[R] Counting occurances of a letter by a factor

[Davis, Brian]

I'm trying to find a more elegant way of doing this.  What I'm trying to accomplish is to count the frequency of letters (major / minor alleles)  in  a string grouped by the factor levels in another column of my data frame.

Ex.
> DF<-data.frame(c("CC", "CC", NA, "CG", "GG", "GC"), c("L", "U", "L", "U", "L", NA))
> colnames(DF)<-c("X", "Y")
> DF
     X    Y
1   CC    L
2   CC    U
3 <NA>    L
4   CG    U
5   GG    L
6   GC <NA>

I have an ugly solution, which works if you know the factor levels of Y in advance.

> ans<-rbind(table(unlist(strsplit(as.character(DF[DF[ ,'Y'] == 'L', 1]), ""))),
+ table(unlist(strsplit(as.character(DF[DF[ ,'Y']  == 'U', 1]), ""))))
> rownames(ans)<-c("L", "U")
> ans
  C G
L 2 2
U 3 1


I've played with table, xtab, tabulate, aggregate, tapply, etc but haven't found a combination that gives a more general solution to this problem.

Any ideas?

Brian