Implementation

1
Implementation

• Dr Z He
• University of Ulster

2
Lecture 5 Implementation

• Programming Paradigms
• Types of programming languages
• Choosing a programming language
• Structured programming (using procedural/tradition
  al languages)
• Object-oriented programming (OO languages)
• Features of programming languages (procedural
  languages)

3
Types of Programming Languages

• Very difficult to evaluate or compare
• Can be classified in many ways
• Application
• Business/data processing COBOL
• Scientist/engineer FORTRAN
• Systems software C
• Real-time/embedded Ada
• Level
• High level Cobol, Fortran, Ada
• Low level assembly
• Both high and low level C

4
Choosing Programming Languages

• Selecting a language for a project depends on
  many factors
• Select the language that has been used and is
  familiar
• Select the language with which you have
  considerable experiences
• Select the language the same as the existing
  systems
• Select a language according to the type of the
  final system
• Select the language as specified in contract
• Select the language with supporting tools (IDE,
  project management)
• Select a language according to the language
  characteristics expected
• programming in small or large?
• separate compilation?
• Comments/data abstraction tool?
• Module interface control?
• Configuration management?

5
Structured Programming

• Programs should only be built from three
  constructs
• Sequences
• written in order in which the statements are to
  be executed
• Selections
• written as if-then-else or switch-case
• Repetitions
• written as while-do, do-while, for

6
Structured Programming

• Characteristics of structured programs
• They have only one entry and exit
• None of the constructs consists of more than
  three boxes (switch-case can be considered as a
  compound if-then)
• The entry is at the start and the exit is at end
• Avoid using goto statements whenever possible
• Abstraction
• most important idea in structured programming
• subsystems can be seen as black boxes
• a procedure can be seen as a black box (single
  entry and single exit)
• three standard constructs have single entry and
  single exit
• A definition of structured programming
• the systematic use of (control) abstraction in
  programming

7
Object-Oriented Programming

• Pure OOP languages
• designed for OOP, Smalltalk, Eiffel, Java
• Hybrid languages
• extended to OOP, C
• OOP languages support
• encapsulation of objects
• computation by message-passing
• polymorphism and dynamic binding
• classification (define classes of common objects)
• inheritance and subclassing (subclasses)
• class libraries (permits customisation and
  extension through subclassing)
• Object-based languages
• support some areas but not inheritance (e.g.
  Visual Basic 5.0)

8
Object Oriented Programming

• Derived from SIMULA-67 and later SmallTalk-80
• Pascal, C etc.
• program contains a number of routines
  manipulating certain data structures
• To change the state of this data, we call the
  routines passing data as parameters
• SmallTalk, C etc.
• Objects are instances of classes
• Classes encapsulate common properties of a set of
  objects - data, state, operations (services or
  methods)
• Operations change the state of the object
• To execute an operation, we send a message to the
  object
• So far the concepts are object-based
• Object-Oriented - add inheritance

9
Object Oriented Programming
human being (base class
inherits from (is_a)
derived part
parent (derived class)
incremental part
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Object Oriented Programming

• class human_being
• public
• int age
• float height
• enum male, female gender
• // declarations of operations on human_being
• class parent public human_being
• public
• char name_of_spouse2-
• int number_of_children
• // declaration of operations on parent

11
Object Oriented Programming

• class is the same as struct except that class
  supports inheritance
• Advantages of C over C
• Data abstraction and procedural abstraction are
  more complete (C has really only procedural
  abstraction)
• Inheritance leads to fewer errors
• Maintenance should be easier
• Reuse is facilitated to a greater extent

12
Features of Programming Languages

• Ref Bell et al, ch 10
• Features of Programming Languages
• Declarations
• indicate the names allowed in the program.
• places constraints on what you can do - type
  checking
• e.g. In C, int n allows n to take on integer
  values.
• e.g. The operator can be used to add integers
  or real numbers (In Ada - vectors as well)
• The compiler knows from the variable types which
  is meant - binding
• In object oriented programming, binding is done
  at runtime
• Some languages do not require declarations.
  Rather the first occurrence of that variable is
  also its declaration e.g BASIC, FORTRAN. -
  Problems?
• Explicit declarations reduces errors and makes
  programs more readable and maintainable

13
Features of programming languages

• Types
• New types can be declared. (enumerated types)
• Operations on the declared type are only allowed
  if they have been defined
• e.g. In C,
• enum colour RED, GREEN, BLUE c,d
• In this example, c and d can only take on the
  values given. The compiler will point out any
  deviations from this.
• Type Checking is not always waterproof e.g. Most
  Pascal compilers allow you to treat pointers as
  integers, allowing you to directly access memory.
  (Ada is safe in this respect)

14
Features of programming languages

• Structured Types
• arrays e.g. int a10 declares a as an array of
  10 integers
• Records (Struct in C)
• e.g. In C,
• struct person
• unsigned long item_num
• unsigned quantity
• double price

15
Features of programming languages

• Pointers
• Good for defining dynamic structures such as
  lists or trees.
• A pointer contains an address
• e.g. linked list
• struct cell
• double value
• struct cell next

p
p
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Features of programming languages

• A binary tree -the definition for a node will be
• typedef struct item
• unsigned long item_num
• unsigned quantity
• double price
• struct item left_branch
• struct item right_branch
• item_t
• Iteration
• While
• In C,
• while (expression)
• statement
• Condition is checked first.

17
Features of programming languages

• Do
• cf. Repeat..Until in Pascal
• do
• statement
• while (expression)
• This time the expression is evaluated after the
  statement.
• For
• as in Pascal
• for (exp-i exp-t exp-s )
• statement
• exp-i is the initial value, exp-t the test value,
  exp-s the step value

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Features of programming languages

• Selection
• If condition then statement else statement
• E.g. in C,
• if (count lt MAX_COUNT)
• count
• else
• printf(Overflow\n
• count 0
• else is optional here and in most languages.
• case expression of
• (list of values and actions)
• end

19
Features of programming languages

• In C,
• switch (expression)
• statement
• E.g.
• switch (n)
• case 0
• ...
• break
• case 1
• ...
• break
• GOTO
• Mostly allowed, but not recommended

20
Features of programming languages

• What is wrong with GOTO? (ref Bell, Morrey
  Pugh pp 42-46)
• not necessary
• experimental evidence show harder to understand
• clarity expressive power
• ease of reading
• checking - single entry/exit vs multiple
  entry/exit
• Are there any arguments in favour
• exception handling
• performance (in exceptional circumstances)

21
Features of programming languages

• PARAMETER PASSING
• Most programming languages have the idea of
  procedures (functions in C) which allow
  parameters to be passed in and out of them
• Actual Parameter is substituted wherever the
  corresponding formal parameter occurs in the
  procedure body

22
Features of programming languages

• E.g.
• .
• .sz size(len,wdth,hght)
• .
• / Compute the height of a rectangular box /
• double size(double length,
• double width,
• double height)
• double size1, girth
• girth 2 (width height)
• size1 length girth
• return size1

Actual arguments
Formal arguments
23
Features of programming languages

• SEPARATE COMPILATION
• This is where parts of the program can be
  compiled separately
• E.g. FORTRAN - individual routines, Turbo Pascal
  - units, Modula-2 - modules, Ada - packages
• C can be grouped into compilation units, which
  are then linked to produce an executable file
• This allows use of modular programming