General-purpose function for sorting data frame

[Kevin Wright <kwright@eskimo.com>]

One of the often asked questions on s-news and r-help relates to sorting
a data frame in ascending/descending order by multiple columns.

I have created a function that does the job very flexibly and in a way that
should be easy to remember.  For example, to sort the Oats data.frame (nlme
library) by nitro (decreasing) and Variety (increasing) :

sort.data.frame(Oats, ~ -nitro + Variety)

Feedback and improvements are welcome.

sort.data.frame <- function(form,dat){
  # Author: Kevin Wright
  # Some ideas from Andy Liaw
  #   http://tolstoy.newcastle.edu.au/R/help/04/07/1076.html

  # Use + for ascending, - for decending.  
  # Sorting is left to right in the formula
  
  # Useage is either of the following:
  # library(nlme); data(Oats)
  # sort.data.frame(~-Variety+Block,Oats) # Note: levels(Oats$Block)
  # sort.data.frame(Oats,~nitro-Variety)

  # If dat is the formula, then switch form and dat
  if(inherits(dat,"formula")){
    f=dat
    dat=form
    form=f
  }
  if(form[[1]] != "~")
    stop("Formula must be one-sided.")

  # Make the formula into character and remove spaces
  formc <- as.character(form[2]) 
  formc <- gsub(" ","",formc) 
  # If the first character is not + or -, add +
  if(!is.element(substring(formc,1,1),c("+","-")))
    formc <- paste("+",formc,sep="")

  # Extract the variables from the formula
  if(exists("is.R") && is.R()){
    vars <- unlist(strsplit(formc, "[\\+\\-]"))    
  }
  else{
    vars <- unlist(lapply(unpaste(formc,"-"),unpaste,"+"))
  }
  vars <- vars[vars!=""] # Remove spurious "" terms

  # Build a list of arguments to pass to "order" function
  calllist <- list()
  pos=1 # Position of + or -
  for(i in 1:length(vars)){
    varsign <- substring(formc,pos,pos)
    pos <- pos+1+nchar(vars[i])
    if(is.factor(dat[,vars[i]])){
      if(varsign=="-")
        calllist[[i]] <- -rank(dat[,vars[i]])
      else
        calllist[[i]] <- rank(dat[,vars[i]])
    }
    else {
      if(varsign=="-")
        calllist[[i]] <- -dat[,vars[i]]
      else
        calllist[[i]] <- dat[,vars[i]]
    }
  }
  dat[do.call("order",calllist),]

}