Transcript Document

Chapter 8
High-Level Programming
Languages
Compilers
High-level language
A language that provides a richer (more
English like) set of instructions
Compiler
A program that translates a high-level
language program into machine code
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Compilers
Figure 8.1 Compilation process
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Interpreters
Interpreter
A translating program that translates and executes
the statements in sequence
– Assembler or compiler produce machine code as
output, which is then executed in a separate step
– An interpreter translates a statement and then
immediately executes the statement
– Interpreters can be viewed as simulators
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Java
• Introduced in 1996 and became instantly
popular
• Portability was of primary importance
• Java is compiled into a standard machine
language called Bytecode
• A software interpreter called the JVM
(Java Virtual Machine) takes the Bytecode
program and executes it
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Portability
Portability
The ability of a program to be run on different machines
Compiler portability
A program in a standardized language can be compiled
and run on any machine that has the appropriate compiler
Bytecode portability
A program translated into Bytecode can be run on
any machine that has a JVM
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Portability
Figure 8.2
Portability
provided by
standardized
languages versus
interpretation by
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Bytecode
Portability
Figure 8.2
Portability
provided by
standardized
languages versus
interpretation by
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Bytecode
Programming Language
Paradigms
Imperative or procedural model
– Program describes the processing
– FORTRAN, COBOL, BASIC, C, Pascal,
Ada, and C++
Functional model
– Program is written terms of mathematical
functions
– LISP, Scheme (a derivative of LISP), and ML
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Programming Language
Paradigms
Logic model
– Program consists of facts about objects and rules that
question the relationships among facts
– PROLOG
Object-oriented model
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– Program consists of a set of objects and the
interactions among the objects
– Smalltalk and Simula
– C++ is as an imperative language with some objectoriented features
– Java is an object-oriented language with some
imperative features
Programming Language
Paradigms
We examine procedural and object-oriented
languages in the rest of this chapter by looking at
the functionality provided in these languages
We give examples in different languages to show
how syntax used to provide the functionality
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Functionality of Imperative
Languages
Sequence
Executing statements in sequence until an instruction is
encountered that changes this sequencing
Selection
Deciding which action to take
Iteration (looping)
Repeating an action
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Boolean Expressions
Boolean expression
A sequence of identifiers, separated by compatible
operators, that evaluates to true or false
A Boolean expression can be
– A Boolean variable
– An arithmetic expression followed by a relational
operator followed by an arithmetic expression
– A Boolean expression followed by a Boolean
operator followed by a Boolean expression
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Boolean Expressions
Variable
A location in memory that is referenced by
an identifier that contains a data value
Thus, a Boolean variable is a location in
memory that can contain either true or false
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Boolean Expressions
<
<=
>
>=
!= or <> or /=
= or ==
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Strong Typing
Data type
A description of the set of values and the basic set
of operations that can be applied to values of the
type (e.g. Integer, real, chars, boolean, strings)
Strong typing
The requirement that only a value of the proper
type can be stored into a variable
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Declarations
Declaration
A statement that associates an identifier with a
variable, an action, or some other entity within
the language that can be given a name; the
programmer can refer to that item by name
Reserved word
A word in a language that has special meaning
Case-sensitive
Uppercase and lowercase letters are considered
the same
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Declaration Example
Assignment statement
Assignment statement
An action statement (not a declaration) that says to
evaluate the expression on the right-hand side of
the symbol and store that value into the place
named on the left-hand side
Named constant
A location in memory, referenced by an identifier,
that contains a data value that cannot be changed
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Input/Output Structures
Pseudocode algorithms used the expressions
Read or Get and Write or Print
High-level languages view input data as a stream
of characters divided into lines
Key to the processing
The data type determines how characters are to
be converted to a bit pattern (input) and how a bit
pattern is to be converted to characters (output)
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Input/Output Structures
Read name, age, hourlyWage
name is a string;
age is an integer;
hourlyWage is a real
The data must be a string, an integer, and a
real in that order.
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Control Structures
Control structure
An instruction that determines the order in
which other instructions in a program are
executed
Can you name the ones we defined in the
functionality of pseudocode?
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Selection Statements
Figure 8.3 Flow of control of
if statement
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Selection Statements
The if statement allows the program to test the state of the
program variables using a Boolean expression
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Blocks
Note
the
symbols
used to
indicate
blocks
in each
language
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Cascading Ifs
If (temperature > 90)
Write "Texas weather: wear shorts"
Else If (temperature > 50)
Write "A little chilly: wear a light jacket"
Else If (temperature > 32)
Write "Philadelphia weather: wear a heavy coat"
Else
Write "Stay inside"
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Looping Statements
Figure 8.4
Flow of control of while statement
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Looping Statements
A count-controlled loop
Set sum to 0
Set count to 1
While (count <= limit)
Read number
Set sum to sum + number
Increment count
Write "Sum is " + sum
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Why is it
called a
count-controlled
loop?
Looping Statements
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Looping Statements
An event-controlled loop
Set sum to 0
Set allPositive to true
While (allPositive)
Read number
If (number > 0)
Set sum to sum + number
Else
Set allPositive to false
Write "Sum is " + sum
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Why is it
called an
event-conrolled
loop?
