first observation and count (find sequence length)

Question

I have the following data.frame.

df <- data.frame(x = c("abc","def","def","def", "ghi", "ghi"))

How can I count the number of observation of the column?

observations meaning different values of the variable or total number of rows? — Diego Aguado, Feb 16 '16 at 07:07
i would like to show 1,1,2,3,1,2 in another column, it would be the number of appreance. — Raphael Li, Feb 16 '16 at 07:15
You can use `with(df, ave(seq_along(x), x, FUN= seq_along))` — akrun, Feb 16 '16 at 07:38

score 3 · Accepted Answer · answered Feb 16 '16 at 07:56

If the sequence by group ('x' column) is needed as the output, we can use ave from base R. We group by 'x', and get the sequence (seq_along)

with(df, ave(seq_along(x), x, FUN= seq_along))
#[1] 1 1 2 3 1 2

With dplyr, we can use row_number() after grouping by 'x'.

library(dplyr)
df %>%
    group_by(x) %>%
    mutate(Seq = row_number())

1 Answers1