R help - Mar 2005 - Search and convert string function

Hi all,
I want to do this kind of function In R enviroment :
For example :
R <- 4
testString <- "I love $R"
then search this testString, when find "$R",replace "$R" to
R ,and because
the value of R is 4
So the final string I want to get is "I love 4"
How can I implement? Thanks advance

Michael

peng shen <michael_shen <at> hotmail.com> writes:

: 
: Hi all,
: I want to do this kind of function In R enviroment :
: For example :
: R <- 4
: testString <- "I love $R"
: then search this testString, when find "$R",replace "$R"
to R ,and because
: the value of R is 4
: So the final string I want to get is "I love 4"
: How can I implement? Thanks advance
: 

Here is one way to do string interpolation:

R> interp <- function(x, e = parent.frame(), pre = "\\$", post =
"" ) {
+    for(el in ls(e)) {
+       tag <- paste(pre, el, post, sep = "")
+       if (length(grep(tag, x))) x <- gsub(tag, eval(parse(text = el), e),
x)
+    }
+    x
+ }
 
R> # a test
R> R <- 4
R> x <- "I love $R"
R> interp(x)
[1] "I love 4"
 
R> # another test
R> y <- "I love ${R}"
R> interp(y, pre = "\\${", post = "}")
[1] "I love 4"

On Sun, 6 Mar 2005, peng shen wrote:
> Hi all,
> I want to do this kind of function In R enviroment :
> For example :
> R <- 4
> testString <- "I love $R"
> then search this testString, when find "$R",replace
"$R" to R ,and because
> the value of R is 4
> So the final string I want to get is "I love 4"
> How can I implement? Thanks advance
sub("$R", R, testString, fixed = TRUE)


-- 
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

peng shen [mailto:michael_shen at hotmail.com] wrote:
> R <- 4
> testString <- "I love $R"
> So the final string I want to get is "I love 4".  How can I
implement?
I've written an interpolater function "g.p" with these additional
features:
 - Loops through all occurences of the escape character ($) rather than
all
   external variables.
 - Variable names are terminated by any non-alphanumeric char or a
"silent" $$.
 - Variables may come from the parent environment or from named
arguments.
 - Pastes together its unnamed arguments with sep="", useful for long
strings.

Example:
R> R <- 4
R> testString <- "I love $R"
R> g.p(testString)
   [1] "I love 4"

Fancier example:
R> var1 <- 7
R> var2 <- 5
R> g.p("Add $var1 to $var2 to calc",
       "ulate $var3",
       var3=var1+var2)
   [1] "Add 7 to 5 to calculate 12"

Function definition:
g.p <- function(..., esc="\\$", sep="", collapse="
", parent=1) {
  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 <- if (v=="") paste(substring(s,1,r-1), substring(s,r+2),
sep="")
else
         gsub(paste(esc,v,sep=""),
              as.character(eval.parent(parse(text=v), parent)), s)
  }
  s
}

-- David Brahm (brahm at alum.mit.edu)