any() and all() on empty list?
Paul Rubin
http
Wed Mar 29 09:15:03 CEST 2006
"Paul McGuire" <ptmcg at austin.rr._bogus_.com> writes:
> > Usually, "for all X in S, PRED(x) is true" means:
> > there does not exist X in S so that PRED(x) is false.
> >
> How do you get this "usually" stuff? I would agree that this is usually
> implemented as a short-circuited loop through the list, that breaks out at
> the first False value. But I would not be quick to equate "commonality of
> implementation" with "meaning".
See <http://en.wikipedia.org/wiki/For_all>:
Generally, then, the negation of a propositional function's universal
quantification is an existential quantification of that propositional
function's negation; symbolically,
\lnot\ \forall{x}{\in}\mathbf{X}\, P(x) \equiv\
\exists{x}{\in}\mathbf{X}\, \lnot\ P(x)
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