Create a sequential number (counter) for rows within each group of a dataframe [duplicate]

Answer 1

The misleadingly named ave() function, with argument FUN=seq_along, will accomplish this nicely -- even if your personid column is not strictly ordered.

df <- read.table(text = "personid date measurement
1         x     23
1         x     32
2         y     21
3         x     23
3         z     23
3         y     23", header=TRUE)

## First with your data.frame
ave(df$personid, df$personid, FUN=seq_along)
# [1] 1 2 1 1 2 3

## Then with another, in which personid is *not* in order
df2 <- df[c(2:6, 1),]
ave(df2$personid, df2$personid, FUN=seq_along)
# [1] 1 1 1 2 3 2