lecture 5: part 2 classes and data abstraction. objects models of things in the real world defined...

Lecture 5: Part 2

Classes and Data Abstraction

Objects

Models of things in the real world Defined in classes

Class name is the object name Example: Library Book Use methods to interact with objects

Methods

Used for one of the following reasons To change the state of an object – the information

stored in the object To calculate a result To retrieve a particular data item that is stored in

an object To get data from the user To display the result of an operation

Methods for a Library Book

Data associated with a library book Whether it is checked out Who has checked it out When it is due Title of the book

What should the methods be?

Invoking methods

Create instance of class LibraryBook book1;

To call a method, use the dot notation objectName.methodName(argumentlist)

book1.setTitle(“The Da Vinci Code”); Member access operators

Dot operator (.) for class members

Strings

string Class – part of the standard template library stores multiple characters

Examples string name; string flower;

To create objects – called class instantiationClassName variableName(arguments); string flower(“tulip”);

Assigning value later

Long versionstring flower;

string name;

flower = string(“Rose”);

name = string(“Pete”); Shorter versionflower = “Rose”;

name = “Pete”;

Special assignment for

strings

Creating examples

Assume: char buffer[] = “hi”; string s0(“string”); string s1; string s2(s0); string s3(buffer); string s4(buffer, 1); string s5(5, ‘f’);

string

Ø

string

hi

h

fffff

String Concatenation

Use the plus operator to join Strings together ‘+’ is the concatenation operator Example:

string fullName = name + “ “ + flower;

fullName holds the value “Pete

Rose”

String Methods

str.length() – Finds the length of the string (str.size() does the same thing)

str.at(6) – Gets the character at position 6 (the first character is at position 0)

str.subString(5) – Gets the part of str starting a position 5

str.substr (5, 2) – Gets the 2 characters of str starting at position 5

More String Methods

str.find(“is”) – Determines the position of the first occurrence of “is” in str. If the “is” does not occur in the string, returns string::npos

str.find(“i”, 5) – Determines the position of the first occurrence of “i” in str starting the search at position 5

String Methods Examples

string saying(“C++ is fun!”);

saying.length(); What is the result?

the integer 11

saying.at(0); What is the result?

the letter C

saying.at(saying.length() – 1); What is the result?

the character !

String Methods Examples

saying.substr(5, saying.length()); What is the result?

The string “is fun!”saying.substring(0, 3); What is the result?

The string “C++”saying.find(“C++”); What is the result?

The integer 0saying.find(“c++”); What is the result?

The integer string::npos

saying = “C++ is fun!”

Displaying and Storing Method Results Use assignment statements to store the result of a

method call int posBlank = saying.find(“ “);

posBlank stores the value 3 String firstWord = saying.substr(0, posBlank);

firstWord store the value “C++”

Including the method call in the output statement cout << “The first blank is a position “

<< saying.find(“ “));

Object-Oriented Design

Program divided into 2 parts Application

File that contains main Create and manipulate one or more worker objects

Worker classes Contain methods that perform different kinds of

operations Example: string

6.1 Introduction

Object-oriented programming (OOP) Encapsulates data (attributes) and functions (behavior) into

packages called classes Information hiding

Class objects communicate across well-defined interfaces Implementation details hidden within classes themselves

User-defined (programmer-defined) types: classes Data (data members) Functions (member functions or methods) Similar to blueprints – reusable Class instance: object

Developing a Worker Class

The class definition Data field declarations Constructor definitions Method definitions

Library Book Example

Data Fields title author date due who checked the book

out

Methods get title get author get due date get lendee calculate overdue fine calculate days until due check out renew check in

Data Fields Store the information associated with an object of that

class May be referenced by any method in the class Values of the fields represent the state of the object Also called instance or class variables Data field declaration

typeName dataFieldName; Examples

int pennies; string month;

Library Book Declaration

class LibraryBook {private:

string title; string author; MyDate dueDate; string borrower;

public:};

titleauthordate duewho checked the book out

Constructor Definitions

Method that is called when a new object is created

Purpose is to initialize the data fields May be multiple constructors with different

sets of arguments

Library Book – adding the constructorclass LibraryBook {

private: string title; string author; string borrower; MyDate dueDate;

public: // Construtor LibraryBook(string t, String a);};

Method Declarations

Method prototype species the name of the method data type of the value returned parameters in parentheses Form

resultType methodName([parameter list]) Example:

double getPrice(); void setPrice(double);

Library Book – Adding the Methodsclass LibraryBook { private: string title; string author; string borrower; MyDate dueDate; public: // Construtor LibraryBook(string t, String a); // Return title string getTitle();

// Returns dueDate MyDate getDueDate(); // Checks out book void checkout(string name, MyDate due);

// Checks book in void checkin();};

get titleset due datecheck outcheck in

Found in the file: LibraryBook.h

Calling Methods

A call to a method is considered a statement For methods return void

Example: book1.checkin(); For methods returning a value (non-void), you

should store the value in a variable or use the value Example: string out = “Title – “ + book1.getTitle();

