Splitting the string at the last sub-string

Prof Brian Ripley wrote:

substring(str, c(1, 26), c(25,length(str)))
 nchar(str) surely?
Yes, or anything larger:  I actually tested 10000.
 regexps can be rather slow though. Here's two functions:
But that's not the way to do this repeatedly for the same pattern. (It is 
normally compiling regexps that is slow, and regexpr is vectorized.) Not 
that I would call 300us `slow'.
byRipley =
function(str,sub){
  lp=attr(regexpr(paste(".*",sub,sep=""),str),'match.length')
  return(substring(str, c(1, lp+1), c(lp,nchar(str))))
}

byJarek =
function(str,sub){
  y = unlist(strsplit(str,sub))
  return(cbind(paste(y[-length(y)], sub,  sep="", collapse = ""),
y[length(y)]))
}

 and a quick test:

system.time(for(i in 1:100000){byJarek(str,sub)})
[1] 15.55  0.10 16.06  0.00  0.00

system.time(for(i in 1:100000){byRipley(str,sub)})
[1] 30.28  0.07 31.86  0.00  0.00

Baz

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Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
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Splitting the string at the last sub-string

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