How to group by different columns and find the percentage based on one column?

Question

I want to group the following data table by var1 and var2 and then find the percentage in var2:

data <- as.data.table(l         


        

Answer 1

This should give you what you want:

data <- data[, .N, by = .(var1, var2)][, ratio:=N/sum(N), by = var2]

which results in:

> data
   var1 var2 N     ratio
1:   x1   y1 2 0.6666667
2:   x2   y1 1 0.3333333
3:   x1   y2 1 0.5000000
4:   x2   y2 1 0.5000000