[R] Counting observations of a combined factor

[David Winsemius]

On Sep 19, 2009, at 5:39 AM, Sam Player wrote:

> #I have a dataset with two factor. I want to combine those factors  
> into a single factor and count the number of data values for each  
> new factor. The following gives a comparable dataframe:
>
> a <- rep(c("a", "b"), c(6,6))
> b <- rep(c("c", "d"), c(6,6))
> df <- data.frame(f1=a, f2=b, d=rnorm(12))
> df
>
> # I use the 'interaction' function to combine factors f1 and f2:
>
> df2 <- data.frame(f3=interaction(df[,"f1"], df[,"f2"]), d=df[,"d"])
> df2
>
> # A count of the first data.frame using factor f1 returns the kind  
> of results I am looking for:
>
> count <- as.data.frame(table(df$f1))
> count
>
> #  Var1 Freq
> #1    a    6
> #2    b    6
>
> # As does a count using factor2:
>
> count2 <- as.data.frame(table(df$f2))
> count2
>
> #  Var1 Freq
> #1    a    6
> #2    b    6
>
> # The same procedure on the second dataframe does not treat the  
> levels of factor f3 discreetly, instead giving all possible  
> combinations of f1 and f2.

We appear to have a different understanding of the term "discrete".  
The interaction function produces all possible combinations of factors  
and then table() counts the occurrences of such.

>
> count3 <- as.data.frame(table(df2$f3))
> count3
>
> #  Var1 Freq
> #1  a.c    6
> #2  b.c    0
> #3  a.d    0
> #4  b.d    6
>
> I need the results to be:
>
> #  Var1 Freq
> #1    a    6
> #2    b    6

Puzzled. You already have such. Why would you want the interaction  
function to behave differently?
Did you just want to create a label from f1 and f2? That can be  
achieved:

 > df2 <- df
 > df2$f12 <- with( df2, paste(f1,f2,sep=".") )
 > df2
    f1 f2           d f12
1   a  c -0.52902802 a.c
2   a  c -1.07351118 a.c
3   a  c  0.63463011 a.c
4   a  c  0.26857599 a.c
5   a  c  1.57677999 a.c
6   a  c  1.08645153 a.c
7   b  d -0.60400852 b.d
8   b  d -0.06611533 b.d
9   b  d  1.00787048 b.d
10  b  d  1.48289305 b.d
11  b  d  0.54658888 b.d
12  b  d -0.67630052 b.d

 > count3 <- as.data.frame(table(df2$f12))
 > count3
   Var1 Freq
1  a.c    6
2  b.d    6





>
> # Any suggestions?
>
> -- 
> Sam Player, B.Sc.(Hons.) B.A.
> Ph.D. Candidate, Faculty of Agriculture, Food & Natural Resources,  
> University of Sydney
>
> Email: splayer at usyd.edu.au
>
> Agroecosystems Research Group
> Room 214 J.R.A. McMillan Building A05
> University of Sydney NSW 2006, Australia
>
> Angkor Research Program
> Room 305 Old Teachers College A22
> University of Sydney NSW 2006, Australia
>
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> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

David Winsemius, MD
Heritage Laboratories
West Hartford, CT