[R] Coding problem: How can I extract substring of function call within the function

[Erik Iverson]

Are you sure you just don't want to tell them about the :: operator?  It sounds easier than what you're proposing. 

E.g.,

base::mean(c(1:10, NA))

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf Of Pitt, Joel
Sent: Thursday, August 13, 2009 3:48 PM
To: r-help at r-project.org
Subject: [R] Coding problem: How can I extract substring of function call within the function

In order to ease my students into the R environment I am developing a package which installs a variety of utility functions as well as slightly modified versions of some standard R functions -- e.g. mean, hist, barplot, .... In my versions of these standard R functions I either add options or alter some defaults that seem to create difficulties for most of my students -- for example, when they do barcharts for two dimensional tables they generally want the bars to be side-by-side and stumble over the standard default of besides=F, and my version of mean by default reports a mean value in the presence of NA's after warning of that presence (but retains the option of setting na.rm=F). (I don't doubt that some (if not many) of you will doubt the wisdom of this, and I would be happy to discuss this in more detail on other occasions.) You might want to think of my replacement R functions as a kind of "training wheels" for R, and, in the spirit of training wheels I include a funct!
 ion in my package that allows a user to revert to the standard version of one or all functions without unloading the package (and loosing its additional functionality). However, I want to add a function that allows a user to revert to running the standard R version of a given function on a one-off basis and that's where my problem comes up.
 
I believe that it should be possible to write a function rStd with the usage rStd(x,...) where x is a function -- e.g. mean, hist, barchart, and the remaining parameters would be any of the parameters that should be passed to the unmodified version of mean, hist, barchart... The problem I have is how to get ahold of that collection of parameters as a single character string. Now I know that sys.calls()[[1]] will give me the full text of the initial call, but the problem is to detach the ... above from that as a text string. If I could do that I'd be done.
 
Here's the incomplete code with comments -- see the gap set off by astericks. 
 
    rStd=function(x,...){
    if(missing(x))  # must have a specified function
    {
       cat("Error: No function specified\n");
       return(invisible(NULL));
    }
    z=as.character(substitute(x));
    # must include code here to check that z is the name
    # of one of our altered functions
    # if z is an altered function, e.g., "mean"
    # then concatenating "x" with z gives the overlaid
    # function -- e.g. xmean is the standard mean
    #***************************************************************************
    # Now we need to get a hold of the ... text                                  *
    #                                                                                             *
    w=sys.calls()[[1]];  # this gets me the whole text of the call          *
    #                                                                                             *
    # and now here's where the problem arises                                 *
    # how to I get the ... text if I could get it and say                          *
    # assign it to the variable params                                               *
    # I could then set                                                                      *
    #                                                                                             *
    cmd=sprintf("x%s(%s)",z,params) # see remarks above about z
   # and then it's done....
    eval(parse(text=cmd),sys.frame());
}
 
 
Any help would be much appreciated.
 
Regards
Joel
Joel Pitt, Ph.D.
Associate Professor & Chair
Mathematics & Computer Science
Georgian Court University
900 Lakewood Avenue
Lakewood, NJ 08540
732-987-2322

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