Implement lag() and lead()

[msberends]

Getting more and more enthusiastic about this package 😃

For lag() and lead(), also used often in data wrangling:

lag <- function (x, n = 1L, default = NA, ...) {
  if (length(n) != 1 || !is.numeric(n) || n < 0) {
    stop("n must be a nonnegative integer scalar")
  }
  if (n == 0) {
    return(x)
  }
  xlen <- length(x)
  n <- pmin(n, xlen)
  out <- c(rep(default, n), x[seq_len(xlen - n)])
  attributes(out) <- attributes(x)
  out
}

lead <- function (x, n = 1L, default = NA, ...) {
  if (length(n) != 1 || !is.numeric(n) || n < 0) {
    stop("n must be a nonnegative integer scalar")
  }
  if (n == 0) {
    return(x)
  }
  xlen <- length(x)
  n <- pmin(n, xlen)
  out <- c(x[-seq_len(n)], rep(default, n))
  attributes(out) <- attributes(x)
  out
}

Reprex:

test <- factor(letters[1:10])

lag(test)
#> [1] <NA> a    b    c    d    e    f    g    h    i   
#> Levels: a b c d e f g h i j

lead(test)
#> [1] b    c    d    e    f    g    h    i    j    <NA>
#> Levels: a b c d e f g h i j

identical(dplyr::lag(test), lag(test))
#> [1] TRUE

identical(dplyr::lead(test), lead(test))
#> [1] TRUE