Why does ifelse() return single-value output? [duplicate]

Question

Those two functions should give similar results, don't they?

f1 <- function(x, y) {
   if (missing(y)) {
      out <- x
   } else {
      out <- c(x, y)
   }
   return(out)
}

f2 <- function(x, y) ifelse(missing(y), x, c(x, y))

Results:

> f1(1, 2)
[1] 1 2
> f2(1, 2)
[1] 1

Answer 1

This is not related to missing, but rather to your wrong use of ifelse. From help("ifelse"):

ifelse returns a value with the same shape as test which is filled with elements selected from either yes or no depending on whether the element of test is TRUE or FALSE.

The "shape" of your test is a length-one vector. Thus, a length-one vector is returned. ifelse is not just different syntax for if and else.