[R] How to get variable names in a function?

[Heinz Tuechler]

At 10:32 08.02.2005 -0800, Berton Gunter wrote:
>If you pass your function a NAMED list, then the following works:
>
>demofn<-function(varlist)
>{
>	nm<-names(varlist)
>    for (i in seq(along=varlist))
>       {cat('\n',nm[i]) 
>       print(table(varlist[[i]]))
>    }
>}
>demofn(list(bravo=bravo, charly=charly))
>
>If you don't pass a named list, then you need to restrict and know the form
>of the expression that is the varlist argument in order to substitute() and
>deparse it correctly to get the identifiers you want. For example, if you
>knew that varlist were an expression of the form:
>list(var1,var2,var3,...) 
>then you could get the ith identifier vari via:
>
>deparse((as.list(substitute(varlist))[-1])[[i]]) 
>
>HOWEVER, this is probably inefficient and **clearly** clumsy, undesirable,
>and almost certain to fail (so don't do this!). 
>
>If the number of tables is small enough that you could simply list them as
>arguments (as opposed to constructing the list of vectors to be tabled in
>some way), then the function call could be of the form function(...) and the
>... arguments could be processed as discussed in section 3.1 of V&R's S
>PROGRAMMING. That is, your example call would be of the form:
>demofn(bravo,charly), and you can forgo lists in the call altogether. This
>strategy actually also works for long constructed lists of arguments using
>do.call() -- see it's help file and V&R again for details.
>
>
>-- Bert Gunter
>Genentech Non-Clinical Statistics
>South San Francisco, CA
> 
>"The business of the statistician is to catalyze the scientific learning
>process."  - George E. P. Box
> 
Thank you for your detailled answer. I see that it is not as easy as I
expected and I will think about a convenient way to do it.
I tried also the opposit approach i.e. to pass a vector of names to the
function like c("bravo", "charly") but until now I did not solve it either.
A third way worked, but seemed to me less generally applicable.
I stored all the names of the variables of a data.frame in a vector, made
the selection of the variables by match() and passed the indices of the
selected variables to the function. By this method I can then access each
variable by its index and also its name via the vector of the stored names.
It seemed to me a crude method of a beginner like me and I still hope to
find a better one.
Your suggestion with the named list may be the best, if I find a practical
way to produce this named list by some function from a simple list without
retyping each name. 

Thanks again,

Heinz
> 
>
>> -----Original Message-----
>> From: r-help-bounces at stat.math.ethz.ch 
>> [mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Heinz Tuechler
>> Sent: Tuesday, February 08, 2005 8:45 AM
>> To: r-help at stat.math.ethz.ch
>> Subject: [R] How to get variable names in a function?
>> 
>> Hello,
>> 
>> applying a function to a list of variables I face the 
>> following problem:
>> Let's say I want to compute tables for several variables. I 
>> could write a
>> command for every single table, like
>> bravo<-c(1,1,2,3,5,5,5,);charly<-c(7,7,4,4,2,1)
>> table(bravo); table(charly)
>> > table(bravo); table(charly)
>> bravo
>> 1 2 3 5 
>> 2 1 1 3 
>> charly
>> 1 2 4 7 
>> 1 1 2 2 
>> The results are two tables with the names of the variables above each.
>> If I want to do the same thing by a function I find no way to get the
>> variable names above the tables. 
>> demofn<-function(varlist)
>>     {for (i in seq(along=varlist))
>>        {cat(deparse(varlist[i])) # < - - - - how to change this?
>>         print(table(varlist[i]))}}
>> > demofn(list(bravo, charly))
>> list(c(1, 1, 2, 3, 5, 5, 5))
>> 1 2 3 5 
>> 2 1 1 3 
>> list(c(7, 7, 4, 4, 2, 1))
>> 1 2 4 7 
>> 1 1 2 2 
>> > 
>> 
>> Thanks,
>> Heinz Tüchler
>> 
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>
>
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