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Today's Topics:
1. Haskell Help (Andrei M.)
----------------------------------------------------------------------
Message: 1
Date: Thu, 10 Feb 2011 17:24:44 +0000 (GMT)
From: "Andrei M." <[email protected]>
Subject: [Haskell-beginners] Haskell Help
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset="iso-8859-1"
I need to write two functions for a coursework that will behave as follows...
unction 1)removeTautologies :: Formula->Formula
If in a clause, a literal and its negation are found, it means that the clause
will be true, regardless of the value
finally assigned to that propositional variable. Consider the following example:
(A v B v -A) ^ (B v C v A)
The first clause contains the literals A and -A. This means that the clause
will always be true, in which case
it can be simplify the whole set to simply (B v C v A) .
I was tinking of using something like
?removeTautologies (f:fs)=filter rTf:removeTautologies fs
?????
where rT-is supposed to take the firs Literal from the clasue and
search for a similar one,if one si found we compare the values if not
the we go to the second literal.
->Function 2)pureLiteralDeletion :: Formula->Formula
This is a little bit complicate but from What I get this function is suppose to
implement a simplification step that assumes
as true any atom in a formula that appears exclusively in a positive or
negative form (not both). Consider the formula:
(P v Q v R) ^ (P v Q v -R) ^ (-Q v R)
Note that in this formula P is present but -P is not. Using Pure Literal
Deletion it can be assumed that the value of P will be True thus
simplifying the formula to (-Q v R). If the literal were false then the
literal would simply be deleted from the clauses it appears in. In that
case any satisfying model for the resulting formula would also be a
satisfying model for the formula when we assume that the literal is true.
removeTautologies :: Formula -> Formula
removeTautologies = map rt.map head.group.sort
???? where rt ((x, x') : (y, y') : rest) | x' == y' = rt rest
????????????????????????????? | otherwise = (x, x') : rt ((y, y') : rest)
pureLiteralDeletion :: Formula -> Formula
pureLiteralDeletion = map lD.map head.group.sort.concat
???? where lD ((x, x') : (y, y') : rest) | x' == y' && x/=y = lD rest
???????????????????????????????????????? | otherwise = (x, x') : lD ((y, y') :
rest)
this is what I've witten so far but they don't wok poperly....any sugestions?
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