Sum of rows based on column value

I want to sum rows that have the same value in one column: ``` > df <- data.frame("1"=c("a","b","a","c","c"), "2"=c(1,5,3,6,2), "3"=c(3,3,4,5,2)) > df X1 X2 X3 1 a 1 3 2 b 5 3 3 a 3 4 4 c 6 5 5 c 2 2 ``` For one column (X2), the data can be aggregated to get the sums of all rows that have the same X1 value: ``` > ddply(df, .(X1), summarise, X2=sum(X2)) X1 X2 1 a 4 2 b 5 3 c 8 ``` How do I do the same for X3 and an arbitrary number of other columns except X1? This is the result I want: ``` X1 X2 X3 1 a 4 7 2 b 5 3 3 c 8 7 ```