[R] multicolumn sort on dataframe?

[Anne York]

On Fri 26 Mar 2004, Jeff D. Hamann wrote:
                                                                                
>I couldn't find any reference to this in the FAQ, but is it 
>possible to sort a  dataframe by multiple columns?
                                                                                
>I've created some code, similar to the following:
                                                                                
>nspr.code <- sp.results$sp.code[order( sp.results$sp.code )]
>nspr.tpa <- sp.results$tpa[order( sp.results$sp.code )]
                                                                                
>nspr.code <- as.character( levels( nspr.code ) )[nspr.code]
>nspr.tpa <- as.numeric( levels( nspr.tpa ) )[nspr.tpa]
                                                                                
>hope <- as.data.frame( cbind( nspr.code, as.numeric(nspr.tpa) ) )


A simple way to sort multiple columns is to paste them 
together and sort the resulting character vector. THat way 
you only have to do one sort. This is a very old method 
taught to me in the first computer course I ever took (date 
censored); the instructor attributed the method to Von 
Neumann but I have no reference. 

You have to be careful choosing the sep character in paste. 

Here is an example


> set.seed(78) 
> foo = data.frame(x= sample(letters[1:3],5,replace=TRUE),
                   y= sample(1:5,5,replace=TRUE))
> foo
  x y
1 c 3
2 c 2
3 b 2
4 c 1
5 c 3

Sorting on y and then by x: 

> my.order=order(foo.paste=paste(foo[,2],foo[,1],sep="/"))
> my.order
[1] 4 3 2 1 5


> my.order=order(paste(foo[,1],foo[,2],sep="/"))
> foo[my.order,]
  x y
3 b 2
4 c 1
2 c 2
1 c 3
5 c 3
>