Download - Chapter 8 High-Level Programming Languages. 2 Compilers High-level language A language that provides a richer (more English like) set of instructions

Chapter 8

High-Level Programming Languages

2

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

3

Compilers

Figure 8.1 Compilation process

4

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

5

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

6

Portability

PortabilityThe ability of a program to be run on different machines

Compiler portabilityA program in a standardized language can be compiled and run on any machine that has the appropriate compiler

Bytecode portabilityA program translated into Bytecode can be run on any machine that has a JVM

7

Portability

Figure 8.2 Portability provided by standardized languages versus interpretation by Bytecode

8

Portability

Figure 8.2 Portability provided by standardized languages versus interpretation by Bytecode

9

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

10


Logic model– Program consists of facts about objects and rules that

question the relationships among facts– PROLOG

Object-oriented model– Program consists of a set of objects and the

interactions among the objects– Smalltalk and Simula – C++ is as an imperative language with some object-

oriented features– Java is an object-oriented language with some

imperative features

11


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

12

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

13

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

14

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

15

Boolean Expressions

<<=>

>=!= or <> or /=

= or ==

16

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

17

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

Declaration Example

19

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

20

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)

21

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 areal in that order.

22

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?

23

Selection Statements

Figure 8.3 Flow of control of if statement

24

Selection Statements

The if statement allows the program to test the state of the program variables using a Boolean expression

25

Blocks

Notethe symbolsused toindicateblocksin eachlanguage

26

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"

ElseWrite "Stay inside"

27

Looping Statements

Figure 8.4 Flow of control of while statement

28

Looping Statements

A count-controlled loop

Set sum to 0Set count to 1While (count <= limit)

Read numberSet sum to sum + numberIncrement count

Write "Sum is " + sum

Why is itcalled acount-controlledloop?

29

Looping Statements

30

Looping Statements

An event-controlled loop

Set sum to 0Set allPositive to trueWhile (allPositive)

Read numberIf (number > 0)

Set sum to sum + numberElse

Set allPositive to falseWrite "Sum is " + sum

Why is it called anevent-conrolledloop? What is theevent?

31

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

32


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

33


Figure 8.5 Subprogram flow of control

34


Figure 8.5 Subprogram flow of control

35


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

36


Think of arguments as being placed on a message board

37


38

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

39

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)

40

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

41

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

42


Declare Record

43


Declare record variableUse record variable

44


Figure 8.8 Array variable tenThings accessed from 0..9

An Array

45


Declare Array

46


Access an Array

47

Functionality of Object-Oriented 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

48

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

49

Review

Rememberthepattern?

Fromproblemto generaldescriptionto classdefinition toprogram

50

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

51


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


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.52


• 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:

53

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); }

54

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.

55

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; }

The subclasses SavingsAccount and CheckingAccount inherit the properties of BankAccount.

56

Download - Chapter 8 High-Level Programming Languages. 2 Compilers High-level language A language that provides a richer (more English like) set of instructions

Top Related