Find a percentage based on multiple columns of criteria in R

Question

I have multiple columns and I would like to find the percentage of a one column in the other columns are the same. For example;

ST  cd  variable
1   1   23432
1         


        

Answer 1

library(data.table)

DT <- data.table(read.table(text = "ST  cd  variable
1   1   23432
1   1   2345
1   2   908890
1   2   350435
1   2   2343432
2   1   9999
2   1   23432 ", header = TRUE))

DT[, percentage := variable / sum(variable) , by = list(ST, cd)]

##    ST cd variable percentage
## 1:  1  1    23432 0.90902743
## 2:  1  1     2345 0.09097257
## 3:  1  2   908890 0.25227624
## 4:  1  2   350435 0.09726856
## 5:  1  2  2343432 0.65045519
## 6:  2  1     9999 0.29909366
## 7:  2  1    23432 0.70090634