R help - Jan 2011 - question on aggregate

an example available on the net goes like
> df
  identifier quantity
1          1       10
2          1       20
3          2       30
4          1       15
5          2       10
6          3       20
> aggregate(df$quantity, by=list(df$identifier), sum)
  Group.1  x
1       1 45
2       2 40
3       3 20


I'd like Group.1 to retain the name "identifier" and would like to
control what "x" get called in the output.  Thanks.

On Mon, Jan 10, 2011 at 8:23 PM, analyst41 at hotmail.com
<analyst41 at hotmail.com> wrote:
> an example available on the net goes like
>
>> df
> ?identifier quantity
> 1 ? ? ? ? ?1 ? ? ? 10
> 2 ? ? ? ? ?1 ? ? ? 20
> 3 ? ? ? ? ?2 ? ? ? 30
> 4 ? ? ? ? ?1 ? ? ? 15
> 5 ? ? ? ? ?2 ? ? ? 10
> 6 ? ? ? ? ?3 ? ? ? 20
>> aggregate(df$quantity, by=list(df$identifier), sum)
> ?Group.1 ?x
> 1 ? ? ? 1 45
> 2 ? ? ? 2 40
> 3 ? ? ? 3 20
>
>
> I'd like Group.1 to retain the name "identifier" and would
like to
> control what "x" get called in the output. ?Thanks.
Try these:
> aggregate(quantity ~ identifier, df, sum)
  identifier quantity
1          1       45
2          2       40
3          3       20
>
> aggregate(df["quantity"], df["identifier"], sum)
  identifier quantity
1          1       45
2          2       40
3          3       20
>
> aggregate(list(Quantity = df$quantity), df["identifier"], sum)
  identifier Quantity
1          1       45
2          2       40
3          3       20

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