selecting individuals in long format in r

Question

I have a data with 100 patients and each patient has values from 7 days (1 to 7). How can I select only patients according another variable only in day 1?

df <- data.frame(id = c(1, 1, 1, 2, 2, 2),
             day = c(1, 2, 3, 1, 2, 3),
             RRT = c(0, 1, 0, 1, 0, 0))

id    day   RRT
1      1     0
1      2     1
1      3     0
2      1     1 
2      2     0
2      3     0

I want select only id whom had RRT at day = 0.

Answer 1

You can do this many ways, for example by using the filter function within dplyr. Assuming you want all ids if RRT does not equal 0 and day equals 1:

library(dplyr)

df %>% 
filter(day == 1 & RRT != 0)

id day RRT
2   1   1