What is the
event?
Subprogram Statements
We can give a section of code a name and
use that name as a statement in another
part of the program
When the name is encountered, the
processing in the other part of the program
halts while the named code is executed
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Subprogram Statements
What if the subprogram needs data from the
calling unit?
Parameters
Identifiers listed in parentheses beside the
subprogram declaration; sometimes called formal
parameters
Arguments
Identifiers listed in parentheses on the
subprogram call; sometimes called actual
parameters
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Subprogram Statements
33 Figure 8.5 Subprogram flow of control
Subprogram Statements
34 Figure 8.5 Subprogram flow of control
Subprogram Statements
Value parameter
A parameter that expects a copy of its
argument to be passed by the calling unit
Reference parameter
A parameter that expects the address of its
argument to be passed by the calling unit
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Subprogram Statements
Think of arguments as being placed on a message board
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Subprogram Statements
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Recursion
Recursion
The ability of a subprogram to call itself
Base case
The case to which we have an answer
General case
The case that expresses the solution in terms of a
call to itself with a smaller version of the problem
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Recursion
For example, the factorial of a number is defined
as the number times the product of all the numbers
between itself and 0:
N! = N * (N  1)!
E.g. 5!=1*2*3*4*5=120
Base case
Factorial(0) = 1 (0! is 1)
General Case
Factorial(N) = N * Factorial(N-1)
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Asynchronous Processing
Asynchronous processing
Not synchronized with the program's action
– Clicking has become a major form of input
to the computer
– Mouse clicking is not within the sequence
of the program
– A user can click a mouse at any time during
the execution of a program
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Composite Data Types
Records
A named heterogeneous collection of items
in which individual items are accessed by
name
Arrays
A named homogeneous collection of items
in which an individual item is accessed by its
position (index) within the collection
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Composite Data Types
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Declare Record
Composite Data Types
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Declare record variable
Use record variable
Composite Data Types
An Array
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Figure 8.8
Array variable
tenThings
accessed
from 0..9
Composite Data Types
Declare Array
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Composite Data Types
Access an Array
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Functionality of ObjectOriented Languages: Review
Object class (problem-solving phase)
A description of a group of objects with similar
properties and behaviors
Object (problem-solving phase)
An entity or thing that is relevant in the context of a
problem
Instantiate
Create an object from a class
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Review
Encapsulation
A language feature that enforces information hiding
(second definition)
Class (implementation phase)
A language construct that is a pattern for an object and
provides a mechanism for encapsulating the properties and
actions of the object class
Object (implementation phase)
An instance of a class
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Review
Remember
the
pattern?
From
problem
to general
description
to class
definition to
program
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Inheritance and Polymorphism
Inheritance
A construct that fosters reuse by allowing an
application to take an already-tested class and
derive a class from it that inherits the properties
the application needs
Polymorphism
The ability of a language to have duplicate method
names in an inheritance hierarchy and to apply the
method that is appropriate for the object to which
the method is applied
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Inheritance and Polymorphism
Inheritance and polymorphism work together
How?
They combine to allow the programmer to build
useful hierarchies of classes that can be put into a
library to be reused in different applications
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Inheritance and Polymorphism
e.g. If a Dog is commanded to speak(), it
may emit a bark, while if a Pig is asked to
speak(), it may respond with an oink. Both
inherit speak() from Animal, but their
subclass methods override the methods of
the superclass, known as overriding
polymorphism. Adding a walk method to
Animal would give both Pig and Dog objects
the same walk method.
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Inheritance and Polymorphism
• To inherit is to derive traits from preceding generations. In the
object-oriented programming world, the term is associated
with a kind of software reuse. With inheritance, new classes
can be derived from existing classes, using the existing
classes as building blocks. The new class inherits properties
and methods from the base class. The new class can also
add its own properties and methods.
• Inheritance can be understood from the following skeleton
example. In this case a parent or a base class called
BankAccount is declared as follows:
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Inheritance example
class BankAccount{
protected:
char* Name;
char* SSNum;
char* AccountNum;
float Balance;
public:
BankAccount (char* name, char* ssnum);
void Deposit (float amount, char* accnum);
void Withdraw(float amount, char* accnum);
void PrintBalance(char* accnum);
}
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Inheritance example
The base class BankAccount has four variables:
• Name: which stores the name of account holder,
• SSNum: the account holder’s social security number,
• AccountNum: the account number, and
• Balance: the account balance.
• The methods Deposit and Withdraw are used to make a deposit and
withdrawal from the bank account. The PrintBalance method prints
the balance in the account. The BankAccount class in itself is not
sufficient to carry out all the transactions on the account. Generally,
there are two types of accounts: the checking account, which
facilitates day to day transactions, and the savings account, which
accrues interest on the saved amount.
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Inheritance example
We therefore derive two subclasses that inherit from the above parent
class. They are SavingsAccount and Checking Account.
class SavingsAccount: public BankAccount{
private:
float InterestRate;
float MinimumBalance;
}
class CheckingAccount: public BankAccount{
private:
float MonthlyFee;
}
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The subclasses SavingsAccount and CheckingAccount inherit the
properties of BankAccount.