Related to my other question - given the following data frame
df0 <- data.frame (id = c(1, 1, 1, 2, 2, 2, 3, 3, 3, 3),
val = c(0, 1, 1, 0, 0, 0, 1, 0, 1, 1))
what is the most straightforward way (i.e. one liner) to add a third column in which, for each id, sequence numbers appear only when val == 1, like the following?
id val seq
1 1 0 NA
2 1 1 1
3 1 1 2
4 2 0 NA
5 2 0 NA
6 2 0 NA
7 3 1 1
8 3 0 NA
9 3 1 2
10 3 1 3
Using data.table:
require(data.table)
setDT(df0)[val == 1L, seq := seq_len(.N), by=id]
# id val seq
# 1: 1 0 NA
# 2: 1 1 1
# 3: 1 1 2
# 4: 2 0 NA
# 5: 2 0 NA
# 6: 2 0 NA
# 7: 3 1 1
# 8: 3 0 NA
# 9: 3 1 2
# 10: 3 1 3
.N contains the number of observations per group. Start with the HTML vignettes for more.
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