William M. Fitchen <William_Fitchen at oxy.com> wrote on 12/19/01:
> I am new to R and am coming from a Perl background. I have had trouble
> figuring out from the documentation how to interpolate a variable into a
> quoted string (if it's possible).... In perl I could write:
> $name = "John";
> print STDOUT "Hi $name, how's it going";
> which would output the following:
> Hi John, how's it going?
I just wrote such a function; here is its current form:
g.p <- function(..., esc="\\$", sep="", collapse="
") {
a <- lapply(list(...), as.character)
n <- names(a); if (is.null(n)) n <- rep("", length(a))
s <- do.call("paste", c(a[n==""], sep=sep,
collapse=collapse))
for (i in which(n != "")) s <-
gsub(paste(esc,n[i],sep=""), a[[i]], s)
while ((r <- regexpr(paste(esc,"\\w*",sep=""), s)) >
0) {
v <- substring(s, r+1, r+attr(r,"match.length")-1)
s <- gsub(paste(esc,v,sep=""),
as.character(eval(parse(text=v))), s)
}
s
}
It does three things:
1) Concatenates all unnamed arguments (with sep="" by default);
2) Uses any named arguments such as var1=7 to interpolate $var1 in the string;
3) Interpolates any remaining $var2 constructions using all visible variables.
Note I didn't allow "." in variable names, for consistency with
Perl.
William's example can be accomplished with either:
R> g.p("Hi $name, how's it going", name="John")
or:
R> name <- "John"
R> g.p("Hi $name, how's it going")
Comments welcome!
--
-- David Brahm (brahm at alum.mit.edu)
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