[R] character strings with embedded commands: perl "/gee" ?

[ivo welch]

joshua wiley told me how to do this.  so, here is something I find quite useful:



estring <- function (e2)
{
  syntax <- regmatches(e2, gregexpr("(?<=\\{\\{).*?(?=\\}\\})", e2,
perl=TRUE))[[1]]
  res <- lapply(syntax, function(p) eval(parse(text=p)))
  res <- lapply(1:length(res), function(i) {
    e2 <<- gsub(paste0("{{", syntax[i], "}}"), res[i], e2, fixed=TRUE)
  })
  e2
}

abort.estring <- function(s) abort(estring(s))

abort <- function (...) {
   cat(..., file=stderr)
   stop(simpleError("Aborting --- use options(error=recover) and
traceback() for debugging\n")
}
die <- abort

`%or%` <- function(e1, e2) { if (!e1) { if (is.character(e2))
abort.estring(e2) else eval(e2) }}
`%and%` <- function(e1, e2) { if (e1) { if (is.character(e2))
abort.estring(e2) else eval(e2) }}


together, this is useful for such constructs as

f <- function(a1) {
   (is.data.frame(a1)) %or% "a1 is not a data frame but a {{class(a1)}}"
   (exists("column", a1)) %or% "sorry, but column named 'column' does
not exist.  choose from {{names(a1)}} instead"
   (is.numeric(a1$column)) %or% "column a1 is not numeric"
}

I also find it useful to have an optional length argument for
is.numeric or is.character, which requires the argument to be of
length x, but this is another story.

/iaw






On Fri, Feb 8, 2013 at 4:01 PM, ivo welch <ivo.welch at anderson.ucla.edu> wrote:
> dear R experts---I am trying to replicate a perl feature. I want to be
> able to embed R commands inside a character string, and have the
> string be printed with the command executed.  my perl equivalent is
>
>   my $a=10;
>   my $teststring = "the expression, $a+1, is ::$a+1::, but add one
> more for ::$a+2::\n";
>   $teststring =~ s/::(.*?)::/$1/gee;
>   print $teststring;
>
> of course, R does not use '$' for variable names.  my ultimate goal is
> to write something like
>
>   cat("d is a ::class(d):: with names ::names(d)::")
>
> of course, I know I can write this as cat("d is a", class(d), "with
> names", names(d)), but I also want to be define %or% so that I can
> write
>
>   (is.data.frame(d)) %or% "d is a ::class(d):: with names ::names(d)::" ;
>
> operators don't take variable arguments afaik.  :-(.
>
> advice appreciated.
>
> regards,
>
> /iaw
> ----
> Ivo Welch (ivo.welch at gmail.com)