determining which id's have a time point that's earlier than another time point in R

I am trying to find a way to return subject_ids with time_2 that’s earlier than time_1

So in this case the code would return subject id’s 191-1245 and 191-2365 because both their time_2 is earlier than time_1.

>Solution :

A dplyr solution:

library(dplyr)
df %>% 
  filter(strptime(Time_2, '%H:%M') < strptime(Time_1, '%H:%M')) %>% 
  pull(subject_id)
[1] "191-1245" "191-2365"