Postconditions

Comments that precede a method Tell user of the class what will be true after the

method is called Part of the documentation Examples

Method returns the title of the book // postcondition: returns the title of this book

Method checks in a book // postcondition: sets borrower to empty string and

dueDate to default value

6.6 Class Scope and Accessing Class Members Class scope

Data members, member functions Within class scope

Class members Immediately accessible by all member functions Referenced by name

Outside class scope Referenced through handles

Object name, reference to object, pointer to object

File scope Nonmember functions

6.6 Class Scope and Accessing Class Members Function scope

Variables declared in member function Only known to function Variables with same name as class-scope

variables Class-scope variable “hidden”

Access with scope resolution operator (::)

ClassName::classVariableName

Variables only known to function they are defined in

Variables are destroyed after function completion

Library Book Method definitionsConstructor Form: className::methodName([parameters]) Purpose is to initialize the data fieldsLibraryBook::LibraryBook(string t, string a)

{

title = t;

author = a;

borrower = “"; // book is not currently

// checked out

dueDate = MyDate();

}

Writing code for Library Book Methodsstring LibraryBook::getTitle(){

return title;}Date LibraryBook::getDueDate(){

return dueDate;}void LibraryBook::checkout(string name, MyDate due){

borrower = name;dueDate = due;

}void LibraryBook::checkin(){

borrower = “”;dueDate = MyDate();

}

Found in the file: LibraryBook.cpp

Ways to Create Stringsstring s; Default constructor. Creates an empty string

string s (str); Copy constructor. Creates a new string s as a copy of another string, str

string s(str,

indx);

Creates a new string s from characters starting at index indx of str

string s(str,

indx, count);

Creates a new string s initialized by at most count characters from str, starting at index indx in str

string s(cstr); Creates a new string s initialized with characters from the cstring cstr

string s(charArray,

count);Creates a new string s initialized with at most count characters from char array charArray

string s(count,

ch);

Creates a new string s initialized with count instances of character ch

Access to string Elements

c = s[i] Indexed access with no range checking. Character at index i is returned

c = s.at(i) Indexed access with range checking. Character at index i is returned. Throws an out_of_range excepetion if i ≥ s.size()

string size methods

s.length() Returns the number of characters currently in s

s.size() Same as s.length()

s.resize(newSize, padChar)

Changes the size of s to newSize, filling with repetitions of the character padChar if necessary

s.empty() Returns true if s is empty, else returns false

s.capacity() Returns the number of characters that s can contain without having to reallocate

string Search and Substringss.find(str) Returns the integer index of the first position of the

first occurrence of string str in s

s.find(str,

pos)

Returns the integer index of the first position of the first occurrence of string str in s, with the search starting at position pos of s

s.find_first_of

(delim, pos)

Returns the integer index of the first position of the first occurrence of any character from the string delim, with the search starting at position pos of s

s.find_first_not_of(delim, pos)

Returns the integer index of the first position of the first occurrence of any character not in the string delim, with the search starting at position pos of s

s.substr(pos, len)

Returns a string object that represents a substring of s of at most len characters, starting at position pos of s. If pos is too large, an out_of_range exception is thrown

string Comparisonss1 == s2 Returns true if all characters of s1 and s2 are

pairwise equal, else turns false

s1 != s2 Returns true if not all characters of s1 and s2 are pairwise equal, else returns false

s1 < s2 Returns true if s1 comes before s2 lexicographically, else returns false

s1 > s2 Returns true if s1 comes after s2 lexicographically, else returns false

s1 <= s2 Same as !(s1 > s2)

S1 >= s2 Same as !(s1 < s2)

Lexicographic ordering compares characters at corresponding positions sequentiallyuntil a position i is found where s1[i] ≠ s2[i]. Then the expression s1 < s2 has the same Boolean value as s1[i] < s2[i].

string I/O Operationsos << str Places the characters from string str onto

stream os

is >> str Extracts characters from stream is into string str. Leading whitespace characters are skipped, and input stops at the first trailing whitespace character

getline(is,

str, delimiter)

Reads characters from stream is into string str up to end-of-file or until the character delimiter is extracted. The delimiter is removed from is and discarded. Note: getline is not a member of the string class. It is a stand-alone, global function.

More complete list of class methods: http://www.msoe.edu/eecs/cese/resources/stl/string.htm

fig08_13.cpp(1 of 4)

1 // Fig. 8.13: fig08_13.cpp

2 // Standard library string class test program.

