Aggregate data in one column based on values in another column

Question

I know there is an easy way to do this...but, I can\'t figure it out.

I have a dataframe in my R script that looks something like this:

A      B    C
1.2         


        

Answer 1

Here is a solution using data.table for memory and time efficiency

library(data.table)
DT <- as.data.table(df)
DT[, list(totalB = sum(B), num = .N), by = A]

To subset only rows where C==1 (as per the comment to @aix answer)

DT[C==1, list(totalB = sum(B), num = .N), by = A]