Repeat sequence every nth element x times

Question

I have a vector e.g:

v <- c(1, 2, 3, 4)

I want to repeat the sequence of every nth element x times e.g:

x=2
n= 2

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

I know that

rep(v, times=n)
[1] 1, 2, 3, 4, 1, 2, 3, 4

rep(v, each=n)
[1] 1, 1, 2, 2, 3, 3, 4, 4

Thanks!

Answer 1

You could split the vector and then repeat:

fun <- function(v, m, n) {
  unlist(by(v, ceiling(seq_along(v) / m), rep, n), use.names = FALSE)
}

v <- c(1, 2, 3, 4)

fun(v, 2, 2)
# [1] 1 2 1 2 3 4 3 4

fun(v, 3, 3)
# [1] 1 2 3 1 2 3 1 2 3 4 4 4

Answer 2

We may create a grouping index with gl and use tapply

f_rep <- function(v, x, n) 
  {
  unname(unlist(tapply(v, as.integer(gl(length(v), x,
        length(v))), rep, times = n)))
}

-testing

> x <- 2
> n <- 2
> f_rep(v, x, n)
[1] 1 2 1 2 3 4 3 4
> f_rep(v, 3, 3)
 [1] 1 2 3 1 2 3 1 2 3 4 4 4

Answer 3

Another split option:

unlist(rep(split(v,(seq_along(v)-1) %/% n), each = x), use.names = FALSE)

#[1] 1 2 1 2 3 4 3 4

Answer 4

With sequence:

v <- 9:1
x <- 2L
n <- 3L

v[sequence(rep(n, x*(length(v) %/% n)), rep(seq(1, length(v), n), each = x))]
#>  [1] 9 8 7 9 8 7 6 5 4 6 5 4 3 2 1 3 2 1

If it needs to handle vectors whose lengths are not multiples of n:

v <- 1:5
x <- 3L
n <- 3L
v[
  sequence(
    c(
      rep(n, x*(length(v) %/% n)),
      rep(length(v) %% n, x)
    ),
    c(
      rep(seq(1, length(v), n), each = x),
      length(v) - length(v) %% n + 1L
    )
  )
]
#>  [1] 1 2 3 1 2 3 1 2 3 4 5 4 5 4 5