[R] summing columns of data frame by group

[Daniel O'Shea]

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