[R] weighted.mean and tapply (again)

[Dan Bolser]

I read answers to questions including the words "tapply" and
"weighted.mean", but I didn't understand either the problem (data) or the
solution provided.

Here is my question ...

> dat[1:10,]
  GROUP  VALUE FREQUENCY
1     2      2        78
2     2      3        40
3     2      4        16
4     2      5         3
5     2      6         1
6     2      8         1
7     3      3        19
8     3      4        10
9     3      5        19
1     3      6         4


For each GROUP, I would like to calculate the weighted.mean of VALUE using
the FREQUENCY as the weight, so for the snippet of data shown that would
be...

group.2 <- weighted.mean(c(2,3,4,5,6,8),c(78,40,16,3,1,1))
group.3 <- weighted.mean(c(3,4,5,6),    c(19,10,19,4))

> cbind(rbind(2,3),rbind(group.2,group.3))
        [,1]     [,2]
group.2    2 2.654676
group.3    3 4.153846

I would like to use tapply to automatically do this across the whole
dataset (dat) - which includes lots of other distinct grouping factors,
however, like I said, I couldn't understand (and therefore apply to my
data) any of the other solutions I found, so any help here would be
greatly appreciated!

All the best,
Dan.