a <- c("a", "aa", "aaa", "aaaa", "aa", "a", "aaa", "a", "bbbb", "bb", "bbb", "bb", "b", "b", "bbbb", "bbb", "c", "c", "cccc", "cc", "cccc", "ccc", "cc", "c") number <- c(1, 2, 3, 4, 5, 6, 7, 8, 9, 1, 2, 4, 5, 6, 7, 8, 1, 2, 3, 4, 5, 6, 7, 8) df <- data.frame(a, number) df str(df)

a is a factor with 12 levels. I want to group all "a", "aa", "aaa", "aaaa" into one level just called A. "b', "bb", "bbb", "bbbb" into one level called B and the same thing with the cs as well. So at the end I only have a facor with 3 levels instead of 12. How do I do that in R studio?