I have a data.table dt. This data.table is sorted first by column date (my grouping variable), then by column age:
library(data.table)
setkeyv(dt, c("date", "age")) # Sorts table first by column "date" then by "age"
> dt
date age name
1: 2000-01-01 3 Andrew
2: 2000-01-01 4 Ben
3: 2000-01-01 5 Charlie
4: 2000-01-02 6 Adam
5: 2000-01-02 7 Bob
6: 2000-01-02 8 Campbell
My question is: I am wondering if it's possible to extract the first 2 rows for each unique date? Or phrased more generally:
How to extract the first n rows within each group?
In this example, the result in dt.f would be:
> dt.f = ???????? # function of dt to extract the first 2 rows per unique date
> dt.f
date age name
1: 2000-01-01 3 Andrew
2: 2000-01-01 4 Ben
3: 2000-01-02 6 Adam
4: 2000-01-02 7 Bob
p.s. Here is the code to create the aforementioned data.table:
install.packages("data.table")
library(data.table)
date <- c("2000-01-01","2000-01-01","2000-01-01",
"2000-01-02","2000-01-02","2000-01-02")
age <- c(3,4,5,6,7,8)
name <- c("Andrew","Ben","Charlie","Adam","Bob","Campbell")
dt <- data.table(date, age, name)
setkeyv(dt,c("date","age")) # Sorts table first by column "date" then by "age"
dt[dt[, .I[1:2], by = date]$V1]is much faster – Misreckon