[R] Particular Cross tables

[Prof Brian D Ripley]

On Sat, 18 May 2002, Kenneth Cabrera wrote:

> Dear R experts:
>
> I have this data base (data frame) with three variables
> where f1 and f2 are factors.
> I want to obtain a table with the sum of the third variable
> at each cross element of the two factors.
>
> Would you help me with any function idea?
>
> Original data base (data frame)
>
> f1 f2 v1
> 1   2  10
> 1   1  20
> 1   2  30
> 1   3  40
> 2   1  50
> 2   2  60
> 1   1  70
> 3   1  80
> 3   2  90
> 2   2  10
>
> I would like to obtain this matrix (sum of the crossed elements with
> zeros where I don't have any crossed factor)
>
>                     Factor 1
>                    90  40 40
> Factor 2      50  70  0
>                    80  90  0

> xtabs(v1~f1+f2, tab)
   f2
f1   1  2  3
  1 90 40 40
  2 50 70  0
  3 80 90  0

> attach(tab)
> (res <- tapply(v1, list(f1, f2), sum))
   1  2  3
1 90 40 40
2 50 70 NA
3 80 90 NA
> res[is.na(res)] <- 0
> res
   1  2  3
1 90 40 40
2 50 70  0
3 80 90  0

(You can add names to the dimnames by

> names(dimnames(res)) <- c("factor 1", "factor 2")
> res
        factor 2
factor 1  1  2  3
       1 90 40 40
       2 50 70  0
       3 80 90  0
)


> Is it possible to obtain any kind of function (not only sum())?
> (like sd(), for example?)

In the second form, yes, e.g.

(res <- tapply(v1, list(f1, f2), sd))

but cells with zero and one entries will both be NA, so you may want to use
the counts given by  xbats(~f1+f2, tab) to distringuish them.

Brian

-- 
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 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595


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