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

Ramon Diaz-Uriarte rdiaz at cnio.es
Fri Jan 5 19:02:07 CET 2007

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 
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 = "")))
    ## do more stuff


h <- function(x, fpost) {
    calledf <- get(paste("f.", fpost, sep = ""))
    ## 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?



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

PGP KeyID: 0xE89B3462

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