cond.R

#' Chained Conditional Element Selection
#'
#' \code{cond} takes even number of arguments representing a series of
#' \code{test}/\code{value} pairs, and returns a value with the same shape as
#' every \code{test}. The returned element is filled with the elements selected
#' from every \code{value} depending on whether its corresponding \code{test} is
#' \code{TRUE} or \code{FALSE}.
#'
#' @param ... An even number of \code{test}/\code{value} pairs. Every \code{test}
#'   is coerced to logical mode. \code{test} arguments should be the same shape.
#' @param default Atomic value to use if no \code{test} was satisfied, or \code{NA} if not specified.
#' @examples
#' chars <- letters[1:5]
#' cond(chars %in% c("a", "c"),
#'      as.character(1:5),
#'      chars == "e",
#'      "X",
#'      default = chars)
#' @seealso \code{\link{ifelse}}
cond <- function(..., default = NA, env = parent.frame()) {
  args <- eval(substitute(alist(...)))

  if (length(args) == 0) {
    return(default)
  } else if (length(args) %% 2 != 0) {
    stop("cond requires an even number of arguments")
  }

  test <- as.logical(eval(args[[1]], envir = env))
  ans  <- test
  ok   <- !(nas <- is.na(test))

  if (any(test[ok]))
    ans[test & ok] <- rep(eval(args[[2]], envir = env),
                          length.out = length(ans))[test & ok]

  if (any(!test[ok]))
    ans[!test & ok] <- do.call(cond, c(args[-1:-2], list(default = default, env = env)))[!test & ok]

  ans[nas] <- NA
  ans
}

Note: a control-flow cond can be found here: https://gist.github.com/DASpringate/8595464

Cool stuff. I used it in my work. Also I just learned that dplyr has the same (?) functionality with dplyr::case_when() https://www.rdocumentation.org/packages/dplyr/versions/0.5.0/topics/case_when