[R] sorting a data.frame using a vector

[Liaw, Andy]

> From: Peter Dalgaard
> 
> Toralf Kirsten <tkirsten at izbi.uni-leipzig.de> writes:
> 
> > Hi all,
> > I'm looking for an efficient solution (speed and memory) for the
> > following problem:
> > Given
> > - a data.frame x containing numbers of type double
> >    with nrow(x)>ncol(x) and unique row lables and
> > - a character vector y containing a sorted order labels
> > 
> > Now, I'd like to sort the rows of the data.frame x w.r.t. 
> the order of
> > labels in y.
> > 
> > example:
> > x <- data.frame(c(1:4),c(5:8))
> > row.names(x)<-LETTERS[1:4]
> > y <- c("C","A","D","B")
> > 
> > 
> > My current solution is like this:
> > if(!is.null(y) && is.vector(y)) {
> >     nObj <- length(y)
> >     for (i in 1:nObj) {
> >       sObj <- y[i]
> >       k <- c(1:nrow(x))[row.names(x)==sObj]
> >       if (i != k) {
> >         names <- row.names(x)
> >         tObj <- row.names(x[i,])
> >         temp <- x[i,]
> >         x[i,] <- x[k,]
> >         x[k,] <- temp
> >         names[i] <- sObj
> >         names[k] <- tObj
> >         row.names(x) <- names
> >      }
> >    }
> > }
> > 
> > But I'm not happy with it because it is not really efficient. Any
> > other suggestions are welcome!
> 
> Anything wrong with x[y,] ???

Well... sometimes:

> nm <- as.character(sample(1:1e5))
> x <- data.frame(x1=rnorm(1e5), row.names=1:1e5)
> system.time(x[nm, , drop=FALSE], gcFirst=TRUE)
[1] 155.13   0.01 156.10     NA     NA
> system.time(x2<-x[match(nm, rownames(x)), , drop=FALSE], gcFirst=TRUE)
[1] 0.37 0.00 0.37   NA   NA
> all(rownames(x2) == nm)
[1] TRUE
> R.version
         _              
platform i386-pc-mingw32
arch     i386           
os       mingw32        
system   i386, mingw32  
status                  
major    2              
minor    0.1            
year     2004           
month    11             
day      15             
language R              

Cheers,
Andy 
 
> -- 
>    O__  ---- Peter Dalgaard             Blegdamsvej 3  
>   c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
>  (*) \(*) -- University of Copenhagen   Denmark      Ph: 
> (+45) 35327918
> ~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)             FAX: 
> (+45) 35327907
> 
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