Apply a function over all combinations of arguments - r

Question

I would like to be able to apply a function to all combinations of a set of input arguments. I have a working solution (below) but would be surprised if there's not a better / more generic way to do this using, e.g. plyr, but so far have not found anything. Is there a better solution?
# Apply function FUN to all combinations of arguments and append results to
# data frame of arguments
cmapply <- function(FUN, ..., MoreArgs = NULL, SIMPLIFY = TRUE,
USE.NAMES = TRUE)
{
l <- expand.grid(..., stringsAsFactors=FALSE)
r <- do.call(mapply, c(
list(FUN=FUN, MoreArgs = MoreArgs, SIMPLIFY = SIMPLIFY, USE.NAMES = USE.NAMES),
l
))
if (is.matrix(r)) r <- t(r)
cbind(l, r)
}
examples:
# calculate sum of combinations of 1:3, 1:3 and 1:2
cmapply(arg1=1:3, arg2=1:3, 1:2, FUN=sum)
# paste input arguments
cmapply(arg1=1:3, arg2=c("a", "b"), c("x", "y", "z"), FUN=paste)
# function returns a vector
cmapply(a=1:3, b=2, FUN=function(a, b) c("x"=b-a, "y"=a+b))

Answer 1

This function isn't necessarily any better, just slightly different:
rcapply <- function(FUN, ...) {
## Cross-join all vectors
DT <- CJ(...)
## Get the original names
nl <- names(list(...))
## Make names, if all are missing
if(length(nl)==0L) nl <- make.names(1:length(list(...)))
## Fill in any missing names
nl[!nzchar(nl)] <- paste0("arg", 1:length(nl))[!nzchar(nl)]
setnames(DT, nl)
## Call the function using all columns of every row
DT2 <- DT[,
as.data.table(as.list(do.call(FUN, .SD))), ## Use all columns...
by=.(rn=1:nrow(DT))][ ## ...by every row
, rn:=NULL] ## Remove the temp row number
## Add res to names of unnamed result columns
setnames(DT2, gsub("(V)([0-9]+)", "res\\2", names(DT2)))
return(data.table(DT, DT2))
}
head(rcapply(arg1=1:3, arg2=1:3, 1:2, FUN=sum))
## arg1 arg2 arg3 res1
## 1: 1 1 1 3
## 2: 1 1 2 4
## 3: 1 2 1 4
## 4: 1 2 2 5
## 5: 1 3 1 5
## 6: 1 3 2 6
head(rcapply(arg1=1:3, arg2=c("a", "b"), c("x", "y", "z"), FUN=paste))
## arg1 arg2 arg3 res1
## 1: 1 a x 1 a x
## 2: 1 a y 1 a y
## 3: 1 a z 1 a z
## 4: 1 b x 1 b x
## 5: 1 b y 1 b y
## 6: 1 b z 1 b z
head(rcapply(a=1:3, b=2, FUN=function(a, b) c("x"=b-a, "y"=a+b)))
## a b x y
## 1: 1 2 1 3
## 2: 2 2 0 4
## 3: 3 2 -1 5

Answer 2

A slight simplification of your original code:
cmapply <- function(FUN, ..., MoreArgs = NULL)
{
l <- expand.grid(..., stringsAsFactors=FALSE)
r <- .mapply(FUN=FUN, dots=l, MoreArgs = MoreArgs)
r <- simplify2array(r, higher = FALSE)
if (is.matrix(r)) r <- t(r)
return(cbind(l, r))
}
This does not require a do.call.
It does miss the SIMPLIFY and USE.NAMES arguments, but the way you are using it seems to make the arguments not usable anyway: if SIMPLIFY = FALSE, the rbind() will fail, and USE.NAMES = TRUE does not do anything because the names get lost after the rbind() anyway.