[R] Counting observations of a combined factor
Peter Ehlers
ehlers at ucalgary.ca
Sat Sep 19 16:27:31 CEST 2009
Sam,
Depending on what your ultimate aim is, perhaps you just want to add
the 'drop=TRUE' argument to your interaction call.
Peter
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.
>
> 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
>
> # Any suggestions?
>
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