Programming Paradigms • Procedural • Functjonal • Logic • Object-Oriented
Specifying the WHAT • Describe the Inputs – Specifjc values – Propertjes • Describe the Outputs (as above) • Describe the Relatjonships Between I x O – As a possibly infjnite table – Equatjons and other predicates between input and output expressions – For a given input, output may not be unique CS784 2
Specifying the HOW • Describe the Inputs – Specifjc values – Propertjes • Describe HOW the Outputs are produced • Models of existjng computers – Program State – Control Flow • A Few Abstractjons – Block Structure – Recursion via a Stack CS784 3
Procedural programming • Describes the details of HOW the results are to be obtained, in terms of the underlying machine model. • Describes computatjon in terms of – Statements that change a program state – Explicit control fmow • Synonyms – Imperatjve programming – Operatjonal • Fortran, C, … – Abstractjons of typical machines – Control Flow Encapsulatjon • Control Structures • Procedures – No return values • Functjons – Return one or more values • Recursion via stack CS784 4
Procedural Programming: State • Program State – Collectjon of Variables and their values – Contents of variables change • Expressions – Not expected to change Program State • Assignment Statements • Other Statements • Side Efgects CS784 5
C, C++, C#, Java • Abstractjons of typical machines • Control Flow Encapsulatjon – Control Structures – Procedures • No return values – Functjons • Return one or more values – Recursion via stack • Betuer Data Type support CS784 6
Illustratjve Example • Expression (to be computed) : a + b + c • Recipe for Computatjon – Account for machine limitatjons – Intermediate Locatjon • T := a + b; T := T + c; – Accumulator Machine • Load a; Add b; Add c – Stack Machine • Push a; Push b; Add; Push c; Add CS784 7
Declaratjve Programming • Specifjes WHAT is to be computed abstractly • Expresses the logic of a computatjon without describing its control fmow • Declaratjve languages include – logic programming, and – functjonal programming. • ofuen defjned as any style of programming that is not imperatjve. CS784 8
Imperatjve vs Non-Imperatjve • Functjonal/Logic style clearly separates WHAT aspects of a program (programmers’ responsibility) from the HOW aspects (implementatjon decisions). • An Imperatjve program contains both the specifjcatjon and the implementatjon details, inseparably inter-twined. CS784 9
Procedural vs Functjonal • Program: a sequence of • Program: a collectjon of instructjons for a von functjon defjnitjons Neumann m/c. (m/c independent). • Computatjon by • Computatjon by term instructjon executjon. rewritjng. • Iteratjon. • Recursion. • Modifjable or updatable • Assign-only-once variables.. variables. CS784 10
Functjonal Style : Illustratjon • Defjnitjon: Equatjons sumto(0) = 0 sumto(n) = n + sumto(n-1) • Computatjon: Substjtutjon and Replacement sumto(2) = 2 + sumto (2-1) = 2 + sumto(1) = 2 + 1 + sumto(1-1) = 2 + 1 + sumto(0) = 2 + 1 + 0 = … = 3 CS784 11
Paradigm vs Language Imperatjve Style Functjonal Style tsum := 0; func sumto(n: int): int; i := 0; if n = 0 while (i < n) do then 0 i := i + 1; else n + sumto(n-1) tsum := tsum + I fj od endfunc; No Side-efgect Storage effjcient CS784 12
Bridging the Gap • Imperatjve is not always faster, or more memory effjcient than functjonal. • E.g., tail recursive programs can be automatjcally translated into equivalent while-loops. func xyz(n : int, r : int) : int; if n = 0 then r else xyz(n-1, n+r) fj endfunc CS784 13
Analogy: Styles vs Formalisms • Iteratjon • Regular Expression • Tail-Recursion • Regular Grammar • General Recursion • Context-free Grammar CS784 14
Logic Programming Paradigm 1. edge(a,b). 2. edge(a,c). 3. edge(c,a). 4. path(X,X). 5. path(X,Y) :- edge(X,Y). 6. path(X,Y) :- edge(X,Z), path(Z,Y). CS784 15
