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CS1022 Computer Programming & Principles. Lecture 3.1 Set Theory (1). Plan of lecture. Why set theory? Sets and their properties Membership and definition of sets “Famous” sets Types of variables and sets Sets and subsets Venn diagrams Set operations. Why set theory?.

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CS1022 Computer Programming & Principles

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## CS1022Computer Programming & Principles

Lecture 3.1

Set Theory (1)

### Plan of lecture

• Why set theory?

• Sets and their properties

• Membership and definition of sets

• “Famous” sets

• Types of variables and sets

• Sets and subsets

• Venn diagrams

• Set operations

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### Why set theory?

• Set theory is a cornerstone of mathematics

• Provides a convenient notation for many concepts in computing such as lists, arrays, etc. and how to process these

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### Sets

• A set is

• A collection of objects

• Separated by a comma

• Enclosed in {...} (curly brackets)

• Examples:

• {Edinburgh, Perth, Dundee, Aberdeen, Glasgow}

• {2, 3, 11, 7, 0}

• {CS1015, CS1022, CS1019, SX1009}

• Each object in a set is called an element of the set

• We use italic capital letters to refer to sets:

• C = {2, 3, 11} is the set C containing elements 2, 3 and 11

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### Sets – indices

• Talk about arbitrary elements, where each subscript is a different integer:

• {ai, aj, ..., an}

• Talk about systematically going through the set, where each superscript is a different integer:

• {a1i, a2j, ..., a7n}

• {Edinburgh1, Perth2, Dundee3, Aberdeen4, Glasgow5}

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### Properties of sets

• The order of elements is irrelevant

• {1, 2, 3} = {3, 2, 1} = {1, 3, 2} = {2, 3, 1}

• There are no repeated elements

• {1, 2, 2, 1, 3, 3} = {1, 2, 3}

• Sets may have an infinite number of elements

• {1, 2, 3, 4, ...} (the “...” means it goes on and on...)

• What about {0, 4, 3, 2, ...}?

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### Membership and definition of sets

• Membership of a set

• a S– represents that ais an element of set S

• a  S – represents that ais not an element of set S

• For large sets we can use a property (a predicate!) to define its members:

• S = {x : P(x)} – S contains those values for x which satisfy property P

• N = { x : x is an odd positive integer} = {1, 3, 5, ...}

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### Why set theory?

• Example: check if an element occurs in a collection

search though

collection by

superscript.

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### Names of “famous” sets

• Some sets have a special name and symbol:

• Empty set: has no element, represented as { } or 

• Natural numbers: N = {1, 2, 3, ...}

• Integers: Z = {..., -3, -2, -1, 0, 1, 2, 3, ...}

• Rational numbers: Q = {p/q : p, q  Z, q  0}

• Real numbers: R = {all decimals}

• N.B.: in some texts/books 0 N

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### Types (of variables) and sets

• Many modern programming languages require that variables be declared as belonging to a data type

• A data type is a set with a selection of operations on that set

• Example: type “int” in Java has operations +, *, div, etc.

• When we declare the type of a variable we state what set the value of the variable belongs to and the operations that can be applied to it.

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### Sets and subsets

• Some sets are contained in other sets

• {1, 2, 3} is contained in {1, 2, 3, 4, 5}

• N (natural numbers) is contained in Z (integers)

• Set A is a subset of set B if every element of A is in B

• We represent this as A  B

• Formally,

A  B if, and only if, x ((x  A) (x  B))

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### Venn diagrams

• A diagram to represent sets and how they relate

• A set is represented as an oval, a circle or rectangle

• With or without elements in them

• Venn diagrams show area of interest in grey

• Venn diagram showing a set and a subset

D

C

A

B

3

1

472

2

John Venn

D C

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### Set equality (1)

• Two sets are equal if they have the same elements

• Formally, A and B are equal if A  Band B  A

• That is,

x ((x  A) (x  B)) and y ((y  B) (y  A))

• We represent this as

A = B

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### Set equality (2)

• Let A = {n : n2 is an odd integer}

• Let B = {n : n is an odd integer}

• Show that A = B

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### Set equality (3)

Proof has two parts

• Part 1: all elements of A are elements of B

• Part 2: all elements of B are elements of A

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### Set operations: union (1)

• The union of sets A and B is

A  B = {x : x  Aorx  B}

• That is,

• Those elements belonging to Atogether with

• Those elements belonging to Band

• (Possibly) those elements belonging to both A and B

• N.B.: no repeated elements in sets!!

