根据数据表中的组和子组的计数创建变量

Question

I have many student records. I need to create two new variable. One should display the count of Unitcode (ie enrolments) for each student_ID for each Year .

One should display the count of Fail (ie Grade=='Fail') for each student_ID for each Year . See the example of records for three students below:

   student_ID=c(rep("1001",8),rep("1002",3),rep("1005",11))
   Year=c(rep(2011,4),rep(2012,4),2011,2012,2013,rep(2011,4),rep(2012,3),rep(2013,4))
   Grade=c(rep("Fail",2),rep("Pass",3),rep("Fail",3),rep("Pass",7),rep("Fail",2),rep("Pass",5))
   Unitcode<-c(1201:1222)
   record<-data.table(student_ID, Year, Grade, Unitcode)

If someone could assist with counting new variables that would be greatly appreciated.

Answer 1

A similar option using dplyr would be

library(dplyr)
record %>%
     group_by(student_ID, Year) %>%
     summarise(unitcodes=n(), fails=sum(Grade=='Fail'))