Programming challenge: wildcard exclusion in cartesian products

wkehowski at cox.net wkehowski at cox.net
Thu Mar 16 20:20:42 CET 2006


What I have in mind is the efficient, <enumerated> generation of the
complement S^n/WC(S^n). A good program should initialize, generate, and
terminate.

T=cartprodex(S,n,WC); //initialize
for all i in T do
  what you want with i
  test to see if any more
  terminate if not

and it should do this without explicitly generating WC and then
complementing. For example, if the cardinality of S is m, and the WC is
just '*a*b*', with a != b, then EX(S^n):=S^n\WC(S^n) has cardinality
(m-1)^(n-1)*(m+n-1). Specifically, if m=5 and n=10, then |EX|=3670016
while |S^10|=9765625, so that |EX|/|S^10| is about 0.3758. In general
the program should directly generate EX from arbitrary WC. Of course,
in practice the WC should themselves occur in a logically consistent
manner, but let's just assume they're a given.




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