Numbers in a string

In S+ strsplit() has a keep=TRUE/FALSE argument to
specify whether to return the substrings that match
the pattern or to return the substrings between
matches to the pattern (the default).  E.g.,
strings <- c("abcde. 11 abc 5.31e+34, (1.45)",
"AB15E9SDF654VKBN?dvb.65")
number.pattern <- "[0-9]+\\.[0-9]+e[+-][0-9]+|[0-9]+\\.[0-9]+|[0-9]+"
strsplit(strings, number.pattern, keep=TRUE)
[[1]]:
[1] "11"       "5.31e+34" "1.45"    

[[2]]:
[1] "15"  "9"   "654" "65"
strsplit(strings, number.pattern, keep=FALSE)
[[1]]:
[1] "abcde. " " abc "   ", ("     ")"      

[[2]]:
[1] "AB"        "E"         "SDF"       "VKBN?dvb."

In R and S+ gregexpr can tell you the start points
and lengths of each match, but it is a pain to
pass this information to substring() to get the
matches themselves.  Should [g]regexpr() have a
value= argument like grep has?

In R the gsubfn package can do this sort of thing.
I don't know if it worth adding more to base R's
strsplit().

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com
-----Original Message-----
From: r-help-bounces at r-project.org 
[mailto:r-help-bounces at r-project.org] On Behalf Of Petr Savicky
Sent: Thursday, December 16, 2010 8:42 AM
To: r-help at r-project.org
Subject: Re: [R] Numbers in a string

On Thu, Dec 16, 2010 at 06:17:45AM -0800, Dieter Menne wrote:
Petr Savicky wrote:
One of the suggestions in this thread was to use an 
external program.
A possible solution without negation in Perl is

  @a = ("AB15E9SDF654VKBN?dvb.65" =~ m/[0-9]/g);
  print @a, "\n";
  15965465

Which is

 gsub("[^0-9]", "", "AB15E9SDF654VKBN?dvb.65")

as Henrique suggested.
I agree. The Perl code was a reply to a question, whether the 
same can be
done by describing the required elements and not by 
describing the ones to
be removed. This could be useful, if we want to extract 
elements described
by a more complex regular expression. A more accurate, although not
complete and definitely not the best, extraction of 
nonnegative numbers
in Perl may be done as follows

  @a = ("abcde. 11 abc 5.31e+34, (1.45)" =~ 
m/[0-9]+\.[0-9]+e[+-][0-9]+|[0-9]+\.[0-9]+|[0-9]+/g);
  print join(" ", @a), "\n";
  11 5.31e+34 1.45

Can something similar be done in R either specifically for numbers or
for a general regular expression?

Going back to the original question, the answer depends on 
the complexity of
extracting numbers in a concrete situation. If possible, 
using functions
within R is suggested (gsub(), strsplit(), ...). On the other 
hand, there
are cases, where an external tool can be helpful. See also R-intro
Chapter 7 Reading data from files, which says

  There is a clear presumption by the designers of R that you will be
  able to modify your input files using other tools, such as 
file editors
  or Perl to fit in with the requirements of R.

Petr Savicky.

______________________________________________
R-help at r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide 
http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.

Numbers in a string

Thread (14 messages)