There is now a blog post that attempts to answer the question in the subject line:
http://www.burns-stat.com/the-three-dots-construct-in-r/ Pat On 17/01/2013 14:36, Ivan Calandra wrote:
Dear users, I'm trying to learn how to use the "...". I have written a function (simplified here) that uses doBy::summaryBy(): # 'dat' is a data.frame from which the aggregation is computed # 'vec_cat' is a integer vector defining which columns of the data.frame should be use on the right side of the formula # 'stat_fun' is the function that will be run to aggregate stat.group <- function(dat, vec_cat, stat_fun){ require(doBy) df <- summaryBy(as.formula(paste0(".~",paste0(names(dat)[vec_cat],collapse="+"))), data=dat, FUN=stat_fun) return(df) } Example, works fine: my_data <- structure(list(cat = structure(c(1L, 1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L, 2L), .Label = c("A", "B"), class = "factor"), varnum = c(-0.754816565434373, -1.94101630973709, -0.102461836059522, -0.519952759645808, -1.73772800855664, -1.13939178585609, 0.522356715260142, -0.701428514907824, 1.45197576541159, 0.0844567413828095)), .Names = c("cat", "varnum"), row.names = c(NA, -10L), class = "data.frame") stat.group(dat=my_data, vec_cat=1, stat_fun=mean) Now summaryBy() has an "..." argument and I would like to use it. For example, I would like to be able to add the trim argument to my call like this: stat.group(dat=my_data, vec_cat=1, stat_fun=mean, trim=0.2) I know I can do it using this "..." but I have no idea how to do it. I've tried to search for it, but a search with "..." doesn't yield interesting results! Thank you in advance for your help! Ivan
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