Pre and post condition rules
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Pre and Post Condition Rules. Definition : If R and S are two assertions, then R is said to be stronger than S if R -> S (R implies S). Example : the assertion i < 0 is stronger than the assertion i < 3 because i < 0 implies that i < 3 ( i < 0 is true implies that i < 3 is true)

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Pre and Post Condition Rules

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Pre and post condition rules

Pre and Post Condition Rules

  • Definition : If R and S are two assertions, then R is said to be stronger than S if R -> S (R implies S).

    • Example :

      • the assertion i < 0 is stronger than the assertion i < 3 because i < 0 implies that i < 3 ( i < 0 is true implies that i < 3 is true)

      • draw the set and subset diagram to get a better “feel,” if needed

    • Note that if R is stronger than S, then all states that satisfy R will satisfy S. But there is at least one state that satisfies S which will not satisfy R. So the number of states that satisfies S is larger than that of R. Thus one may view the notion of “stronger” as “more selective” because less states will satisfy the stronger condition.

    • Of course, if R is strongerthan S, then S is weaker than R


Precondition strengthening

Precondition Strengthening

  • Rule 1: If P’ is stronger than P and if {P}C{Q} triple is correct, then with the strengthened precondition assertion, {P’}C{Q} triple is also correct

    • Example :

      • if {P}C{Q} is correct, for P which is (x>0), then for P’, which asserts (x>2), the triple {P’}C{Q} will also be correct.

  • Rule 1 - More formally :

    • P’ -> P (strengthening p to p’)

    • {P}C{Q}

    • {P’}C{Q}

  • Example : if {x <5} x:= x+1 {x < 6} is correct, then strengthening {P} to {x < 3} should give us {x<3} x:=x+1 {x<6} as also correct because :

    • {x<3} -> {x<5}

    • {x<5} x := x+1 {x<6}

    • {x<3} x := x+1 {x<6}


Post condition weakening

Post Condition Weakening

  • Rule 2: If Q -> Q’ and {P}C{Q} triple is correct then {P}C{Q’} triple is correct

    • Rule 2 : Formally we have:

      • {P}C{Q}

      • Q -> Q’

      • {P}C{Q’}

    • Example :

      • if { } max := b {max=b} then show { } max:=b {max >= b}

        { } max := b {max =b}

        {max = b} -> {max >= b}

        { } max:=b {max >=b }


Conjunction and and disjunction or rules

Conjunction (AND) and Disjunction (OR) Rules

  • Rule 3: If C is a piece of code, {P}C{Q} AND {P’}C{Q’} are correct (note that both of the conditions have to be True simultaneously), then {P AND P’} C {Q AND Q’} is also correct

    • Formally :

      • {P} C {Q}

      • {P’} C {Q’}

      • {P AND P’} C {Q AND Q’}

  • Rule 4:If C is a piece of code, {P}C{Q} AND {P’}C{Q’}, then {P OR P’} C {Q OR Q’} is also correct

    • Formally :

      • {P} C {Q}

      • {P’} C {Q’}

      • {P OR P’} C {Q OR Q’}


Example using conjunction rule more

Example using Conjunction Rule & more

  • Problem : given the following Hoare Triples:

    • { } x:= x+1 { x = x+1 } AND [note that x:= x + 1 has multiple meanings here !]

    • {x>0} x:= x +1 {x > 0}

    • show that {x>0} x:=x+1 {x > -1}

  • Proof : (a little more detailed than needed)

    • a) using conjunction rule, we get {x>0} x:= x+ 1{x=x+1 AND x>0}

    • b) using the weakening the post-condition rule, we have {x=x+1 and x>0} -> {x>0}; thus the conjunction triple {x>0} x:=x+1 {x=x+1 AND x>0} also imply {x>0} x:=x+1 {x>0} is correct by weakening the post condition

      • Furthermore {x > 0 } -> { x > - 1} (or x > 0 is stronger than x > -1)

    • Therefore we have {x>0} x := x+1 {x>-1} (by weakening the post condition)

    • Alternatively : note that the weakening of post condition can be achieved through dropping x=x+1 and also directly weakening x > 0 to x > -1.


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