A length-4 vector, the first two positions give the range and
the next two give the positions in x where the max and min occurred.
This is almost equivalent to c(range(x), which.min(x), which.max(x)).
Note that the type is not strictly preserved, but no loss should occur. In particular,
logical x results in an integer result, and a double x will
have double values for which.min(x) and which.max(x).
A completely empty, logical x returns c(NA, NA, NA, NA) as an integer vector.