Pfthb

I'm trying, for each row, to calculate the difference with the closest previous row belonging to the same group which meets a certain criterion.

Suppose I have the following dataframe:

s <- read.table(text = "Visit_num Patient Day Admitted
1 1 2015/01/01 Yes
2 1 2015/01/10 No 
3 1 2015/01/15 Yes
4 1 2015/02/10 No 
5 1 2015/03/08 Yes
6 2 2015/01/01 Yes 
7 2 2015/04/01 No
8 2 2015/04/10 No
9 3 2015/04/01 No
10 3 2015/04/10 No", header = T, sep = "")

For each Visit_num and for each Patient, I'd like to get the difference with the closest row for which the patient was admitted (i.e. Yes). Note column day is ordered by day, and time unit for this example is days.
Here is what I wanted my dataframe to look like:

Visit_num Patient Day Admitted Diff_days
 1 1 2015/01/01 Yes NA
 2 1 2015/01/10 No 9
 3 1 2015/01/15 Yes 14
 4 1 2015/02/10 No 26
 5 1 2015/03/08 Yes 52
 6 2 2015/01/01 Yes NA
 7 2 2015/04/01 No 90
 8 2 2015/04/10 No 99
 9 3 2015/04/01 No NA
 10 3 2015/04/10 No NA

Any help is appreciated.

Here is an option with tidyverse. Convert the 'Day' to Date class, arrange by 'Patient', 'Day', grouped by 'Patient' get the difference of adjacent 'Day', create a group 'grp' based on the occurrence of 'Yes' in 'Admitted' and take the cumulative sum of 'Diff_days'

library(tidyverse)
s %>%
 mutate(Day = ymd(Day)) %>%
 arrange(Patient, Day) %>% 
 group_by(Patient) %>% 
 mutate(Diff_days = c(NA, diff(Day))) %>% 
 group_by(grp = cumsum(lag(Admitted == "Yes", default = TRUE)), add = TRUE) %>% 
 mutate(Diff_days = cumsum(replace_na(Diff_days, 0))) %>%
 ungroup %>%
 select(-grp) %>%
 mutate(Diff_days = na_if(Diff_days, 0))
# A tibble: 8 x 5
# Visit_num Patient Day Admitted Diff_days
# <int> <int> <date> <fct> <dbl>
#1 1 1 2015-01-01 Yes NA
#2 2 1 2015-01-10 No 9
#3 3 1 2015-01-15 Yes 14
#4 4 1 2015-02-10 No 26
#5 5 1 2015-03-08 Yes 52
#6 6 2 2015-01-01 Yes NA
#7 7 2 2015-04-01 No 90
#8 8 2 2015-04-10 No 99