create a sequence that replicates itself excluding the last element-CodePudding

Is there a way to create a sequence that replicates itself excluding the last element without a loop? Say we have a starting sequence 4:1 and the function fun that generates the rest of the sequence like:

> fun(4:1)
[1] 4 3 2 1 4 3 2 4 3 4

CodePudding user response：

Using sequence.

5L - sequence(4:1)
# [1] 4 3 2 1 4 3 2 4 3 4

As function:

f <- \(n) (n   1) - sequence(n:1)
f(4)
# [1] 4 3 2 1 4 3 2 4 3 4

f2 <- \(seq) seq[1]   1 - sequence(seq)
f2(4:1)
# [1] 4 3 2 1 4 3 2 4 3 4

For sequences not ending with 1 maybe

f3 <- \(s) unlist(list(s, lapply(1:diff(range(s)) - 1, \(i) s[-((length(s) - i):length(s))])))
f3(5:3)
# [1] 5 4 3 5 4 5

CodePudding user response：

One way to solve your problem:

v = 4:1
unlist(lapply(4:1, rep_len, x=rev(seq_along(v)))

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

if you prefer a custom function:

fun = function(x) {
  nx = rev(seq_along(x))
  unlist(lapply(x, rep_len, x=nx))
}

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

CodePudding user response：

Using matrix, also works with lower bounds > 1

fun <- function(x){lx <- length(x); m <- matrix(x, nrow=lx, ncol=lx) 
                   m[upper.tri(m, diag=T)[, lx:1]]}

fun(4:1)
 [1] 4 3 2 1 4 3 2 4 3 4

fun(5:2)
 [1] 5 4 3 2 5 4 3 5 4 5

CodePudding user response：

fun <- function(x) {
  x |>
  length() |>
  seq() |>
  sapply(function(y) x[1:y]) |>
  rev() |>
  purrr::simplify()
}

fun(4:1)
#> [1] 4 3 2 1 4 3 2 4 3 4

CodePudding user response：

We can use head() iteratively on the vector:

foo = function(x) {
  lapply(rev(seq_along(x)), head, x = x) |> unlist()
}

foo(6:2)
# [1] 6 5 4 3 2 6 5 4 3 6 5 4 6 5 6
foo(4:1)
# [1] 4 3 2 1 4 3 2 4 3 4