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

Question

How can we generate unique id numbers within each group of a dataframe? Here's some data grouped by "personid":

personid date measurement
1         x     23
1         x     32
2         y     21
3         x     23
3         z     23
3         y     23

I wish to add an id column with a unique value for each row within each subset defined by "personid", always starting with 1. This is my desired output:

personid date measurement id
1         x     23         1
1         x     32         2
2         y     21         1
3         x     23         1
3         z     23         2
3         y     23         3

I appreciate any help.

Answer 1

Some dplyr alternatives, using convenience functions row_number and n.

library(dplyr)
df %>% group_by(personid) %>% mutate(id = row_number())
df %>% group_by(personid) %>% mutate(id = 1:n())
df %>% group_by(personid) %>% mutate(id = seq_len(n()))
df %>% group_by(personid) %>% mutate(id = seq_along(personid))

---

You may also use getanID from package splitstackshape. Note that the input dataset is returned as a data.table.

getanID(data = df, id.vars = "personid")
#    personid date measurement .id
# 1:        1    x          23   1
# 2:        1    x          32   2
# 3:        2    y          21   1
# 4:        3    x          23   1
# 5:        3    z          23   2
# 6:        3    y          23   3

Answer 2

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