R help - Apr 2011 - replace an expression with its value, or read macros

I know my subject line seems odd, but I want to replace an expression—such as a
variable—with its value. For example, I want to paste() some strings together,
assign the result to a variable s, then use the value of s as a variable to hold
another value. Something like this:

import_subjects <- function (start, iterations) {
for (i in 1:iterations) { # iterate from start to end
s <- paste("s", i, sep="") # make the data.frame's
name
fn <- paste(i, ".txt", sep="") # make the filename
s <- read.delim(fn, header = FALSE) # now put it all together
# …except that I don't want to reassign s, I want to evaluate it and make
its value a variable with its own value.
}
}

If this were Lisp I'd use backquotes (or "backticks", as some say)
and commas—examples of read macros—to do this, but I can't find anything
analogous for R. How might I accomplish this in R?

Sincerely,

Frank Tamborello, PhD
W. M. Keck Postdoctoral Fellow
School of Biomedical Informatics
University of Texas Health Science Center at Houston


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On Fri, Apr 8, 2011 at 11:45 AM, Franklin Tamborello II
<Franklin.Tamborello at uth.tmc.edu> wrote:
> I know my subject line seems odd, but I want to replace an expression?such
as a variable?with its value. For example, I want to paste() some strings
together, assign the result to a variable s, then use the value of s as a
variable to hold another value. Something like this:
>
> import_subjects <- function (start, iterations) {
> for (i in 1:iterations) { # iterate from start to end
> s <- paste("s", i, sep="") # make the
data.frame's name
> fn <- paste(i, ".txt", sep="") # make the filename
> s <- read.delim(fn, header = FALSE) # now put it all together
> # ?except that I don't want to reassign s, I want to evaluate it and
make its value a variable with its own value.
> }
> }
>
> If this were Lisp I'd use backquotes (or "backticks", as some
say) and commas?examples of read macros?to do this, but I can't find
anything analogous for R. >How might I accomplish this in R?
As a first note, most people who ask this sort of question would be
better off putting the sets of data into a single list rather than
named variables.

However, it is possible to do what you want.  One approach is to use
assign().  Another is to use bquote(), which is related to the Lisp
backquote macro: eval(bquote(.(s) <- read.delim(.(fn),header=FALSE)),
where .() is analogous to comma.

      -thomas

-- 
Thomas Lumley
Professor of Biostatistics
University of Auckland

On Thu, Apr 7, 2011 at 7:45 PM, Franklin Tamborello II
<Franklin.Tamborello at uth.tmc.edu> wrote:
> I know my subject line seems odd, but I want to replace an expression?such
as a variable?with its value. For example, I want to paste() some strings
together, assign the result to a variable s, then use the value of s as a
variable to hold another value. Something like this:
>
> import_subjects <- function (start, iterations) {
> for (i in 1:iterations) { # iterate from start to end
> s <- paste("s", i, sep="") # make the
data.frame's name
> fn <- paste(i, ".txt", sep="") # make the filename
> s <- read.delim(fn, header = FALSE) # now put it all together
> # ?except that I don't want to reassign s, I want to evaluate it and
make its value a variable with its own value.
> }
> }
>
> If this were Lisp I'd use backquotes (or "backticks", as some
say) and commas?examples of read macros?to do this, but I can't find
anything analogous for R. How might I accomplish this in R?
>
Here are a few possibilities:

# 1 - assign
assign(s, read.delim(fn, header = FALSE)

# 2 - .GlobalEnv
.GlobalEnv[[s]] <- read.delim(fn, header = FALSE)

# 3 - gsubfn's fn$ supports backticks
# Here we need gsubfn::fn to disambiguate the two fn's
library(gsubfn)
eval(gsubfn::fn$parse(text = " `s` <- read.delim(fn, header = FALSE)
"))

-- 
Statistics & Software Consulting
GKX Group, GKX Associates Inc.
tel: 1-877-GKX-GROUP
email: ggrothendieck at gmail.com