R - Keep first observation per group identified by multiple variables (Stata equivalent “bys var1 var2 : keep if _n == 1”)

Question

So I currently face a problem in R that I exactly know how to deal with in Stata, but have wasted over two hours to accomplish in R.

Using the data.frame below, the

Answer 1

The package dplyr makes this kind of things easier.

library(dplyr)
mydata %>% group_by(id, day) %>% filter(row_number(value) == 1)

This command requires more memory in R than in Stata: rows are not suppressed in place, a new copy of the dataset is created.