Thanks, everybody!
*Felipe Nunes*
CAPES/Fulbright Fellow
PhD Student Political Science - UCLA
Web: felipenunes.bol.ucla.edu
On Mon, Jan 16, 2012 at 2:33 PM, Marc Schwartz <marc_schwa...@me.com> wrote:
>
> On Jan 16, 2012, at 4:13 PM, Felipe Nunes wrote:
>
> > Dear all,
> >
> > I have a data set in which the same unit appears 2 or 3 or 4 times. I
> need
> > to aggregate this data to maintain only one unit by row. But I need to do
> > that based on a comparison between the values of such units. I can't
> find a
> > function to help me on that. I appreciate any help. Below I provide an
> > example of what I want:
> >
> > This is my data:
> >
> > Units Var1 Var2 Var3
> > 1 B 2 2
> > 1 C 1 3
> > 2 D 3 3
> > 2 C 2 1
> > 2 A 1 2
> > 3 C 2 1
> > 3 A 1 3
> > 4 B 3 3
> > 4 C 2 2
> > 4 A 1 1
> >
> > I want to produce a new data in which I keep, for each unit, the row with
> > max number for Var2. For instance,
> >
> > Units Var1 Var2 Var3
> > 1 B 2 2
> > 2 D 3 3
> > 3 C 2 1
> > 4 B 3 3
> >
> > Thanks,
>
>
> There are likely to be multiple approaches, but the first that comes to
> mind is combining ?merge with ?aggregate.
>
> > DF
> Units Var1 Var2 Var3
> 1 1 B 2 2
> 2 1 C 1 3
> 3 2 D 3 3
> 4 2 C 2 1
> 5 2 A 1 2
> 6 3 C 2 1
> 7 3 A 1 3
> 8 4 B 3 3
> 9 4 C 2 2
> 10 4 A 1 1
>
> > merge(aggregate(Var2 ~ Units, max, data = DF), DF)
> Units Var2 Var1 Var3
> 1 1 2 B 2
> 2 2 3 D 3
> 3 3 2 C 1
> 4 4 3 B 3
>
>
> The use of aggregate() gets you the max value of Var2 for each group
> within Units:
>
> > aggregate(Var2 ~ Units, max, data = DF)
> Units Var2
> 1 1 2
> 2 2 3
> 3 3 2
> 4 4 3
>
> merge() then allows you to add back the columns from 'DF', matching on
> 'Units' and 'Var2', which are the common columns between the two data
> frames. Note that the ordering of the columns is changed a bit here. That
> is easily adjusted with:
>
> > merge(aggregate(Var2 ~ Units, max, data = DF), DF)[, c(1, 3, 2, 4)]
> Units Var1 Var2 Var3
> 1 1 B 2 2
> 2 2 D 3 3
> 3 3 C 2 1
> 4 4 B 3 3
>
>
> Note that you would want to use the 'na.rm = TRUE' argument to max() in
> the call to aggregate() if in fact any of your Var2 arguments are NA.
>
> HTH,
>
> Marc Schwartz
>
>
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