[R] manipulate group data using column name

[Gabor Grothendieck]

Try this:

fa$sum <- ave(fa$X1, fa$X3, FUN = sum)


On 2/8/07, sun <flyhyena at yahoo.com> wrote:
> Hi,
>
>  Maybe this is a trivial question but I can not figure out a good solution.
>
> I have a data frame fa and want to add a new column "sum" with the sum value
> of fa$X1 grouped by fa$X3.
>
> > fa
>   X1 X2 X3
> 1   1 11  1
> 2   2 12  1
> 3   3 13  1
> 4   4 14  2
> 5   5 15  2
> 6   6 16  2
> 7   7 17  3
> 8   8 18  3
> 9   9 19  3
> 10 10 20  3
>
> fa$X3 is the index of group
>
> i can
>
> >aggregate(fa[,"X1"],list(fa$X3),sum)
>  Group.1  x
> 1       1  6
> 2       2 15
> 3       3 34
>
> then I want to add a new column "sum" in fa and assign the aggregated result
> to the new column. Is there a solution without using loops?
> or maybe there is some way can even avoid aggregate operation?
>
> Thanks.
>
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