[R] Data Frame Manipulations

[Petr Pikal]

Hi Michael

On 1 Dec 2004 at 11:50, michael watson (IAH-C) wrote:

> Hi
> 
> I have used merge() to merge two data frames, very much like
> performing a SQL join.  Now I want to do a few different SQL-style
> things and I wondered if there were functions to do it... 
> 
> Is there a "group by" style function?  For example if I merge() two
> data frames and end up with multiple rows for each "id", and want to
> take the average of the values of a particular column?  I know I can
> probably put something together using merge() and by() and then
> munging the results together myself, but is there something in R to
> perform this automatically?

aggregate, by, *apply but AFAIK only after merge

> 
> The second thing I'd like to do is like a cross-tab query; that is
> when after a merge() I end up with multiple rows for a particular
> "id", and want to cross-tab the data so that the multiple values
> become columns and I end up with one row for each "id" again e.g.

reshape in base or reShape in Hmisc is probably what you need

Cheers
Petr


> 
> ID	Val
> 1	5
> 1	10
> 2	15
> 2	20
> 
> Becomes
> 
> ID	Val1	Val2
> 1	5	10
> 2	15	20
> 
> Thanks in advance!
> 
> Mick
> 
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Petr Pikal
petr.pikal at precheza.cz