Chapter 8 High-Level Programming Languages. 2 Compilers High-level language A language that provides...
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Transcript of Chapter 8 High-Level Programming Languages. 2 Compilers High-level language A language that provides...
Chapter 8
High-Level Programming Languages
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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
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
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Portability
Figure 8.2 Portability provided by standardized languages versus interpretation by Bytecode
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Portability
Figure 8.2 Portability provided by standardized languages versus interpretation by Bytecode
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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– 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
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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
Declaration Example
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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 areal 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
Notethe symbolsused toindicateblocksin eachlanguage
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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"
ElseWrite "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 0Set count to 1While (count <= limit)
Read numberSet sum to sum + numberIncrement count
Write "Sum is " + sum
Why is itcalled acount-controlledloop?
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Looping Statements
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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?
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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
Figure 8.5 Subprogram flow of control
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Subprogram Statements
Figure 8.5 Subprogram flow of control
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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
Declare Record
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Composite Data Types
Declare record variableUse record variable
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Composite Data Types
Figure 8.8 Array variable tenThings accessed from 0..9
An Array
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Composite Data Types
Declare Array
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Composite Data Types
Access an Array
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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
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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
Rememberthepattern?
Fromproblemto generaldescriptionto classdefinition toprogram
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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
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.52
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; }
The subclasses SavingsAccount and CheckingAccount inherit the properties of BankAccount.
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