R help - Aug 2005 - how to (combine / recode / merge) two factor.

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

interaction(df$f1, df$f2)

?interaction

laurent buffat wrote:
> 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
> 
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