[R] regexpr and parsing question
ggrothendieck at gmail.com
Wed Jan 31 00:52:11 CET 2007
And here is an alternative to the regular expressions (although again
I don't think you really need any of this):
> capture.output(dput(strsplit("col1 col2 col3", " ")[]))
 "c(\"col1\", \"col2\", \"col3\")"
On 1/30/07, Gabor Grothendieck <ggrothendieck at gmail.com> wrote:
> Both spaces and tabs are whitespace so this
> should be good enough (unless you can
> have empty fields):
> read.table("myfile.dat", header = TRUE)
> See the sep= argument in ?read.table .
> Although I don't think you really need this, here are
> some regular expressions for processing a header
> into the form you asked for. The first line places
> quotes around the names, the second one inserts
> commas and the last one adds c( and ).
> s <- gsub('(\\S+)', '"\\1"', 'col1 col2 col3')
> s <- gsub("(\\S+) ", "\\1, ", s)
> sub("(.*)", "c(\\1)", s)
> On 1/30/07, Kimpel, Mark William <mkimpel at iupui.edu> wrote:
> > The main problem I am trying to solve it this:
> > I am importing a tab delimited file whose first line contains only one
> > column, which is a descriptor of the form "col_1 col_2 col_3", i.e. the
> > colnames are not tab delineated but are separated by whitespace. I would
> > like to parse this first line and make such that it becomes the colnames
> > of the rest of the file, which I am reading into R using read.delim().
> > The file is so huge that I must do this in R.
> > My first question is this: What is the best way to accomplish what I
> > want to do?
> > My other questions revolve around some failed attempts on my part to
> > solve the problem on my own using regular expressions. I thought that
> > perhaps I could change the first line to "c("col_1", "col_2", "col_3")
> > using gsub. I was having trouble figuring out how R uses the backslash
> > character because I know that sometimes the backslash one would use in
> > Perl needs to be a double backslash in R.
> > Here is a sample of what I tried and what I got:
> > a<-"col_1 col_2 col_3"
> > > gsub("\\s", " " , a)
> >  "col_1 col_2 col_3"
> > > gsub("\\s", "\\s" , a)
> >  "col_1scol_2scol_3"
> > As you can see, it looks like R is taking a regular expression for
> > "pattern", but not taking it for "replacement". Why is this?
> > Assuming that I did want to solve my original problem with gsub and then
> > turn the string into an R object, how would I get gsub to return
> > "c("col_1", "col_2", "col_3") using my original string?
> > Finally, is there a way to declare a string as a regular expression so
> > that R sees it the same way other languages, such as Perl do, i.e. make
> > the backslash be interpreted the same way? For someone who is just
> > learning regular expressions as I am, it is very frustrating to read
> > about them in references and then have to translate what I've learned
> > into R syntax. I was thinking that instead of enclosing the string in
> > "", one could use THIS.IS.A.REGULAR.EXPRESSION(), similar to the way we
> > use I() in formulae.
> > These are a bunch of questions, but obviously I have a lot to learn!
> > Thanks,
> > Mark
> > Mark W. Kimpel MD
> > (317) 490-5129 Work, & Mobile
> > (317) 663-0513 Home (no voice mail please)
> > 1-(317)-536-2730 FAX
> > ______________________________________________
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