• Examples:

{1, 2, 3, 4}  {4, 3, 2, 1} = {1, 2, 3, 4}

{a, b, c}  {1, 2} = {a, 1, b, 2, c}

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### Set operations: union (2)

• Venn diagram (area of interest in grey)

B

A

A B

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### Set operations: intersection (1)

• The intersection of sets A and B is

A  B = {x : x  Aandx  B}

• That is,

• Only those elements belonging to both AandB

• Examples:

{1, 2, 3, 4}  {4, 3, 2, 1} = {1, 2, 3, 4}

{a, b, c}  {1, 2} = { } =  (empty set)

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### Set operations: intersection (2)

• Venn diagram (area of intersection in darker grey)

A

B

A B

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### Set operations: complement (1)

• The complement of a set B relative to a set A is

A – B = A \ B = {x : x  Aandx  B}

• That is,

• Those elements belonging to Aandnot belonging to B

• Examples:

{1, 2, 3, 4} – {4, 3, 2, 1} = { } =  (empty set)

{a, b, c} – {1, 2} = {a, b, c}

{1, 2, 3} – {1, 2} = {3}

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### Set operations: complement (2)

• Venn diagram (area of interest in darker grey)

B

A

A– B

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### Universal set

• Sometimes we deal with subsets of a large set U

• U is the universal set for a problem

• In our previous Venn diagrams, the outer rectangle is the universal set

• Suppose A is a subset of the universal set U

• Its complement relative to U is U – A

• We represent as U – A = A = {x : x  A}

A

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### Set operations: Symmetric difference

• Symmetric difference of two sets A and B is

AB = {x : (x  Aandx  B)or(x  Bandx  A)}

That is:

• Elements in A and not in Bor

• Elements in B and not in A

Or: elements in A or B, but not in both (grey area)

A

B

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### Examples

Let

A = {1, 3, 5, 7} B = {2, 4, 6, 8} C = {1, 2, 3, 4, 5}

Find

• A  C

• B  C

• A – C

• B C

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### Examples

Let

A = {1, 3, 5, 7} B = {2, 4, 6, 8} C = {1, 2, 3, 4, 5}

Find

• A  C = {1, 3, 5, 7}  {1, 2, 3, 4, 5} = {1, 2, 3, 4, 5, 7}

• B  C

• A – C

• B C

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### Examples

Let

A = {1, 3, 5, 7} B = {2, 4, 6, 8} C = {1, 2, 3, 4, 5}

Find

• A  C = {1, 3, 5, 7}  {1, 2, 3, 4, 5} = {1, 2, 3, 4, 5, 7}

• B  C = {2, 4, 6, 8}  {1, 2, 3, 4, 5} = {2, 4}

• A – C

• B C

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### Examples

Let

A = {1, 3, 5, 7} B = {2, 4, 6, 8} C = {1, 2, 3, 4, 5}

Find

• A  C = {1, 3, 5, 7}  {1, 2, 3, 4, 5} = {1, 2, 3, 4, 5, 7}

• B  C = {2, 4, 6, 8}  {1, 2, 3, 4, 5} = {2, 4}

• A – C = {1, 3, 5, 7} – {1, 2, 3, 4, 5} = {7}

• B C

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### Examples

Let

A = {1, 3, 5, 7} B = {2, 4, 6, 8} C = {1, 2, 3, 4, 5}

Find

• A  C = {1, 3, 5, 7}  {1, 2, 3, 4, 5} = {1, 2, 3, 4, 5, 7}

• B  C = {2, 4, 6, 8}  {1, 2, 3, 4, 5} = {2, 4}

• A – C = {1, 3, 5, 7} – {1, 2, 3, 4, 5} = {7}

• B C = (B – C)  (C – B) = ({2, 4, 6, 8} – {1, 2, 3, 4, 5})  ({1, 2, 3, 4, 5} – {2, 4, 6, 8}) = {6, 8}  {1, 3, 5} = {1, 3, 5, 6, 8}

• N.B.: ordering for better visualisation!

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### Information modelling with sets

• We can build an information model with sets

• “Model” means we don’t care how it is implemented

• Essence: what information is needed

• Example: information model for student record

• NAME = {namei, ...., namen}

• ID = {idi, ...., idn}

• COURSE= {coursei, ...., coursen}

• Student Info:

(namej, idk, courses), where namej NAME, idk ID,and

courses  COURSE.

• Student Database is a set of student info:

R = {(bob,345,{CS1022,CS1015}),

(mary,222,{SX1009,CS1022,MA1004}),

(jill,246,{SX1009,CS2013,MA1004}),

(mary,247,{SX1009,CS1022,MA1004}), ...}

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### Query the Student Database

• R = {(bob,345,{CS1022,CS1015}),

(mary,222,{SX1009,CS1022,MA1004}),

(jill,246,{SX1009,CS2013,MA1004}),

(mary,247,{SX1009,CS1022,MA1004}), ...}

• Query to obtain a class list. Give set C, where:

C= {(N,I) : (N,I,Courses)  Rand CS1022  Courses}

= {(bob,345), (mary,222), (mary,247), ...}

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### Summary

You should now know:

• What sets are and how to represent them

• Venn diagrams

• Operations with sets

• How to build information models with sets and how to operate with this model

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