[R] count() function

[William Dunlap]

I think you are looking for the function called length().  I cannot recreate
your output, since I don't know what is in NZ_Conifers, but with the built-in
dataset mtcars I get:

  > ddply(mtcars, .(cyl,gear,carb), summarize, MeanWt=mean(wt), N=length(wt))
     cyl gear carb  MeanWt N
  1    4    3    1 2.46500 1
  2    4    4    1 2.07250 4
  3    4    4    2 2.68375 4
  4    4    5    2 1.82650 2
  5    6    3    1 3.33750 2
  6    6    4    4 3.09375 4
  7    6    5    6 2.77000 1
  8    8    3    2 3.56000 4
  9    8    3    3 3.86000 3
  10   8    3    4 4.68580 5
  11   8    5    4 3.17000 1
  12   8    5    8 3.57000 1
  > with(mtcars, sum(cyl==8 & gear==3 & carb==4)) # output line 10
  [1] 5

If all you want is the count of things in various categories, you can use table
instead of ddply and length:
  > with(mtcars, table(cyl, gear, carb))
  , , carb = 1

     gear
  cyl 3 4 5
    4 1 4 0
    6 2 0 0
    8 0 0 0

  , , carb = 2
  
     gear
  cyl 3 4 5
    4 0 4 2
    6 0 0 0
    8 4 0 0
  ...

Using ftable on table's output gives a nicer looking printout, but table's output is easier
to use in a program.

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com


> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf
> Of Christopher R. Dolanc
> Sent: Thursday, April 05, 2012 12:16 PM
> To: r-help at r-project.org
> Subject: [R] count() function
> 
> I keep expecting R to have something analogous to the =count function in
> Excel, but I can't find anything. I simply want to count the data for a
> given category.
> 
> I've been using the ddply() function in the plyr package to summarize
> means and st dev of my data, with this code:
> 
> ddply(NZ_Conifers,.(ElevCat, DataSource, SizeClass), summarise,
> avgDensity=mean(Density), sdDensity=sd(Density), n=sum(Density))
> 
> and that gives me results that look like this:
> 
>     ElevCat DataSource SizeClass avgDensity   sdDensity            n
> 1    Elev1        FIA    Class1   38.67768  46.6673478    734.87598
> 2    Elev1        FIA    Class2   27.34096  23.3232470    820.22879
> 3    Elev1        FIA    Class3   15.38758   0.7088432     76.93790
> 4    Elev1        VTM    Class1   66.37897  70.2050817  24958.49284
> 5    Elev1        VTM    Class2   39.40786  34.9343269  11782.95152
> 6    Elev1        VTM    Class3   21.17839  12.3487600   1461.30895
> 
> But, instead of "sum(Density)", I'd really like counts of "Density", so
> that I know the sample size of each group. Any suggestions?
> 
> --
> Christopher R. Dolanc
> Post-doctoral Researcher
> University of California, Davis
> 
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