Get first n rows for each date in a dataframe

Question

I am currently trying to subset the first n-observations for each date in my dataset. Let's say n=2 for example purposes. This is what the data set looks like:

Date             Measure  
2019-02-01       5  
2019-02-01       4  
2019-02-01       3  
2019-02-01       6  
   …             …   
2019-02-02       5  
2019-02-02       5  
2019-02-02       2  
   …             …   

I would like to see this output:

Date             Measure  
2019-02-01       5  
2019-02-01       4  
2019-02-02       5  
2019-02-02       5  
   …             … 

Unfortunately, this is not something I am able to do with definitions. I am dealing with over 10 million rows of data, so the solution needs to be dynamic to make the selection of n for each unique date.

Answer 1

An option is to group by 'Date' and slice the sequence of 'n' rows

library(dplyr)
n <- 2
df1 %>%
    group_by(Date) %>%
    slice(seq_len(n))