Hello, R users.
I would like to count the number of triples (r_i, s_j, t_k) with r_i,
s_j, t_k distinct and abs((r_i-t_k)-u)=0 and abs((s_j-t_k)-v)=0, where
r_i, s_j, t_k are the elements of three vectors a,b,c with different
lengths and u,v=1:n. I have solved this problem writing a subroutine in
Fortran an calling .Fortran(). I would like to find another way to use
functions like mapply. I tried this one:
xx <- mapply( function(u,v) {
kk <- outer(a, c, function(s,t) abs((s-t)-u)==0)
ll <- outer(b, c, function(s,t) abs((s-t)-v)==0)
sum( outer( which(kk==TRUE, TRUE)[,2], which(ll==TRUE, TRUE)[,2],
function(s,t) s==t) ) },
rep(1:n, each=n), rep(1:n, times=n) )
xx <- matrix(xx, nrow=n, ncol=n, byrow=TRUE)
It works but it is rather slow. Taking into account that my vectors have
lengths 3000, and n is from 50 to 200, can I do something to improve the
running time of the above code?
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