How to calculate percent change in R when there are some years of data missing?

Question

I'm calculating the percent change of enrollment from academic year to academic year, but there are some academic years missing data, so I don't want it to calculate the change in those instances and keep it as blank instead of calculating a two year difference. I have multiple years, schools, and groups I am doing this by. Example data frame below and the code I am using currently. So I am missing 2016-17 in this example and don't want to calculate it for 17-18 then.

 School      Academic Year  Group   Enrollment pct_change  
 1 School 1  2018-19        Overall 450        ANSWER 
 2 School 1  2017-18        Overall 630        NA
 3 School 1  2015-16        Overall 635        ANSWER
 4 School 1  2014-15        Overall 750        ANSWER
 5 School 1  2013-14        Overall 704        ANSWER 

data <- data %>%
  group_by(School, Group) %>% 
  mutate(pct_change = (((Enrollment-lead(Enrollment, order_by = `Academic Year`))/Enrollment)) * 100) %>%
  ungroup()

Answer 1

An option may be to expand the data for complete year

library(dplyr)
library(tidyr)
data %>%
    separate(`Academic Year`, into = c("Year", "Day"), 
remove = FALSE, convert = TRUE) %>%
    group_by(School, Group) %>%
    complete(Year = full_seq(Year, period = 1)) %>%
     mutate(pct_change = (((Enrollment-lead(Enrollment, 
    order_by = Year))/Enrollment)) * 100) %>%
  ungroup()
    filter(complete.cases(Enrollment)) %>%
    select(-Year, -Day)