Logic Programming • A logic program defjnes a set of relatjons. • This “knowledge” can be used in various ways by the interpreter to solve difgerent “queries”. • In contrast, the programs in other languages • Make explicit HOW the “declaratjve knowledge” is used to solve the query. CS784 16
Append in Prolog • append([], L, L). • append([ H | T ], X, [ H | Y ]) :- • append(T, X, Y). • True statements about append relatjon. • Uses patuern matching. – “[]” and “|” stand for empty list and cons operatjon. CS784 17
Difgerent Kinds of Queries • Verifjcatjon – append: list x list x list • append([1], [2,3], [1,2,3]). • Concatenatjon – append: list x list -> list • append([1], [2,3], R). CS784 18
More Queries • Constraint solving – append: list x list -> list • append( R, [2,3], [1,2,3]). – append: list -> list x list • append(A, B, [1,2,3]). • Generatjon – append: -> list x list x list • append(X, Y, Z). CS784 19
Object-Oriented Style • Programming with Abstract Data Types – ADTs specify/describe behaviors. • Basic Program Unit: Class – Implementatjon of an ADT. • Abstractjon enforced by encapsulatjon.. • Basic Run-tjme Unit: Object – Instance of a class. • Has an associated state. CS784 20
Procedural vs Object-Oriented • Emphasis on procedural • Emphasis on data abstractjon. abstractjon. • Top-down design; Step- • Botuom-up design; wise refjnement. Reusable libraries. • Suited for programming • Suited for programming in the small. in the large. CS784 21
Integratjng Heterogeneous Data • In C, Pascal, etc., use • Union Type / Switch Statement • Variant Record Type / Case Statement • In C++, Java, Eifgel, etc., use • Abstract Classes / Virtual Functjons • Interfaces and Classes / Dynamic Binding CS784 22
Comparison : Figures example • Data • Classes – Square – Square • side • side • area – Circle • (= side * side) • radius – Circle • Operatjon (area) • radius – Square • area • side * side • (= PI*radius*radius) – Circle • PI * radius * radius CS784 23
Adding a new operatjon • Data • Classes – Square • ... • ... • Operatjon (area) • perimeter • Operatjon (perimeter) • (= 4 * side) – Square – Circle • 4 * side • ... • perimeter – Circle • (= 2 * PI * radius) • 2 * PI * radius CS784 24
Adding a new data representatjon • Data • Classes – ... – ... – rectangle – rectangle • length • length • width • width • area • Operatjon (area) • (= length * width) – ... – rectangle • length * width CS784 25
Procedural vs Object-Oriented • New operatjons cause additjve changes in procedural style, but require modifjcatjons to all existjng “class modules” in object-oriented style. • New data representatjons cause additjve changes in object-oriented style, but require modifjcatjons to all “procedure modules”. CS784 26
Object-Oriented Concepts • Data Abstractjon (specifjes behavior) • Encapsulatjon (controls visibility of names) • Polymorphism (accommodates various implementatjons) • Inheritance (facilitates code reuse) • Modularity (relates to unit of compilatjon) CS784 27
Example : Role of interface in decoupling • Client – Determine the number of elements in a collectjon. • Suppliers – Collectjons : Vector, String, List, Set, Array, etc • Procedural Style – A client is responsible for invoking appropriate supplier functjon for determining the size. • OOP Style – Suppliers are responsible for conforming to the standard interface required for exportjng the size functjonality to a client. CS784 28
Client in Scheme • (defjne (size C) (cond ( (vector? C) (vector-length C) ) ( (pair? C) (length C) ) ( (string? C) (string-length C) ) ( else “size not supported”) ))) • (size (vector 1 2 (+ 1 2))) • (size ‘(one “two” 3)) CS784 29
Suppliers and Client in Java Interface Collectjon {int size(); } class myVector extends Vector implements Collectjon { } class myString extends String implements Collectjon { public int size() { return length();} } class myArray implements Collectjon { int[] array; public int size() {return array.length;} } Collectjon c = new myVector(); c.size(); CS784 30
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