[R] Counting observations of a combined factor

[Sam Player]

#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?

-- 
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

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