If all components are different (and no NAs), the ranks are
well defined, with values in seq_len(x). With some values equal
(called ‘ties’), argument ties.method determines the
result at the corresponding indices. The "first" method results
a permutation with increasing values at each index set of ties. The
"random" method puts these in random order, whereas the
default, "average", replaces them by their mean, and
"max" and "min" replace them with their maximum and
minimum respectively, the latter being the typical sports ranking.
NA values are never considered to be equal: for na.last =
TRUE and na.last = FALSE they are given distinct ranks in the
order in which they occur in x.