[R] sorting a data.frame using a vector

[Peter Dalgaard]

"Liaw, Andy" <andy_liaw at merck.com> writes:

> > > 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

Yes, the internals are using  

   pmatch(i, rows, duplicates.ok = TRUE)

and pmatch() is a horrible lot slower than match(). Anyone for a spot
of hardcore optimization?

(Partial matching of character indices is a feature long regretted by
its inventors, but every time we consider killing it, we tend to recoil
in horror upon realizing what it would break...) 

-- 
   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