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## PowerPoint Slideshow about 'Relations' - tolla

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- A binary relationfrom A to B is a subset of A x B
- A binary relation on A is a subset of A x A
- A binary relation is defined by
- Enumerating elements
- Relations definition: x r y x + y is odd
- Binary relations can be
- one-to-one
- one-to-many
- many-to-one
- many-to-many
- An n-ary relation on S1,S2,…,Sn is a subset of S1 x S2 x S3 x … x Sn
- Si are called the domains
- n is called the degree

- Let r be a binary relation on set S

Don’t confuse antisymmetric with “not symmetric”!

Likewise irreflexive and “not reflexive”

Relations built from Relations (1)

- Closure
- Definition: A binary relation r* on a set S is theclosure of a relation r on S with respect to property p if
- r* has property p
- r r*
- r* is a subset of every other relation on S that includes r and has property p.
- Composite
- Definition: Let r be a relation from A to B and s be a relation from B to C. The composite (r s) is the relation consisting of ordered pairs (a,c) where a A, c C, and for which there exists an element b B such that (a,b) r and (b,c) s.
- Definition: Let r be a relation on set A. The powers rn, n = 1,2, … are define recursively by
- r1 = r
- ri+1 = ri r

difference?

- Relations built from Relations (2)
- Example: Let r be the relation on the set of all people in the world that contains (a,b) if ahas metb.
- What is rn?
- Those pairs (a,b) such that there are people x1,x2,…,xn-1 such that a has met x1, x1 has met x2, …, and xn-1 has met b.
- What is r*?
- Those pairs (a,b) such that there is a sequence of people, starting with a and ending with b, in which each person in the sequence has met the next person in the sequence.

Who cares? Wait for graphs!

- Partial Ordering
- Definition: A relation r on a set S is a partial ordering if it is reflexive, antisymmetric, and transitive.
- (S, r) is called a partially ordered set or poset
- The elements a and b of a poset (S, r) are called comparable if either (a,b) r or (b,a) r
- Strict Partial Ordering
- Definition: A relation r on a set S is a strict partial ordering if it is irreflexive, antisymmetric, and transitive.
- Total Ordering
- Definition: A relation r on a set S is a total ordering if it is (S,r) is a poset and every two elements of S are comparable.
- Strict Total Ordering
- Definition: A relation r on a set S is a strict total ordering if it is (S,r) is a strict poset and every two elements of S are comparable.

- Equivalence Relations
- Definition: A relation r on a set S is an equivalence relation if it is reflexive, symmetric, and transitive.
- Two elements related by an equivalence relation are said to be equivalent
- The set of all elements that are related to an element a of S is called the equivalence class of a.
- A partition of a set is a collection of disjoint nonempty subsets of S such that they have S as their union. The equivalence classes of r form a partition of S.

Application: Relation Representation

- Enumeration
- list the ordered pairs
- Zero-One Matrix
- Suppose r is a relation from A {a1,a2,…,am} to B {b1,b2,…,bn}
- r can be represented by matrix Mr = [mij] where
- Digraph
- A relation r on a set S is represented by a directed graph (digraph) that has the elements of S as it vertices and the ordered pairs (a,b) where (a,b) r, as edges.
- So how do we represent digraphs in a computer? Later…

Application: Warshall’s Algorithm (1)

- Stephen Warshall circa 1960
- Algorithm to find the transitive closure of a set S
- transitive closures are particularly interesting in that they provide “connection” information
- Suppose r is a relation on S with n elements
- Let a1, a2, …, an be an arbitrary listing of those elements
- If a,x1,x2,…,xm-1,b is a sequence in the transitive closure, then the xis are called the interior elements of the sequence.
- Warshall’s algorithm is based on the construction of a series of zero-one matrices (W0,W1, …, Wn) where
- where
- there is a sequence from xi to xj using only interior elements {x1,…,xk}

Note: Wn = Mr*

Application: Warshall’s Algorithm (2)

- Example

W0 is the matrix of the relation.

W1 has a 1 as its (i,j)th entry if there is

a sequence from vi to vj moving through

only v1.

Since no edges go into v2, W2 is the same as W1.

W3 has a 1 as its (i,j)th entry if there is

a sequence from vi to vj moving through

only v1, v2, or v3.

W4 has a 1 as its (i,j)th entry if there is

a sequence from vi to vj moving through

only v1, v2, v3, or v4.

