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

Using data.table, assuming the mydata object has already been sorted in the way you require, another approach would be:

library(data.table)
mydata <- data.table(my.data)
mydata <- mydata[, .SD[1], by = .(id, day)]

Using dplyr with magrittr pipes:

library(dplyr)
mydata <- mydata %>%
  group_by(id, day) %>%
  slice(1) %>%
  ungroup()

If you don't add ungroup() to the end dplyr's grouping structure will still be present and might mess up some of your subsequent functions.