[R] Splitting the string at the last sub-string

[Prof Brian Ripley]

On Thu, 15 Sep 2005, Barry Rowlingson wrote:

> 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)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595