Dear Alberto,
Try this:

# colnames and rownames
cn <- colnames(a)
rn <- rownames(a)

# the index
ind <- which(a==0, arr = TRUE)

# the result
data.frame( res = apply(ind, 1, function(x)
                        paste(rn[x[1]],",", cn[x[2]], sep ="")
                                      )
            )

#   res
# 1 A1,F1
# 2 A3,F1
# 3 A2,F2
# 4 A3,F3

See ?which, ?apply and ?paste for more information.

HTH,

Jorge


On Wed, Jul 15, 2009 at 7:15 PM, Alberto Lora M <albertoloram@gmail.com>wrote:

> Dear sir,
>
> I have a matrix like
>
> a<-matrix(c(0,2,0,4,0,6,5,8,0),nrow=3)
> colnames(a)<-c("F1","F2","F3")
> rownames(a)<-c("A1","A2","A3")
> a
>
>   F1 F2 F3
> A1  0  4  5
> A2  2  0  8
> A3  0  6  0
>
> I want to extract all pairs (rownames, columnames) from which the value in
> the matrix is 0
>
> The result should be something like this
>
> A1, F1
> A2, F2
> A3, F1
> A3, F3
>
> how it is possible?
>
> thanks for your help....
>
> Best Regards
>
> Alberto
>
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>
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