SQL-like decode statement for R

R_SQL_Decode.r

decode <- function(x, ...) {

  odds <- function(x) { unlist(x[1:length(x) %% 2 == 1][1:floor(length(x)/2)]) }
  even <- function(x) { unlist(x[1:length(x) %% 2 == 0]) }
  last <- function(x) { unlist(if(length(x) %% 2 == 1) tail(x,1)) }
  
  interpret_args <- function(x) { if(is.call(x)) {eval(x)} else if(is.name(x)) {as.character(x)} else {x} }
  
  args <- eval(substitute(alist(...)))
  args <- lapply(args, interpret_args)
  
  targets      <- odds(args)
  replacements <- even(args)
  default      <- last(args)
  
  res <- x
  
  if(!is.null(default))
    res[! x %in% targets & ! is.na(x)] <- default

  for(i in seq_along(targets)) {
    t <- targets[[i]];  r <- replacements[[i]]
    res[res == t | (is.na(t) & is.na(res))] <- r    
  }
  
  if(inherits(x, "factor"))
    res <- as.factor(res)
  
  res
}