[R] apply on large arrays

[Erich Neuwirth]

I have a big contingency table, approximately of size 60*2*500*500,
and I need to count the number of cells containing a count of 1 for each 
of the factors values defining the first dimension.
Here is my attempt:

tab1<-with(pisa1,table(CNT,GENDER,ISCOF,ISCOM))
tab2<-apply(tab1,1:4,function(x)ifelse(sum(x)==1,1,0))
tab3<-apply(tab2,1,sum)

Computing tab2 is very slow.
Is there a faster and/or more elegant way of doing this?
-- 
Erich Neuwirth, University of Vienna
Faculty of Computer Science
Computer Supported Didactics Working Group
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