[R] how to (combine / recode / merge) two factor.

[laurent buffat]

Hi,
I have a data frame with two factors, and I would like to create a new 
one by "combining" them.
I have already a solution, but it is very "heavy", and I'm sure there is 
a basic function which can do the same.
I tried to find it in the R-help, but without result.

See the example :

# just to create a dataframe
f1 <- factor(rep(c("A","B"),3))
f2 <- factor(rep(c("x","y","z"),each=2))
df <- data.frame(f1,f2)
df <- rbind(df,df,df)

# here is what I want do do :
df$f1f2 <- 
factor(as.numeric(df$f1)+(as.numeric(df$f2)-1)*nlevels(df$f1), labels= 
letters[1:(nlevels(df$f1)*nlevels(df$f2))])
# I don't care about the labels.

df :
 > df
   f1 f2 f1f2
1   A  x    a
2   B  x    b
3   A  y    c
4   B  y    d
5   A  z    e
6   B  z    f
11  A  x    a
21  B  x    b
31  A  y    c
41  B  y    d
51  A  z    e
61  B  z    f
12  A  x    a
22  B  x    b
32  A  y    c
42  B  y    d
52  A  z    e
62  B  z    f
 >


Thanks for your helps,

Laurent Buffat