CS114-009 Class 19 Today

Practice with classes Announcements

Turn in algorithm for Project 5 in class todayProject 5 due 11/11 by midnight – e-mail to

vrbsky@cs.ua.edu Make sure you have read Section 10.2

pp. 540-572

Review – creating a class Our class

LARGE INTEGERS Normal integer can only

hold up to 2 billion Want to be able to hold

a number with up to 100 digits in it

Declarationclass INT {

// Attributes

// items needed to

// represent the data

public:

// Methods

// constructors

// basic arithmetic (+, –)

// input and output

};

Sample Class (for large ints) Data representation

100 digits Sign (positive or negative)

Constructors Two constructors Default

INT a, b, c[10]; Construct from an integer

INT a(-37), b=89, c[2]={12,34}; Other operations

Addition Read and Write

Last four methods are better Use actual + and – operators Use actual << and >> operators

Friends (for I/O) Overload << and >> for INT

Will focus on those in CS 124

class INT {int digits[100];char sign;

public:INT( );INT(int num);void Add(INT); // x.Add(y);void Read( ); // x.Read();void Write( ); // x.Write();

INT& operator + (const INT& a);INT& operator - (const INT& a);friend ostream& operator<<

(ostream&, const INT&);friend istream& operator>>

(istream&, INT&);};

class INT {

int digits[100];

char sign;

public:

INT( );

void Read( );

void Write( );

void Add( INT );

};

Try to get this version workingint main( ) {

INT a, b;

a.Read( );

b.Read( );

a.Write( ); cout << endl;

b.Write( ); cout << endl;

a.Add(b);

a.Write( ); cout << endl;

return 0;

}

“Dot” notation We’ve seen this, but haven’t discussed. Classes we’ve worked with: cin, cout, string

cin.eof() // End of File? string num; num.length(); // length of a string num.at(i); // character in num cout.setf(ios::fixed) // set the output format

The ‘dot’ is a way of calling a function for that object.s1.length(); s2.length(); s3.length();

// different objects, different lengths

Code for Constructor

INT::INT( ) {

for (int a=0;a<100;a++)

digits[a] = 0;

sign = 'p';

}

System automatically calls constructor whenever a variable of this type is declared

Constructor initializes all digits to zero and the sign to ‘p’ ‘n’ represents negative

Code for Read & Writevoid INT::Write() {

int a = 0;if (sign == 'n') cout << '-';while (a < 100)

cout << digits[a++];}

void INT::Read() {string num;cin >> num;if (num.at(0) == '-') sign = 'n';if (num.at(0) == '+') sign = 'p';int s = 0;if ( !isdigit( num.at(0) ) ) s = 1;int loc = 100 – (num.length() - s);for (int a=s; a<num.length(); a++) digits[loc++] = num.at(a) – '0';

}

Write simply prints out a leading sign (if the number is negative) followed by 100 digits

Read inputs the variable into a string (temporarily) and then extracts from the string into the array of digits

Code for Add method

void INT::Add(INT num) {if (num.sign == 'n' || sign == 'n') cout << “not done" << endl;else { int a, carry = 0, total; for (a=99; a>=0; a--) {

total = digits[a] + num.digits[a] +

carry;if (total > 9) carry = 1;else carry = 0;digits[a] = total % 10;

}}

}

Syntax INT a, b; a.Add(b);

Only adds numbers if both are positive

Ignores overflow (result greater than 100 digits)

Class Exercise The class we have started is on our web

page: Big_INT.cpp

Download this program, then compile and run

Modify Write( ) so that it does not print leading zeros in the number

Modify Add( ) so that it is also capable of adding two negative numbers

Modify Read( ) so that it erases the previous contents of the object

Addition to Class Exercise:Modify Add( ) so that it is also capable of adding

two negative numbers Consider how “Add()” works. It’s called this way:

INT a, b; // read values…a.Add(b); // adds the value of b to a

void INT::Add(INT num)

if (sign == ‘n’ || num.sign == ‘n’) … What’s the difference between “sign” and

“num.sign”? We’re adding two objects.

One is the parameter “num”.The other one is “this” object.

Call: a.Add(b) adds ‘b’ to ‘a’.

a: 42 b: 13

void INT::Add(INT num)

if (sign == ‘n’ || num.sign == ‘n’) …

Which one is “sign”?

Which one is “num.sign”?

a: 42 b: 13

a.Add(b);

Every Class needs… Functions that give you the (private) values Functions that let you change the values We call them:

Accessors (or “getters”) Usually called “getValue()” (for each “Value” attribute)

Mutators (of “setters”) Usually called “setValue(x)”

Other language:Class data members are called “attributes”Class functions are called “methods”

The Car Class Download the file cars.cpp from our class web page What is the constructor of the Car class?

What does this constructor do, in terms of setting attributes?

Identify the accessors Better known as “getter” methods

Identify the mutators Better known as “setter” methods

Identify the other methods. What do they do? Could the functions outside of the Cars class be rewritten as

methods for the Cars class?

Class Exercise Write the definitions to the following methods, whose

prototypes you’ll find on cars.cpp setCarInfo() – set a car’s information, given the make,

model, type, doors, quantity of the car in stock, cylinder engine, and liters of the engine

load() – reads in information from the file cars.txt, and uses setCarInfo() to set the information for an instance of car Data for each car is on one line. The order of the data:

Make, Model, Type, Doors, Liters, Cylinders, Quantity

Class exercise Download the file box.cpp from our class web

page Write the definitions to the following methods,

whose prototypes you’ll find in box.cpp volume() – given the height, width, and length of a

box, return its volume compare() – compares the volume of a box to another

box; returns true if the volumes of the 2 boxes are equivalent, otherwise returns false.

Insert lines of code into main( ) as indicated to utilize the appropriate member function. (see comments in box.cpp)

End of Class 19