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Representing Relations. Rosen, section 7.3 CS/APMA 202 Aaron Bloomfield. In this slide set…. Matrix review Two ways to represent relations Via matrices Via directed graphs. Matrix review. This is from Rosen, page 201 and 202 We will only be dealing with zero-one matrices

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representing relations

Representing Relations

Rosen, section 7.3

CS/APMA 202

Aaron Bloomfield

in this slide set
In this slide set…
  • Matrix review
  • Two ways to represent relations
    • Via matrices
    • Via directed graphs
matrix review
Matrix review
  • This is from Rosen, page 201 and 202
  • We will only be dealing with zero-one matrices
    • Each element in the matrix is either a 0 or a 1
  • These matrices will be used for Boolean operations
    • 1 is true, 0 is false
matrix transposition
Matrix transposition
  • Given a matrix M, the transposition of M, denoted Mt, is the matrix obtained by switching the columns and rows of M
  • In a “square” matrix, the main diagonal stays unchanged
matrix join
Matrix join
  • A join of two matrices performs a Boolean OR on each relative entry of the matrices
    • Matrices must be the same size
    • Denoted by the or symbol: 
matrix meet
Matrix meet
  • A meet of two matrices performs a Boolean AND on each relative entry of the matrices
    • Matrices must be the same size
    • Denoted by the or symbol: 
matrix boolean product

Matrix Boolean product
  • A Boolean product of two matrices is similar to matrix multiplication
    • Instead of the sum of the products, it’s the conjunction (and) of the disjunctions (ors)
    • Denoted by the or symbol: 
relations using matrices
Relations using matrices
  • List the elements of sets A and B in a particular order
    • Order doesn’t matter, but we’ll generally use ascending order
  • Create a matrix
relations using matrices1
Relations using matrices
  • Consider the relation of who is enrolled in which class
    • Let A = { Alice, Bob, Claire, Dan }
    • Let B = { CS101, CS201, CS202 }
    • R = { (a,b) | person a is enrolled in course b }
relations using matrices2
Relations using matrices
  • What is it good for?
    • It is how computers view relations
      • A 2-dimensional array
    • Very easy to view relationship properties
  • We will generally consider relations on a single set
    • In other words, the domain and co-domain are the same set
    • And the matrix is square
reflexivity
Reflexivity
  • Consider a reflexive relation: ≤
    • One which every element is related to itself
    • Let A = { 1, 2, 3, 4, 5 }

If the center (main) diagonal is all 1’s, a relation is reflexive

irreflexivity
Irreflexivity
  • Consider a reflexive relation: <
    • One which every element is not related to itself
    • Let A = { 1, 2, 3, 4, 5 }

If the center (main) diagonal is all 0’s, a relation is irreflexive

symmetry
Symmetry
  • Consider an symmetric relation R
    • One which if a is related to b then b is related to a for all (a,b)
    • Let A = { 1, 2, 3, 4, 5 }
  • If, for every value, it is the equal to the value in its transposed position, then the relation is symmetric
asymmetry
Asymmetry
  • Consider an asymmetric relation: <
    • One which if a is related to b then b is not related to a for all (a,b)
    • Let A = { 1, 2, 3, 4, 5 }
  • If, for every value and the value in its transposed position, if they are not both 1, then the relation is asymmetric
  • An asymmetric relation must also be irreflexive
  • Thus, the main diagonal must be all 0’s
antisymmetry
Antisymmetry
  • Consider an antisymmetric relation: ≤
    • One which if a is related to b then b is not related to a unless a=b for all (a,b)
    • Let A = { 1, 2, 3, 4, 5 }
  • If, for every value and the value in its transposed position, if they are not both 1, then the relation is antisymmetric
  • The center diagonal can have both 1’s and 0’s
transitivity
Transitivity
  • Consider an transitive relation: ≤
    • One which if a is related to b and b is related to c then a is related to c for all (a,b), (b,c) and (a,c)
    • Let A = { 1, 2, 3, 4, 5 }
  • If, for every spot (a,b) and (b,c) that each have a 1, there is a 1 at (a,c), then the relation is transitive
  • Matrices don’t show this property easily
2004 ioccc winners
2004 IOCCC winners
  • 2004 winners:
    • 2004 anonymous Rendering of a stroked font
    • 2004 arachnid Curses maze displayer/navigator with only line-of-sight visibility
    • 2004 burley A Poker game
    • 2004 gavare A ray tracer
    • 2004 gavin Mini-OS
    • 2004 hibachi A CGI capable HTTP server
    • 2004 hoyle Curses based polynomial graphing with auto-scale
    • 2004 jdalbec Conway's look'n'say sequence split into elements
    • 2004 kopczynski OCR of 8, 9, 10 and 11
    • 2004 newbern Renders arbitary bitmapped fonts
    • 2004 omoikane A CRC inserter
    • 2004 schnitzi Editor animation
    • 2004 sds Space/tab/linefeed steganography
    • 2004 vik1 X Windows car racing game
    • 2004 vik2 Calculates prime numbers using only CPP
  • At http://www1.us.ioccc.org/years.html#2004
combining relations via boolean operators
Combining relations: via Boolean operators
  • Example 4 from Rosen, section 7.3
  • Let:
  • Join:
  • Meet:
combining relations via relation composition
Combining relations: via relation composition
  • Example 4 from Rosen, section 7.3
  • Let:
  • But why is this the case?

