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

Let's not forget that a lot of things can be done in base R, although sometimes not as efficiently as data.table or dplyr:

df$count<-ave(as.integer(df$one),df[,2:3],FUN=function(x) sum(!is.na(x)))
#   one group1 group2 count
#1  one      A      C     1
#2       A      C     1
#3       A      C     1
#4       A      D     0
#5       B      E     1
#6  two      B      E     1
#7       B      F     0
#8       B      F     0