3 #include <iostream> 4 using std::cout;

5 using std::endl; 6 #include <string> 7 using std::string;

8 9 int main() {10 string s1( "happy" );

11 string s2( " birthday" );

12 string s3;

13 14 // test overloaded equality and relational operators

15 cout << "s1 is \"" << s1 << "\"; s2 is \"" << s2

16 << "\"; s3 is \"" << s3 << '\"'

17 << "\n\nThe results of comparing s2 and s1:"

18 << "\ns2 == s1 yields "

19 << ( s2 == s1 ? "true" : "false" )

20 << "\ns2 != s1 yields "

21 << ( s2 != s1 ? "true" : "false" )


21 << "\ns2 > s1 yields "

22 << ( s2 > s1 ? "true" : "false" )

23 << "\ns2 < s1 yields "

24 << ( s2 < s1 ? "true" : "false" )

25 << "\ns2 >= s1 yields "

26 << ( s2 >= s1 ? "true" : "false" )

27 << "\ns2 <= s1 yields "

28 << ( s2 <= s1 ? "true" : "false" );

29 30 // test string member function empty

31 cout << "\n\nTesting s3.empty():\n"; 32 if ( s3.empty() ) {

33 cout << "s3 is empty; assigning s1 to s3;\n";

34 s3 = s1; // assign s1 to s3

35 cout << "s3 is \"" << s3 << "\"";

36 }37 38 // test overloaded string concatenation operator

39 cout << "\n\ns1 += s2 yields s1 = ";

40 s1 += s2; // test overloaded concatenation

41 cout << s1;


42 // test overloaded string concatenation operator

43 // with C-style string

44 cout << "\n\ns1 += \" to you\" yields\n";

45 s1 += " to you";

46 cout << "s1 = " << s1 << "\n\n";

47 48 // test string member function substr

49 cout << "The substring of s1 starting at location 0 for\n"

50 << "14 characters, s1.substr(0, 14), is:\n"

51 << s1.substr( 0, 14 ) << "\n\n";

52 53 // test substr "to-end-of-string" option

54 cout << "The substring of s1 starting at\n"

55 << "location 15, s1.substr(15), is:\n"

56 << s1.substr( 15 ) << '\n';

57


58 // test using subscript operator to create lvalue

59 s1[ 0 ] = 'H';

60 s1[ 6 ] = 'B';

61 cout << "\ns1 after s1[0] = 'H' and s1[6] = 'B' is: "

62 << s1 << "\n\n";

63 64 // test subscript out of range with string member function "at"

65 cout << "Attempt to assign 'd' to s1.at( 30 ) yields:" << endl;

66 s1.at( 30 ) = 'd'; // ERROR: subscript out of range

67 68 return 0;

69 70 } // end main

s1 is "happy"; s2 is " birthday"; s3 is ""

The results of comparing s2 and s1:

s2 == s1 yields false

s2 != s1 yields true

s2 > s1 yields false

s2 < s1 yields true

s2 >= s1 yields false

s2 <= s1 yields true

Testing s3.empty():

s3 is empty; assigning s1 to s3;

s3 is "happy"

s1 += s2 yields s1 = happy birthday

fig08_13.cppoutput (1 of 2)

s1 += " to you" yields

s1 = happy birthday to you

The substring of s1 starting at location 0 for

14 characters, s1.substr(0, 14), is:

happy birthday

The substring of s1 starting at

location 15, s1.substr(15), is:

to you

s1 after s1[0] = 'H' and s1[6] = 'B' is: Happy Birthday to you

Attempt to assign 'd' to s1.at( 30 ) yields:

abnormal program termination

fig08_13.cppoutput (2 of 2)

Example

999999

976001

124100

402976

432402

762800

001761

761432

100124

800762

Orig

inal

Arra

y Arra

y Af

ter

2nd P

ass

Arra

y Af

ter

1st P

ass

Arra

y Af

ter

3rd P

ass

Table

[0] [1] [2] [3] … [n-1]

[0] 800 100

[1] 761 001

[2] 762 432 402

[3]

[4] 124

[5]

[6] 976

[7]

[8]

[9] 999

•Insert original array into the table as shown•Collect the values so they are ordered as shown in Array After 1st Pass

Example

999999999

976976001

762124100

761402976

432432402

124762800

402001761

001761432

100100124

800800762

Orig

inal

Arra

y Arra

y Af

ter

2nd P

ass

Arra

y Af

ter

1st P

ass

Arra

y Af

ter

3rd P

ass

Table

[0] [1] [2] [3] … [n-1]

[0] 800 100 001 402

[1]

[2] 124

[3] 432

[4]

[5]

[6] 761 762

[7] 976

[8]

[9] 999

•Doing the same process for the tens place

Example

999999999999

976976976001

800762124100

762761402976

761432432402

432124762800

402402001761

124001761432

100100100124

001800800762

Orig

inal

Arra

y Arra

y Af

ter

2nd P

ass

Arra

y Af

ter

1st P

ass

Arra

y Af

ter

3rd P

ass

Table

[0] [1] [2] [3] … [n-1]

[0] 001

[1] 100 124

[2]

[3]

[4] 402 432

[5]

[6]

[7] 761 762

[8] 800

[9] 976 999

•Doing the same process for the hundreds place

lecture 5: part 2 classes and data abstraction. objects models of things in the real world defined...

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