
> names(x) <- 1:6
> x
1 2 3 4 5 6
A B C D E F
Levels: A B C D E F
> rep(x, 2)
1 2 3 4 5 6 1 2 3 4 5 6
A B C D E F A B C D E F
Levels: A B C D E F

Repeat a list:

> x <- list(cost=1000.24,urgent="yes")
> x
$cost
[1] 1000.24
$urgent
[1] "yes"
> rep(x,2)
$cost
[1] 1000.24
$urgent
[1] "yes"
$cost
[1] 1000.24
$urgent
[1] "yes"