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foo <- LETTERS[1:4]
foo[4] <- NA
foo
[1] "A" "B" "C" NA
paste(1:4, foo, sep = ", ")

[1] "1, A"  "2, B"  "3, C"  "4, NA"

[1] "1, A" "2, B" "3, C" "4"

sub(', NA$', '', paste(1:4, foo, sep = ", "))
[1] "1, A" "2, B" "3, C" "4"

paste2 <- function(...,sep=", ") {
    L <- list(...)
    L <- lapply(L,function(x) {x[is.na(x)] <- ""; x})
    gsub(paste0("(^",sep,"|",sep,"$)"),"",
                gsub(paste0(sep,sep),sep,
                     do.call(paste,c(L,list(sep=sep)))))
}
foo <- c(LETTERS[1:3],NA)
bar <- c(NA,2:4)
baz <- c("a",NA,"c","d")
paste2(foo,bar,baz)
# [1] "A, a"    "B, 2"    "C, 3, c" "4, d"

gsub(", $", "", paste(1:4, ifelse(is.na(foo), "", foo), sep = ", "))