[R] indexing via a character matrix?

[Prof Brian Ripley]

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.

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
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