[R] Function reference

[Tony Plate]

R does this sort of thing easily without any parse/eval acrobatics needed. 
  E.g., you can do:

 > stu <- function(x) {return( 1 + (2*x*x) - (3*x) )}
 > (x <- 0:3)
[1] 0 1 2 3
 > stu(x)
[1]  1  0  3 10
 > metafun <- function(FUN, data) FUN(data)
 > metafun(stu, x)
[1]  1  0  3 10
 > # if you want to be able use a character-data name for the function:
 > metafun2 <- function(FUN, data) {if (is.character(FUN)) FUN <- 
getFunction(FUN); FUN(data)}
 > metafun2("stu", x)
[1]  1  0  3 10
 > metafun2(stu, x)
[1]  1  0  3 10
 >

What went wrong with your code was that your parse() constructed a list of 
4 expressions, and evaluating that returned the value of the last one:

 > (fun <- "stu")
[1] "stu"
 > paste( fun, "(", x, ")", sep = "" )
[1] "stu(0)" "stu(1)" "stu(2)" "stu(3)"
 > parse( text = paste( fun, "(", x, ")", sep = "" ) )
expression(stu(0), stu(1), stu(2), stu(3))
attr(,"srcfile")
<text>
 >

(Others have observed that in a very large proportion of the situations 
where people reach for parse/eval, there's a neater, cleaner & more direct 
way of doing the job.)

-- Tony Plate

Talbot Katz wrote:
> Hi.
>  
> I'm looking for an R equivalent to something like function pointers in C/C++.  I have a search procedure that evaluates the fitness of each point it reaches as it moves along, and decides where to move next based on its fitness evaluation.  I want to be able to pass different fitness functions to this procedure.  I am trying to find a good way to do this.  I was thinking of passing in the name of the function and then using eval.  However, I haven't gotten this to work the way I'd like it to.  Consider the following example:
>  
>  
>> stu <- function(x) {return( 1 + (2*x*x) - (3*x) )}> (x=0:3)[1] 0 1 2 3> stu(x)[1]  1  0  3 10> (fun="stu")[1] "stu"> eval( parse( text = paste( fun, "(", x, ")", sep = "" ) ) )[1] 10>
>  
>  
> Notice that the function I defined called "stu" will operate on a vector x and return a vector y = stu(x) such that y[i] equals stu(x[i]).  When I tried to pass stu and x to a procedure that would evaluate stu(x) I only get stu(x[N]), when N is the last element of x.  What am I doing wrong?  Is there a better way to pass function references?
>  
> I can get the following to work, but it seems awfully clunky:
>  
>> sapply( 1:length(x), function(i){ return( eval( parse( text = paste( fun, "(", x[i], ")", sep = "" ) ) ) ) } )[1]  1  0  3 10>
>  
> Thanks!
>  
> --  TMK  --212-460-5430 home917-656-5351 cell
> 	[[alternative HTML version deleted]]
> 
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