[R] Function sort.data.frame

[Kevin Wright]

I can never remember how to use "order" to sort the rows of a data frame,
so like any good, lazy programmer, I decided to write my own function.

The idea is to specify a data.frame and a one-sided formula with +/- 
indicating ascending/descending.  For example:
  sort.data.frame(~ +nitro -Variety, Oats)

Since sorting of a data.frame is an oft-asked question on this list, I am
posting my function in hopes that others may find it useful.  

Computing 'on the language' (formulas) is not my strongest point, so the
function can probably be improved.  A similar idea could be used for
matrix objects.  Feedback is welcome.

Kevin Wright




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:
  # sort.data.frame(~Block-Variety,Oats)
  # sort.data.frame(Oats,~-Variety+Block)
  
  # 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
  vars <- unlist(strsplit(formc, "[\\+\\-]"))
  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),]

}


d = data.frame(b=factor(c("Hi","Med","Hi","Low"),levels=c("Low","Med","Hi"),
               ordered=TRUE),
               x=c("A","D","A","C"),y=c(8,3,9,9),z=c(1,1,1,2))
sort.data.frame(~-z-b,d)
sort.data.frame(~x+y+z,d)
sort.data.frame(~-x+y+z,d)
sort.data.frame(d,~x-y+z)