[R] Help with Mahalanobis

[Jose Claudio Faria]

Well, as I did not get a satisfactory reply to the original question I tried to 
make a basic function that, I find, solve the question.

I think it is not the better function, but it is working.

So, perhaps it can be useful to other people.

#
# Calculate the matrix of Mahalanobis Distances between groups
# from data.frames
#
# by: José Cláudio Faria
# date: 10/7/05 13:23:48
#

D2Mah = function(y, x) {

   stopifnot(is.data.frame(y), !missing(x))
   stopifnot(dim(y)[1] != dim(x)[1])
   y    = as.matrix(y)
   x    = as.factor(x)
   man  = manova(y ~ x)
   E    = summary(man)$SS[2] #Matrix E
   S    = as.matrix(E$Residuals)/man$df.residual
   InvS = solve(S)
   mds  = matrix(unlist(by(y, x, mean)), byrow=T, ncol=ncol(y))

   colnames(mds) = names(y)
   Objects       = levels(x)
   rownames(mds) = Objects

   library(gtools)
   nObjects = nrow(mds)
   comb     = combinations(nObjects, 2)

   tmpD2 = numeric()
   for (i in 1:dim(comb)[1]){
     a = comb[i,1]
     b = comb[i,2]
     tmpD2[i] = (mds[a,] - mds[b,])%*%InvS%*%(mds[a,] - mds[b,])
   }

   # Thanks Gabor for the below
   tmpMah = matrix(0, nObjects, nObjects, dimnames=list(Objects, Objects))
   tmpMah[lower.tri(tmpMah)] = tmpD2
   D2 = tmpMah + t(tmpMah)
   return(D2)
}

#
# To try
#
D2M = D2Mah(iris[,1:4], iris[,5])
print(D2M)

Thanks all for the complementary aid (specially to Gabor).

Regards,
-- 
Jose Claudio Faria
Brasil/Bahia/UESC/DCET
Estatistica Experimental/Prof. Adjunto
mails:
  joseclaudio.faria at terra.com.br
  jc_faria at uesc.br
  jc_faria at uol.com.br
tel: 73-3634.2779