Thanks for you contributions. Jonnes' solution (after sorting) works fine
for my purposes but it would be useful to have a function that works for any
numeric prefix. Another case to include would be a signed numeric:
x<-c("+12.3.abc", "-0.12xyz")
Mike
----- Original Message -----
From: "Peter Dalgaard" <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Cc: "R user" <[EMAIL PROTECTED]>; <R-help@stat.math.ethz.ch>; "Mike White"
<[EMAIL PROTECTED]>
Sent: Monday, January 31, 2005 11:05 PM
Subject: Re: [R] Extracting a numeric prefix from a string
> (Ted Harding) <[EMAIL PROTECTED]> writes:
>
> > On 31-Jan-05 R user wrote:
> > > You could use something like
> > >
> > > y <- gsub('([0-9]+(.[0-9]+)?)?.*','\\1',x)
> > > as.numeric(y)
> > >
> > > But maybe there's a much nicer way.
> > >
> > > Jonne.
> >
> > I doubt it -- full marks for neat regexp footwork!
>
> Hmm, I'd have to deduct a few points for forgetting to escape the dot...
>
> > x <- "2a4"
> > y <- gsub('([0-9]+(.[0-9]+)?)?.*','\\1',x)
> > y
> [1] "2a4"
> > as.numeric(y)
> [1] NA
> Warning message:
> NAs introduced by coercion
>
> and maybe a few more for using gsub() where sub() suffices.
>
> There are a few more nits to pick, since "2.", ".2", "2e-7" are also
> numbers, but ".", ".e-2" are not. In fact it seems quite hard even to
> handle all cases in, e.g.,
>
> x <- c("2.2abc","2.def",".2ghi",".jkl")
>
> with a single regular expression. The first one that worked for me was
>
> > r <- regexpr('^(([0-9]+\\.?)|(\\.[0-9]+)|([0-9]+\\.[0-9]+))',x)
> > substr(x,r,r+attr(r,"match.length")-1)
> [1] "2.2" "2." ".2" ""
>
> but several "obvious" attempts had failed.
>
> The problem is that regular expressions try to find the
> longest match, but not necessary of subexpressions, so
>
> > sub('(([0-9]+\\.?)|(\\.[0-9]+)|([0-9]+\\.[0-9]+))?.*','\\1',x)
> [1] "2." "2." ".2" ""
>
> even though
>
> > sub('(([0-9]+\\.?)|(\\.[0-9]+)|([0-9]+\\.[0-9]+))','XXX',x)
> [1] "XXXabc" "XXXdef" "XXXghi" ".jkl"
>
> Actually, this one comes pretty close:
>
> > sub('([0-9]*(\\.[0-9]+)?)?.*','\\1',x)
> [1] "2.2" "2" ".2" ""
>
> It only loses a trailing dot which is immaterial in the present
> context. However, next try extending the RE to handle an exponent
> part...
>
> --
> O__ ---- Peter Dalgaard Blegdamsvej 3
> c/ /'_ --- Dept. of Biostatistics 2200 Cph. N
> (*) \(*) -- University of Copenhagen Denmark Ph: (+45) 35327918
> ~~~~~~~~~~ - ([EMAIL PROTECTED]) FAX: (+45) 35327907
>
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