Programming Languages

1
Programming Languages

• Shyh-Kang Jeng
• Department of Electrical Engineering/
• Graduate Institute of Communication Engineering
• National Taiwan University

2
Generations of programming languages
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Machine Instructions and Mnemonic Techniques
156C 166D 5056 306E C000
LD R5, PRICE LD R6, TAX ADDI R0, R5 R6 ST R0,
TOTAL HLT
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Assemblers and Assembly Languages

• Programs to translate other programs written in
  mnemonic form into machine compatible form
• Assemble machine instructions out of the op-codes
  and operands obtained by translating mnemonics
  and identifiers
• Mnemonic systems for representing programs are
  recognized as programming languages called
  assembly languages

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Assembly Languages

• Second-generation language
• Machine dependent
• Must conform to the machines register
  configuration and instruction set
• Low level primitives
• A programmer is still forced to think in terms of
  the small, incremental steps of the machines
  language

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Design Process of a Product

• Better suited to the use of high-level
  primitives, each representing a concept
  associated with a major feature of the product
• Once the design is complete, the primitives can
  be translated to lower-level concepts relating to
  the details of implementation

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Third-Generation Languages

• High-level primitives
• Machine independent if standard is followed
• FORTRAN
• FORmula TRANslator
• COBOL
• COmmon Business-Oriented Language
• BASIC

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Compilers vs. Interpreters

• Compilers
• Compile several machine instructions into short
  sequences to simulate the activity requested by a
  single high-level primitive
• Produce a machine-language copy of a program that
  would be executed later
• Interpreters
• Execute the instructions as they were translated

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Machine Independence and Beyond

• Restrictions of the machines
• Size of registers and memory cells
• Dialects and standard languages
• ANSI (American National Standards Institute)
  standard
• ISO (International Organization of
  Standardization) standard
• Language extensions

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Evolution of Programming Paradigms
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Imperative Paradigm

• Procedural paradigm
• Develops a sequence of commands that when
  followed, manipulate data to produce the desired
  result
• Approaches a problem by trying to find an
  algorithm for solving it

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Declarative Paradigm

• Emphasizes
• What is the problem?
• Rather than What algorithm is required to solve
  the problem?
• Implemented a general problem-solving algorithm
• Develops a statement of the problem compatible
  with the algorithm and then applies the algorithm
  to solve it

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Functional Paradigm

• Views the process of program development as
  connecting predefined black boxes, each of
  which accepts inputs and produces outputs
• Mathematicians refer to such boxes as functions
• Constructs functions as nested complexes of
  simpler functions

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Functional Paradigm Example
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Examples of LISP Expressions

• (Find_diff
• (Find_Sum Old_balance Credits) (Find_Sum
  Debits))
• (First (Sort List))

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Advantages of Functional Paradigm

• Constructing complex software from predefined
  primitive functions leads to well-organized
  systems
• Provides an environment in which hierarchies of
  abstraction are easily implemented, enabling new
  software to be constructed from large predefined
  components rather than from scratch

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Object-Oriented Paradigm

• OOP
• Encapsulation of data and procedures
• Lists and sorting
• Graphic user interface (GUI)
• Natural modular structure and program reuse
• Communication between modules is accomplished by
  passing messages
• CORBA and software components

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Statements in Programming Languages

• Declarative statements
• Define customized terminology that is used later
  in the program
• Imperative statements
• Describe steps in the underlying algorithms
• Comments
• Enhance the readability of a program

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Composition of a Typical Imperative Program or
Program Unit
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Variables, Literals, Constants

• EffectiveAlt ? Altimeter 645
• Variables
• Example EffectiveAlt, Altimeter
• Literals
• Example 645
• Constants
• Example const int AirportAlt 645

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Data Type

• Refers to
• Interpretation of data
• Operations that can be performed on the data
• Common types
• integer, real, character, Boolean

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Variable Declarations

• Pascal
• Length, width real
• Price, Tax, Total integer
• C, C, Java, C
• float Length, width
• int Price, Tax, Total
• FORTRAN
• REAL Length, Width
• INTEGER Price, Tax, Total

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Data Structure

• Conceptual shape of data
• Common data structure
• Homogeneous array
• Heterogeneous array

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Declaration of a 2D Array

• C
• int Scores29
• Java
• int Scoresnew int 29
• Pascal
• Scores array3..4, 12..20 of integer

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A two-dimensional array
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Declaration of a Heterogeneous Array

• Pascal
• var Employee record
• Name packed array1..8 of char
• Age integer
• SkillRating real
• end
• C
• struct Employee
• char Name8
• int Age
• float SkillRating

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Assignment Statements and Operators

• C, C, Java
• Total Price Tax
• Ada, Pascal
• Total Price Tax
• APL
• Total lt- Price Tax
• Operator precedence
• Operator overloading

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Control Statements

• Alter the execution sequence of the program
• goto is the simplest control statement
• Example
• goto 40
• 20 Total price 10
• goto 70
• 40 if Price lt 50 goto 60
• goto 20
• 60 Total Price 5
• 70 stop

if( Price lt 50 ) then Total Price 5 else
Total Price 10 endif stop
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Control Structure (1)
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Control Structure (2)
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Control Structures (3)
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Comments

• For inserting explanatory statements (internal
  documentation)
• C and Java
• / This is
• a comment
• /
• // This is a comment
• Explain the program, not to repeat it
• Example Total Price Tax

