[R] Programming/scripting with "expressions - variables"

[Frank E Harrell Jr]

Gorjanc Gregor wrote:
> Hello to Rusers!
> 
> I am puzzled with R and I really do not know where to look
> in for my problem. I am moving from SAS and I have difficulties
> in translating SAS to R world. I hope I will get some hints 
> or pointers so I can study from there on.
> 
> I would like to do something like this. In SAS I can write 
> a macro as example bellow, which is afcourse a silly one but 
> shows what I don't know how to do in R.
> 
> %macro test(data, colname, colvalue);
>     data &data;
>         ...
>         &colname="&colvalue";
>         other_&colname="other_&colvalue";
>     run;
> %mend;
> 
> And if I run it with this call:
> %test(Gregor, Gorjanc, 25);
> 
> I get a table with name 'Gregor' and columns 'Gorjanc', 
> and 'other_Gorjanc' with values:
> 
> Gorjanc other_Gorjanc
> "25"    "other_25"
> 
> So can one show me the way to do the same thing in R? 
> 
> Thanks!
> 
> --
> Lep pozdrav / With regards,
>     Gregor GORJANC
> 

Greetings, Gregor, from a fan of Slovenia.

Here is a start.  Multiple variables are often put together in data 
frames.  Your example did not include an input dataset name.  I took 
data to be an input data frame, and added two new variables.  I assume 
that colvalue and colname are character values.  If colvalue has length 
one it will be duplicated for each row of data.

test <- function(data, colname, colvalue) {
   data[[colname]] <- colvalue
   data[[paste('other',colname,sep='_')]] <- paste('other',colvalue,sep='_')
   data
}

Example usage:

data <- data.frame(x=1:5, y=11:15)
data2 <- test(data, 'a', 'b')
data <- test(data, 'a', 'b')  # over-write
test(data, 'a', 'b')          # create data but don't store
with(test(data, ....),{...})        # reference variables in temporary 
dataset

If we know what your ultimate goal is, there may be a much better 
approach than the test function.  In many cases, you do not need to 
create new variables at all.

Franc
-- 
Frank E Harrell Jr   Professor and Chair           School of Medicine
                      Department of Biostatistics   Vanderbilt University