How to get group-level statistics while preserving the original dataframe?

Question

I have the following dataframe

one <- c(\'one\',NA,NA,NA,NA,\'two\',NA,NA)
group1 <- c(\'A\',\'A\',\'A\',\'A\',\'B\',\'B\',\'B\',\'B\')
group2 <- c(\'C\         


        

Answer 1

with data.table:

setDT(df)
df[,count_B:=sum(!is.na(one)),by=c("group1","group2")]

gives:

   one group1 group2 count_B
1: one      A      C       1
2:  NA      A      C       1
3:  NA      A      C       1
4:  NA      A      D       0
5:  NA      B      E       1
6: two      B      E       1
7:  NA      B      F       0
8:  NA      B      F       0

The idea is to sum the true values (1 once converted to integer) where B is not NA while grouping by group1and group2.