sum by group in a data.frame

Question

I'm trying to get the sum of a numerical variable per a categorical variable (in a data frame). I've tried using tapply, but it's doesn't take a whole data.frame.

Here is a working example with some data that looks like this:

> set.seed(667) 
> df <- data.frame(a = sample(c("Group A","Group B","Group C",NA),  10, rep = TRUE), 
                   b = sample(c(1, 2, 3, 4, 5, 6),  10, rep=TRUE), 
                   c = sample(c(11, 12, 13, 14, 15, 16),  10, rep=TRUE))
> df
          a b  c
 1  Group A 4 12
 2  Group B 6 12
 3     <NA> 4 14
 4  Group C 1 16
 5     <NA> 2 14
 6     <NA> 3 13
 7  Group C 4 13
 8     <NA> 6 15
 9  Group B 3 16
 10 Group B 5 16

using tapply, I can get one vector at a time:

> tapply(df$b,df$a,sum)
 Group A Group B Group C 
       4      14       5     

but I am more interested in getting something like this:

         a  b  c
1  Group A  4 12
2  Group B 14 44
3  Group C  5 29

Any help would be appreciated. Thanks.

Answer 1

Use aggregate instead:

 aggregate(df[ , c("b","c")], df['a'], FUN=sum)
        a  b  c
1 Group A  4 12
2 Group B 14 44
3 Group C  5 29

I'm not sure why but you need to pass the second argument to aggregate as a list, so using df$a will error out. It then uses the function on the individual columns in the first argument.