Chapter 5 Programming Languages

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Chapter 5 Programming Languages

• COSC 1309
• Logic Design

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5.1 Historical Prespective

• Before programming languages
• Programmers had to program using the computers
  Opcodes (native machine language)
• Assembly Language was created which used
  mnemonics (short phrases like LD for Load, ST
  for Store JMP for Jump etc)
• Also, special names were given to registers like
  R1, R2, R3 . To registers in the CPU.

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Assembly Language

• In the beginning programmers using assembly
  language would write code on paper and manually
  translate it to machine code.
• The above translation procedure was recognized as
  something that a computer could do so the
  Assembler was born.

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Why other Programming languages

• Unfortunately programs written in assembly
  language
• were machine dependent
• could not be easily transported to another
  machine design because it was built to fit a
  machines particular instruction set and registers
• Programmer was stuck to think in small steps

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Programming Languges

• First generation --- Machine Language
• Second generation --- Assembly Language
• Third generation --- High level languages like
  Fortran and Cobol emerged
• Grace Hopper created the first compiler ( program
  that translates a high level language to machine
  code
• Interpreters also emerged as a way to translate a
  high level language into machine code.

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Programming Paradigms

• Programming languages has developed along
  different paths or alternate approaches to the
  programming process (programming paradigms)
• Imperative paradigm a.k.a procedural paradigm
• Traditional approach finding an algorithm to
  solve the problem and then expressing the
  algorithm as a sequence of commands
• Ex Pascal, Fortran, C, Cobol, BASIC

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Programming Paradigms

• Declarative Paradigm
• Asks the programmer to describe the problem
• The trick is to discover a general problem
  solving algorithm
• Problems can then be expressed in a form
  compatible with this algorithm and then applying
  the algorithm
• Ex Prolog and GPSS

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Programming Paradigms

• Functional Paradigm
• Views the process of program development as
  connecting predefined black boxes
• Each accepts inputs and produces outputs.
• These boxes are referred to as functions
• The primitives of functional languages are
  elementary functions
• Ex ML,Scheme , Lisp --- used in AI research

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Programming Paradigms

• Object oriented languages (OOP)
• Units of data are viewed as active objects
  rather than the passive units envisioned by
  traditional imperative paradigms.
• A template is used called a Class and from that
  class an object is created which has access to
  member variables and functions
• Ex C , Java , C , Smalltalk, Visual Basic

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5.2 Traditional Programming

• This section covers concepts found in the
  imperative object oriented programming
  languages
• Look at Appendix D for a more extensive
  background of imperative OO languages

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5.2 Variables and Data Types

• All locations in main memory have to be
  referenced by a descriptive name rather than a
  numeric address this is known as a variable.
• The type of data and the operations that can be
  performed on that data are called the data
  type.
• Ex int, float, double, char, long, string,
  boolean

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5.2 Data Structure

• Variables in programs are often associated with a
  data structure --- a conceptual shape or
  arrangement of data
• For example
• Text is normally viewed as a long string of
  characters
• Where as sales records may be viewed as a
  rectangular table of numeric values

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5.2 Data Structure

• One common data structure is the homogeneous
  array
• Which is a block of values of the same type such
  as a one-dimensional list or a two dimensional
  table with rows and columns.
• Example declaration of a two dimensional array
• int Scores 2 9
• A heterogeneous array is a block of data in which
  different elements can have different types.

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5.2 Constants and Literals

• Sometimes a fixed or predetermined value is used
  in a program this called a literal
• EX EffectiveAlt ? Altimeter 645
• The 645 is called the literal
• Often literals are not used because they
• Mask the meaning of statements in which they
  appear
• Can complicate the task of modifying the program
  should it become necessary

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5.2 Constants and Literals

• To solve the problems of literals, programming
  languages allow descriptive names to be assigned
  specific, non-changeable values.
• Such names are called constants
• Ex in programming language Ada
• AirportAlt constant Integer 645

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5.2 Assignment Statements

• Once variable names and constants are declared
  then the programmer can begin to describe the
  algorithm.
• This is done by using imperative statements
• Or Assignment Statements which requests that a
  value be assigned to a variable
• More precisely , stored in the memory area
  identified by the variable
• Operator Precedence is maintained PEMDAS in the
  assignment statements

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5.2 Control Statements

• A control statement is an imperative statement
  that alters the execution sequence of the program
• EX
• If then , If then else
• While loop
• Switch statement

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Chapter 5

• 5.4 Language Implementation

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The Translation Process

• The process of converting a program from one
  language to another is called translation.
• Source program the program in its original form
• Object Program -- the translated version of the
  source program
• The Translation Process consists of three
  activities
• Lexical Analysis
• Parsing
• Code Generation

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Lexical Analysis

• The process of recognizing which strings of
  symbols from the source program represent a
  single entity.
• Read the source program symbol by symbol and
  identifies which groups of symbols represent
  single units classifying those units as
• Numeric Values, words, arithmetic operators,
  skipping over comments and so on.
• As each unit is classified, the lexical analyzer
  generates a bit pattern known as a token to
  represent the unit and hands the token to the
  parser.

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Parser

• Views the program in terms of lexical units
  (tokens) rather than individual symbols.
• Groups units into statements
• Identifying the grammatical structure of the
  program and recognizing the role of each
  component.
• Earlier languages required that statements be
  placed in a certain position on the page these
  were called fixed-format languages
• Today the position is not critical thus we call
  most programming languages free-format languages

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Parser

• In order for the parser to identify statements
  regardless of the spacing key words and
  characters must be added
• For example
• Ending each statement with a semicolon and or
  using reserved words like if , else, then, while,
  for ..
• The parsing process is based on a set of syntax
  rules this defines the syntax of the programming
  language.
• One way of expressing the syntax rules is by
  using a syntax diagram which is a pictorial
  representation of a programs grammatical
  structure.

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Syntax diagram of the if then else statement
then
If
Boolean Expression
Statement
else
Statement
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Parser

• Notice that the terms that actually appear in an
  if-then-else statement are enclosed in ovals
  these are called terminals
• Those that appear in rectangles such as Boolean
  Expressions and statements require further
  description these are called non-terminals

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Parser

• Using the syntax rules described on page 239
  figure 5.18 xy X z can be represented in a
  pictorial form by a parse tree. (see fig. 5.19)
• The syntax rules describing a programs
  grammatical structure must not allow two distinct
  parse trees for one string, since this would lead
  to ambiguities.

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Parser

• As a parser analyzes the declarative statements
  in a program, it records the information in these
  statements in a table called the symbol table.
• The symbol table contains such information as
  which variables have been declared and what data
  types and data structures are associated with
  those variables.
• Z?XY if X and Y for example are not of the same
  data type then the parser will choose to convert
  one to the other type like int to real this is
  called Coercion

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Parser

• Coercion is frowned upon by many language
  designers. They argue that the need for coercion
  usually indicates a flaw in the programs design
  and therefore should not be accommodated by the
  parser.
• Most modern languages are Strongly Typed, which
  means that all activities requested by a program
  must involve data of agreeable types without
  coercion.
• Parsers for these languages report all type
  conflicts as errors.

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Code generation

• The final activity in the translation process is
  code generation.
• This is the process of constructing the
  machine-language instructions to implement the
  statements recognized by the parser.
• One of the issues that code generation faces
  besides translating the statements into machine
  code is producing efficient code.
• Example consider x?yz w?xz these
  statements could be translated as individual
  statements which would produce inefficient code.
• The idea is that since x and z are already in the
  CPUs general purpose registers there is no need
  to load them from memory again. Insights such as
  these are called code optimization.
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