[R] Autogenerate tags in tag=value pairs for

[Jon Olav Vik]

Dear list,

Is there a way to programmatically specify tag names for the ... (ellipsis) 
part of the argument list to a function? In other words, a way to do this:

x <- data.frame(A=1:5)

if the name "A" was not hardcoded but given by a variable, and without 
resorting to:

x <- data.frame(1:5)
names(x) <- "A"


A longer example describing my actual problem follows. Thanks in advance for 
any help.

Best regards,
Jon Olav


I want to use function transformBy() in package doBy. The key is that the "... 
Further arguments of the form tag=value" require "tag" to be specified, 
otherwise the output does not include the results of my groupwise calculations.

Quoting the documentation:
" transformBy(doBy)
" Function to make groupwise transformations of data 
" by applying the transform function to subsets of data. 
" 
" Usage
" transformBy(formula, data, ...)
" 
" Arguments
" formula A formula with only a right hand side, see examples below 
" data A data frame 
" ... Further arguments of the form tag=value 

### example ###

# a function to replace NAs with the last non-NA value from above 
filldown <- function(x) {
    notna <- !is.na(x) # elements with values
    ix <- cumsum(notna) # index to previous element (but zeros where we need NA)
    ix[ix==0] <- NA # use [NA] as index to produce NA in output
    return(x[notna][ix]) # for each: return previous value if found, else NA
}
# illustration of how it works
tmp <- c(NA,NA,1,NA,3,NA,NA)
cbind(tmp,filldown(tmp))

# I now want to apply filldown() to subsets of a data frame
# and I want it to work on several columns

# generate a data frame for illustration, 
# with a few non-NA values scattered round
set.seed(5) # repeatable example
x <- data.frame(id = rep(1:4,each=6), v1=NA, v2=NA)
ix <- which(runif(nrow(x))>0.75)
x[ix,2] <- rpois(length(ix),5)
ix <- which(runif(nrow(x))>0.75)
x[ix,3] <- rpois(length(ix),5)
x

library(doBy)
# the hard way -- works as required, 
# but I would like not having to hardcode column names v1 etc.
transformBy(~id,data=x,v1.fd = filldown(v1),v2.fd = filldown(v2))

# does not work because
# output includes only columns explicitly mentioned in the ... argument
transformBy(~id,data=x,function(y) lapply(y,filldown))