Once upon a time, there was a R programmer named Bob who was working on a project to analyze customer behavior. He had a large data frame with information about each customer's purchase history. He needed to extract the first purchase made by each customer to analyze their initial buying behavior.
So, he used the .SD and [1L] functions from the data.table package to select the first record within each group. But, when he looked at the output, he was surprised to see that the first purchases of each customer were all made on the same day - January 1st, 1970!
Bob was confused and thought there was a bug in his code. He checked his code several times, but everything seemed to be correct. Finally, he realized that the date column in his data frame was in the Unix time format and January 1st, 1970 was the starting point for Unix time.
Bob laughed at himself for not realizing this earlier and changed the date format to something more meaningful. From then on, he always made sure to check the format of his data before using it in his analysis.
library(data.table)
setDT(df)
df[, .SD[1L], by = group_variable]
In this code, df is the name of your dataframe and group_variable is the variable that you want to group by. The by argument specifies the grouping variable, and the .SD (data.table shorthand for data.frame) is used to reference the subset of data for each group. The [1L] index takes the first row of each group.