本人研究出来了，愿意和大家分享：
x1 <- c(3, 1, 4, 15, 92)
x2 <- c(3, 1, 4, 1, 5, 9, 2, 6, 5, 3, 5)
names(x1)<-rep(letters[1],times=5)
names(x2)<-rep(letters[2],times=11)
(r2<-rank(c(x1,x2),ties.method="average"))
#其中ties.method有如下选项：ties.method = c("average", "first", "random", "max", "min"))