1

Assume you have the following definition:

abstract class IntSet {
  def incl(x: Int): IntSet    
  def contains(x: Int): Boolean    
  def union(other: IntSet): IntSet
}

case class NonEmpty(elem: Int, left: IntSet, right: IntSet) extends IntSet {
  def incl(x: Int) =
    if (x < elem) NonEmpty(elem, left incl x, right)
    else if (x > elem) NonEmpty(elem, left, right incl x)
    else this

  def contains(x: Int) =
    if (x < elem) left contains x
    else if (x > elem) right contains x
    else true

  def union(other: IntSet) = (left union (right union other)) incl elem
}

object Empty extends IntSet {
  def incl(x: Int) = NonEmpty(x, Empty, Empty)    
  def contains(x: Int) = false    
  def union(other: IntSet) = other
}

and the following proposition has to be proven:

(xs union ys) contains x = xs contains x || ys contains x

From here I deduce two base cases. xs = Empty and ys = Empty. It is the second base case where I got stuck because of the following reasoning:

// substituting ys = Empty
 (xs union Empty) contains x = xs contains x || Empty contains x
// RHS:
 xs contains x || false
 xs contains x
// LHS:
 ((left union (right union Empty)) incl elem) contains x // By definition of NonEmpty.union

How can I reduce the LHS to xs contains x? Do I have to do another Induction Hypothesis on xs union Empty = xs and if so, how can that be used to the expression?

Salailah
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1 Answers1

2

To Prove:

(xs union ys) contains x = xs contains x || ys contains x

Given:

  1. Definitions above
  2. Empty contains x = false
  3. (s incl x) contains x = true
  4. (s incl y) contains x = s contains x ; if x != y

Induction Step and Structural Proof using xs only, not ys, please refer to the original problem

Case I:

if xs = Empty

Left Hand Side:
(Empty union ys) contains x
= ys contains x => Definition of Empty union other

Right Hand Side:
Empty contains x || ys contains x
= false || ys contains x => Definition of Empty contains x
= ys contains x => Truth table of OR ( false OR true = true, false OR false = false)

Left Hand Side = Right Hand Side hence Induction step proved, the statement is true for all subtrees

Case II:
if (xs is NonEmpty(z, l, r) and z = x)

Left Hand Side:
(NonEmpty(x, l, r) union ys) contains x
= ((l union(r union ys)) incl x) contains x => From definition of union on NonEmpty
= true => from (3) above (s incl x) contains x = true

Right Hand Side:
xs contains x || ys contains x
= NonEmpty(x, l, r) contains x || ys contains x => From definition of xs
= true || ys contains x => From definition of contains on NonEmpty
= true => Truth table of OR (true OR false = true, true OR true = true)

Left Hand Side = Right Hand Side

Case III:
if ( xs is NonEmpty(z, l, r) and z < x )

Left Hand Side:
(NonEmpty(z, l, r) union ys) contains x
= (l union(r union ys) incl z) contains x => From definition of union
= (l union(r union ys)) contains x => From (4) above (s incl x) contains y = s contains y; if x != y
= (l contains x || (r union ys) contains x) => From induction step, the statement is true for all subtrees
= (l contains x || r contains x || ys contains x) => From induction step, the statement is true for all subtrees
= r contains x || ys contains x => since z < x, definition of contains (l contains x) returns false

Right Hand Side:
(if z < x)
NonEmpty(z, l, r) contains x || ys contains x
= r contains x || ys contains x => definition of contains,

Left Hand Side = Right Hand Side,

The case if ( xs is NonEmpty(z, l, r) and z > x ) is analogous to Case III above Hence proved

Hitesh Shetty
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