d e f

g h i

a

b

c

d

e

f

g h i

a

b

c

representing relations using directed graphs

1

2

3

4

1

2

3

4

1

2

3

4

Representing relations using directed graphs
  • A directed graph consists of:
    • A set V of vertices (or nodes)
    • A set E of edges (or arcs)
    • If (a, b) is in the relation, then there is an arrow from a to b
  • Will generally use relations on a single set
  • Consider our relation R = { (a,b) | a divides b }
  • Old way:
reflexivity1

2

1

5

3

4

Reflexivity
  • Consider a reflexive relation: ≤
    • One which every element is related to itself
    • Let A = { 1, 2, 3, 4, 5 }

If every node has a loop, a relation is reflexive

irreflexivity1

2

1

5

3

4

Irreflexivity
  • Consider a reflexive relation: <
    • One which every element is not related to itself
    • Let A = { 1, 2, 3, 4, 5 }

If every node does not have a loop, a relation is irreflexive

symmetry1

2

1

5

3

4

Symmetry
  • Consider an symmetric relation R
    • One which if a is related to b then b is related to a for all (a,b)
    • Let A = { 1, 2, 3, 4, 5 }
  • If, for every edge, there is an edge in the other direction, then the relation is symmetric
  • Loops are allowed, and do not need edges in the “other” direction

Called anti-

parallel pairs

Note that this relation is neither reflexive nor irreflexive!

asymmetry1

2

1

5

3

4

Asymmetry
  • Consider an asymmetric relation: <
    • One which if a is related to b then b is not related to a for all (a,b)
    • Let A = { 1, 2, 3, 4, 5 }
  • A digraph is asymmetric if:
  • If, for every edge, there is not an edge in the other direction, then the relation is asymmetric
  • Loops are not allowed in an asymmetric digraph (recall it must be irreflexive)
antisymmetry1

2

1

5

3

4

Antisymmetry
  • Consider an antisymmetric relation: ≤
    • One which if a is related to b then b is not related to a unless a=b for all (a,b)
    • Let A = { 1, 2, 3, 4, 5 }
  • If, for every edge, there is not an edge in the other direction, then the relation is antisymmetric
  • Loops are allowed in the digraph
transitivity1

2

1

5

3

4

Transitivity
  • Consider an transitive relation: ≤
    • One which if a is related to b and b is related to c then a is related to c for all (a,b), (b,c) and (a,c)
    • Let A = { 1, 2, 3, 4, 5 }
  • A digraph is transitive if, for there is a edge from a to c when there is a edge from a to b and from b to c
applications of digraphs mapquest

End

Start

Applications of digraphs: MapQuest
  • Not reflexive
  • Is irreflexive
  • Not symmetric
  • Not asymmetric
  • Not antisymmetric
  • Not transitive
  • Not reflexive
  • Is irreflexive
  • Is symmetric
  • Not asymmetric
  • Not antisymmetric
  • Not transitive
rosen questions 31 32 section 7 3
Rosen, questions 31 & 32, section 7.3

Which of the graphs are reflexive, irreflexive, symmetric, asymmetric, antisymmetric, or transitive

rosen section 7 1 sic question 45 a
Rosen, section 7.1 (sic) question 45 (a)
  • How many symmetric relations are there on a set with n elements?
  • Solution guide explanation is pretty poorly worded
  • So instead we’ll use matrices
rosen section 7 1 sic question 45 a1
Rosen, section 7.1 (sic) question 45 (a)
  • Consider the matrix representing symmetric relation R on a set with n elements:
  • The center diagonal can have any values
  • Once the “upper” triangle is determined, the “lower” triangle must be the transposed version of the “upper” one
  • How many ways are there to fill in the center diagonal and the upper triangle?
  • There are n2 elements in the matrix
  • There are n elements in the center diagonal
    • Thus, there are 2n ways to fill in 0’s and 1’s in the diagonal
  • Thus, there are (n2-n)/2 elements in each triangle
    • Thus, there are ways to fill in 0’s and 1’s in the triangle
  • Answer: there are possible symmetric relations on a set with n elements
quick survey
Quick survey
  • I felt I understood the material in this slide set…
  • Very well
  • With some review, I’ll be good
  • Not really
  • Not at all
quick survey1
Quick survey
  • The pace of the lecture for this slide set was…
  • Fast
  • About right
  • A little slow
  • Too slow
quick survey2
Quick survey
  • How interesting was the material in this slide set? Be honest!
  • Wow! That was SOOOOOO cool!
  • Somewhat interesting
  • Rather borting
  • Zzzzzzzzzzz
biggest software errors
Biggest software errors
  • Ariane 5 rocket explosion (1996)
    • Due to loss of precision converting 64-bit double to 16-bit int
  • Pentium division error (1994)
    • Due to incomplete look-up table (like an array)
  • Patriot-Scud missile error (1991)
    • Rounding error on the time
    • The missile did not intercept an incoming Scud missile, leaving 28 dead and 98 wounded
  • Mars Climate Orbiter (1999)
    • Onboard used metric units; ground computer used English units
  • AT&T long distance (1990)
    • Wrong break statement in C code
  • Therac-25, X-ray (1975-1987)
    • Badly designed software led to radiation overdose in chemotherapy patients
  • NE US power blackout (2003)
    • Flaw in GE software contributed to it
  • References: http://www5.in.tum.de/~huckle/bugse.html, http://en.wikipedia.org/wiki/Computer_bug, http://www.cs.tau.ac.il/~nachumd/verify/horror.html