[R] indexing via a character matrix?

[Roger Levy]

Prof Brian Ripley wrote:
> On Mon, 13 Feb 2006, Roger Levy wrote:
> 
>> Hi,
>>
>> Is it possible to index via a character matrix?  For example, I would
>> like to do the following:
>>
>> cont.table <- table(df1$A,df1$B) # df1 a data frame with factors A and B
>> cont.table[cbind(df2$A,df2$B)]   # df2 a smaller data frame with some
>>                                  # pairings of values for A and B
>>
>> but the second step does not work -- I guess that matrices to be used
>> for indexing this way must be numeric.  Is there a way to index multiple
> 
> Numeric or logical.
> 
>> character tuples out of a contingency table without resorting to writing
>> loops?
> 
> What are you trying to do here?  One possibility is that you meant
> 
> comt.table[df2$A, df2$B]
> 
> and another is
> 
> ind1 <- match(df2$A, df1$A)
> ind2 <- match(df2$B, df1$B)
> cont.table[cbind(ind1, ind2)]
> 
> and I am not certain which.
> 

Hmm, I think neither one is what I wanted.  After thinking more about 
it, I think what I would need is

ind1 <- match(df2$A,levels(df1$A))
ind2 <- match(df2$B,levels(df2$B))
cont.table[cbind(ind1,ind2)]

Does this make sense?  Is there a more natural way to express this?

Many thanks,

Roger