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Procedures

• A procedure is a set of instructions for
  performing a task that can be used as an abstract
  tool by other program units
• Control is transferred to the procedure at the
  time its services are required and then returned
  to the original program unit (calling unit) after
  the procedure is finished
• The process of transferring control to a
  procedure is often referred to as calling or
  invoking the procedure

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Procedures (1)
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Procedures (2)

• Local variables
• Global variables
• Procedures header
• Parameters
• Formal parameters
• Actual parameters

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A C Procedure
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Pass by Value
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Pass by Reference
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Functions

• A program unit similar to procedure unit except
  that a value is transferred back to the calling
  unit
• Example
• Cost 2 TotalCost( Price, TaxRate )

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A C Function
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Input/Output Statements

• I/O statements are often not primitives of
  programming languages
• Most programming languages implement I/O
  operations as procedures or functions
• Examples
• printf( d d\n, value1, value2 )
• cout ltlt value ltlt endl

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A Formatted Output Example in C
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Translation Process
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Lexical Analyzer

• Reads the source program symbol by symbol,
  identifying which groups of symbols represent
  single units, and classifying those units
• 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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Parsing

• Group lexical units (tokens) into statements
• Identify the grammatical structure of the program
  
• Recognize the role of each component
• Fixed-format vs. free-format languages
• Key words and reserved words

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Syntax Diagram

• Pictorial representations of a programs
  grammatical structure
• Nonterminals
• Requires further description
• Terminals

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Syntax Diagram
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Parse Tree

• Pictorial form which represents a particular
  string conforming to a set of syntax diagrams
• The process of parsing a program is essentially
  that of constructing a parse tree for the source
  program
• A parse tree represents the parsers
  understanding of the programmers grammatical
  composition

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Parse Tree
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Dangling else Problem

• if B1
• then ( if B2 then S1 )
• else S2
• if B1
• then ( if B2 then S1
• else S2 )

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Parse Tree (1)
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Parse Tree (2)
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Mini Review

• Draw the parse tree for the expression
• based on the given syntax diagrams

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Answer
Expression
Expression
Term
Factor
Factor
Term
Expression
Term
Factor
Factor
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Mini Review

• Describe the strings that conforms to the
  structure Chacha according to the following
  syntax diagrams

Step
Chacha
Chacha
Turn
Step
forward
backward
Cha
Cha
Cha
backward
forward
Turn
right
swing
Cha
Cha
Cha
left
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Other Translation Topics

• Symbol tables
• Coercion
• Strongly typed language
• Code generation
• Code optimization
• x ? y z
• w ? x z

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Object-Oriented Approach to the Translation
Process
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Linker

• Most programming environments allow the modules
  of a program to be developed and translated as
  individual units at different times
• Linker links several object programs, operating
  system routines, and other utility software to
  produce a complete, executable program (load
  module) that is in turn stored as a file in the
  mass storage system

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Loader

• Places the load module in memory
• Often part of the operating systems scheduler
• Important in multitasking systems
• Exact memory area available to the programs is
  not known until it is time to execute it
• Loader also makes any final adjustments that
  might needed once the exact memory location of
  the program is known (e.g. dealing with the JUMP
  instruction)

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Program Preparation Processes
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Object-Oriented Programming

• Objects and classes
• Abstract data type
• Example (in C) LaserClass laser1, laser2
• Inheritance
• Example (in Java)
• class RechargeableLaser extends
• LaserClass
• Polymorphism
• Encapsulation

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A Class Example
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Constructor
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An Example for Encapsulation in Java or C
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Concurrent Processing

• True parallel processing
• Requires multiple CPUs
• Ada task and Java thread
• Basic actions
• Creating new process
• Handling communication between processes
• Mutually exclusive access
• Critical region
• A data item augmented with the ability to control
  access to itself is called a monitor

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Interaction between processes

• Mutual exclusion a method for ensuring that
  data can be accessed by only one process at a
  time
• Monitor a data item augmented with the ability
  to control access to itself

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Spawning processes
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Additional Reading for Logical Programming

• S. Russell and P. Norvig, Artificial
  Intelligence A Modern Approach, Englewood
  Cliffs, NJ Prentice Hall, 1995

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Logical Deduction

• Either Kermit is on stage or Kermit is sick
• Kermit is not on stage
• Kermit is sick
• Resolution Principle

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Truth Table
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Implication
U
P
Q
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Resolving (P OR Q) and (R OR ? Q) to Produce (P
OR R)
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Clause Form

• First-order predicate logic vs. Higher-order
  predicate logic
• Clause form

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Confirming the Inconsistency of a Set of
Inconsistent Clauses
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Interpretation (1)

• P Kermit is on stage
• Q Kermit has got SARS for two weeks
• R Kermit coughs

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Interpretation (2)
contradiction
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Automatic Reasoning using reduction ad absurdum

• Want to confirm a collection of statements
  implies the statement
• Same as contradicting the statement
• Apply resolution to the original statements and
  the statement until an empty clause occurs
• With inconsistent with the original
  statements, the original statements must imply

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Unification

• The process of assigning values to variables so
  that resolution can be performed

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Prolog (1)

• PROgramming in LOGic
• A Prolog program consists of a collection of
  initial statements upon which the underlying
  algorithm bases its deductive reasoning
• Predicate
• A fact about its argument
• Examples

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Prolog (2)

• Rule
• Goal
• Proposed to the system and the system applies the
  resolution to try to confirm that the goal is a
  consequence of the initial statements

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Prolog Queries
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Exercise

• 2, 5, 8, 22, 28, 29, 31, 37, 49, 53