[R] summing columns of data frame by group

[John Kane]

?by perhaps
--- Daniel O'Shea <dan.oshea at dnr.state.mn.us> wrote:

> I have a data frame and one separate vector that is
> a grouping variable for the data frame.  I would
> like to take all rows of the data frame belonging to
> each group and then sum the columns with out using a
> for statement.
> 
> Something like: take all rows of group 1 then
> apply(group1.data,1,sum), but do this without having
> to do it separately for each group.  Any ideas? 
> thank you.
> 
> Dan
> 
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