索引向量中的重复元素

Question

I would like to take a vector:

x <- c(rep("cat",3), rep("dog",2))

[1] "cat" "cat" "cat" "dog" "dog"

and turn it into:

[1] "cat_a" "cat_b" "cat_c" "dog_a" "dog_b"

Where each unique element is index with "_" plus a letter.

Answer 1

Using base functions you can do

ave(x, x, FUN=function(x) paste(x, letters[seq_along(x)], sep="_"))
# [1] "cat_a" "cat_b" "cat_c" "dog_a" "dog_b"