Application: Warshall’s Algorithm (3)

- How do we calculate the Wis?
- We can compute Wk directly from Wk-1
- Adding vk to Wk-1 can do one of two things:
- Leave a sequence untouched (can’t use vk)
- Wk at (i,j) is 1 only if Wk-1 at (i,j) is a 1
- Add a sequence from vi to vk to vj
- Wk at (i,j) is 1 only if Wk-1 at (i,k) is 1 and Wk-1 at (k,j) is 1
- Algorithm
- W = Mr
- for k = 1 to n
- for i = 1 to n
- for j = 1 to n
- wij = wij (wik wkj)

Application: Relational Databases (1)

- Recall from CS 185
- E-R Modeling
- Attributes
- One-to-One, One-to-many, Many-to-one, and Many-to-Many
- Both “Entity Sets” and “Relations” in Databases are relations in the mathematical sense
- Table is a set of n-tuples (rows)
- No duplicates and No order
- a table is a subset of D1 x D2 x … x Dn where Di is the domain from which attribute Ai takes its value
- therefore a table is an n-ary relation on Dis
- E-R Relations have Di in one table the same as Di for the primary key of another
- Joins the attributes into a new cross-product
- therefore a relation is an m-ary relation on Dis

Application: Relational Databases (2)

- Operations on Relations
- restrict
- Let r be an n-ary relation and c a condition that elements of r must satisfy. Then the restrict operator rc maps the n-ary relation r to the n-ary relation of all n-tuples from r that satisfy the condition c.
- leads to the SQL “where” clause
- project
- The projection Pi1,i2,…,im maps the n-tuple (a1,a2, …,an) to the m-tuple (ai1,ai2,…,aim) where m n.
- leads to the SQL “select” clause
- join
- Let r be a relation of degree m and s a relation of degree n. The join jp(r,s), where p m and p n, is a relation of degree m + n – p that consists of all (m + n – p)-tuples (a1,a2,…,am-p,c1,c2,…,cp,b1,b2,…,bn-p) where the m-tuple (a1,a2,..,am-p,c1,c2,…,cp) r and the n-tuple (c1,c2,…,cp,b1,b2,…,bn-p) s.
- leads to the SQL “from a,b,…,c” clause

- Java Collections Framework provides a collection of container classes
- Example: HashMap, HashSet, …
- Some collections are ordered
- Example: TreeSet, …
- How does Java order the items in the collection?
- By use of the compareTo(Object obj) method
- By definition, compareTo(Object obj) must define a strict total ordering of all elements in the container
- compareTo(Object obj) must meet
- x.compareTo(y) == -1 * y.compareTo(x)
- x.compareTo(y) == y.compareTo(z) == x.compareTo(z)
- x.equals(y) x.compareTo(y) == 0
- failure to meet these requirements will result in unexpected behavior
- for example, Sets with duplicate objects!

antisymmetric

transitive

irreflexive

Application: equals in Java (1)

- According to Java API “The equals method implements an equivalence relation on non-null object references”
- Therefore a.equals(b) must behave the same as b.equals(a)
- Most implementations fail on this property (1)
- class A {
- private int x;
- public boolean equals(Object that) { boolean isEqual = false; if ((that != null) && (that instanceof A)) { A castedThat = (A) that; // perform comparisons on private data isEqual = (this.x == castedThat.x); } return isEqual; }
- }

Reflexive

Symmetric

Transitive

Application: equals in Java (2)

- Most implementations fail on this property (2)
- class B extends A {
- private int y;
- public boolean equals(Object that) { boolean isEqual = false; if ((that != null) && (that instanceof B)) { B castedThat = (B) that; // perform comparisons on private data isEqual = (this.y == castedThat.y); } return (isEqual && super.equals(that)); }
- }

instanceA.equals(instanceB) would return true, but instanceB.equals(instanceA)

would fail the instanceof test and return false!

Application: equals in Java (3)

- Correct Definition (1)
- abstract class T { public final boolean equals(Object that) { boolean isEqual = false; if ((that != null) && (that instanceof T)) { T castedThat = (T) that; if (this.getTypeEquiv().equals( castedThat.getTypeEquiv())) { isEqual = localEquals(that); } } return isEqual; } protected boolean localEquals(Object that) { return true; // to stop the chaining }abstract protected Class getTypeEquiv();
- }

Top of hierarchy!

Application: equals in Java (4)

- Correct Definition (2)
- class A extends T {private int x;
- protected boolean localEquals(Object that) { A castedThat = (A) that;// perform comparisons on private data boolean isEqual = (this.x == castedThat.x); return (isEqual && super.localEquals(that)); } protected Class getTypeEquiv() { Class result = null; try { // will never fail, but must try/catch result = Class.forName(“A”); } catch (ClassNotFoundExeception e) { } return result; }
- }

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