[R] eval(parse(text vs. get when accessing a function

[Bert Gunter]

??

Or to add to what Peter Dalgaard said... (perhaps for the case of many more
functions)

Why eval(parse())? What's wrong with if then? 

g <- function(fpost,x){if(fpost==1)f.1 else f.2 }(x)

or switch() if you have more than 2 possible arguments? I think your remarks
reinforce the wisdom of Thomas's "axiom" . 

Bert Gunter
Genentech Nonclinical Statistics
South San Francisco, CA 94404


-----Original Message-----
From: r-help-bounces at stat.math.ethz.ch
[mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Ramon Diaz-Uriarte
Sent: Friday, January 05, 2007 10:02 AM
To: r-help; rdiaz02 at gmail.com
Subject: [R] eval(parse(text vs. get when accessing a function

Dear All,

I've read Thomas Lumley's fortune "If the answer is parse() you should
usually 
rethink the question.". But I am not sure it that also applies (and why) to 
other situations (Lumley's comment 
http://tolstoy.newcastle.edu.au/R/help/05/02/12204.html
was in reply to accessing a list).

Suppose I have similarly called functions, except for a postfix. E.g.

f.1 <- function(x) {x + 1}
f.2 <- function(x) {x + 2}

And sometimes I want to call f.1 and some other times f.2 inside another 
function. I can either do:

g <- function(x, fpost) {
    calledf <- eval(parse(text = paste("f.", fpost, sep = "")))
    calledf(x)
    ## do more stuff
}


Or:

h <- function(x, fpost) {
    calledf <- get(paste("f.", fpost, sep = ""))
    calledf(x)
    ## do more stuff
}


Two questions:
1) Why is the second better? 

2) By changing g or h I could use "do.call" instead; why would that be
better? 
Because I can handle differences in argument lists?



Thanks,


R.



-- 
Ramón Díaz-Uriarte
Centro Nacional de Investigaciones Oncológicas (CNIO)
(Spanish National Cancer Center)
Melchor Fernández Almagro, 3
28029 Madrid (Spain)
Fax: +-34-91-224-6972
Phone: +-34-91-224-6900

http://ligarto.org/rdiaz
PGP KeyID: 0xE89B3462
(http://ligarto.org/rdiaz/0xE89B3462.asc)



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