[R] Again: Variable names in functions

[Uwe Ligges]

Heinz Tuechler wrote:

> Hello,
> 
> still I have difficulties with variable names in functions. I know the
> famous example form help for deparse/substitute but I will give a simpler
> one to explain my problem.
> I know from Reid Huntsinger (Tue, 8 Feb 2005 12:39:32 -0500) that:
> "Semantically, R is pass-by-value, so you don't really have the names, just
> the values. In implementation, though, R *does* pass names, in part at least
> in order to do "lazy evaluation". You can get them via "substitute" ; see
> the help for that." 
> 
> The output of several functions does not make much sense, if these names do
> not appear (e.g. parameter estimates in Cox-regression, ...)
> Only to give a trivial example I show my problem with the table function.
> 
> As you know, if I call table as follows, the output is labelled properly.
> 
>>charly<-c(rep(1,3),rep(2,7));delta<-c(rep(1:2,5))
>>table(charly, delta)
> 
>       delta
> charly 1 2
>      1 2 1
>      2 3 4
> If I define a trivial function to call table, the output is less satisfying.
> (Of course, I know that this function is useless.)
> 
>>mytable1<-function(x,y){table(x,y)}
>>mytable1(charly, delta)
> 
>    y
> x   1 2
>   1 2 1
>   2 3 4
> If I define the function in the following way, it does what I wish, namely
> it returns output equivalent to the simple call "table(charly, delta)".
> 
>>mytable2<-function(x,y){
> 
> +   cat("table(",as.symbol((deparse(substitute(x)))),
> +   "," ,  as.symbol(deparse(substitute(y))),")\n",
> +   file="temp",sep="",append=F)
> +   eval(parse("temp",n=-1))
> +                       }
> 
>>mytable2(charly, delta)

See argument "dnn" in ?table:


   mytable2 <- function(x,y){
     table(x, y, dnn = c(deparse(substitute(x)),
                         deparse(substitute(y))))
   }

Uwe Ligges



>       delta
> charly 1 2
>      1 2 1
>      2 3 4
> 
> I assume that there is a better way to solve this problem and I would be
> happy about hints, where to find solutions in the documentation.
> 
> Thanks,
> 
> Heinz Tüchler
> 
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