R help - Aug 2007 - summing columns of data frame by group

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

?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
> 
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained,
> reproducible code.
>

Here's one way,
lapply(split(DF, your.vector), function(x) {apply(x, 2, sum)})

> -----Original Message-----
> From: r-help-bounces at stat.math.ethz.ch
> [mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Daniel O'Shea
> Sent: Tuesday, August 21, 2007 3:53 PM
> To: r-help at stat.math.ethz.ch
> Subject: [R] summing columns of data frame by group
>
> 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
>
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>