[R] summing and combining rows

[Christopher R. Dolanc]

Hello,

I have a data set that needs to be combined so that rows are summed by a 
group based on a certain variable. I'm pretty sure rowsum() or rowsums() 
can do this but it's difficult for me to figure out how it will work for 
my data based on the examples I've read.

My data are structured like this:

Plot    SizeClass    Stems
12       Class3            1
12       Class4            3
17       Class3            5
17       Class4            2

I simply want to sum the size classes by plot and create a new data 
frame with a size class called "Class34" or with the SizeClass variable 
removed. I actually do have other size classes that I want to leave 
alone, but combine 3 and 4, so if I could figure out how to do this by 
creating a new class, that would be preferable.

I've also attached a more detailed sample of data.

Thanks,
Chris Dolanc

-- 
Christopher R. Dolanc
Post-doctoral Researcher
University of Montana and UC-Davis

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      Data           Plot      Lat LatCat Elevation ElevCat    Type SizeClass Stems     Area   Density
5171   VTM          39C16 39.76282   Lat6      1500   Elev1 Conifer    Class3     0 751.5347   0.00000
5172   VTM          39C16 39.76282   Lat6      1500   Elev1 Conifer    Class4     1 751.5347  13.30611
5771   VTM          39F11 39.57721   Lat6       500   Elev1 Conifer    Class3     0 751.5347   0.00000
5772   VTM          39F11 39.57721   Lat6       500   Elev1 Conifer    Class4     0 751.5347   0.00000
5787   VTM          39F13 39.56214   Lat6      1500   Elev1 Conifer    Class3     3 694.0784   0.00000
5788   VTM          39F13 39.56214   Lat6      1500   Elev1 Conifer    Class4     1 694.0784   0.00000
5795   VTM          39F14 39.54522   Lat6       900   Elev1 Conifer    Class3     1 763.9850  13.08926
5796   VTM          39F14 39.54522   Lat6       900   Elev1 Conifer    Class4     2 763.9850   0.00000