Chapter 3. Expressions and Interactivity

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Starting Out with C++, 3rd Edition

Chapter 3. Expressions and Interactivity

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3.1 The cin Object

• The cin object reads information types at the keyboard.

• cin is the standard input object• Notice the >> and << operators appear to

point in the direction information is flowing.

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Program 3-1#include <iostream.h>void main(void){

int length, width, area;cout <<"This program calculates the area of a rectangle.\n";cout <<"What is the length of the rectangle? ";cin>>length;cout <<"What is the width of the rectangle? ";cin>>width;area = length * width;cout <<"The area of the rectangle is " << area << ".\n";

}

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Program Output

This program calculates the area of a rectangle.

What is the length of the rectangle? 10 [Enter]

What is the width of the rectangle? 20 [Enter]

The area of the rectangle is 200.

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Program 3-2// This program reads the length and width of a rectangle. // It calculates the rectangle's area and displays // the value on the screen.#include <iostream.h>

void main(void){

int length, width, area;cin >> length;cin >> width;area = length * width;cout << "The area of the rectangle is " << area << endl;

}

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Entering Multiple Values

• The cin object may be used to gather multiple values at once.

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Program 3-3#include <iostream.h>

void main(void){

int length, width, area;cout <<"This program calculates the area of a rectangle.\n";cout <<"Enter the length and width of the rectangle separated by a space. \n";cin >> length>> width;area = length * width;cout <<"The area of the rectangle is " << area << endl;

}

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Program Output

This program calculates the area of a rectangle.

Enter the length and width of the rectangle separated by a space.

10 20 [Enter]The area of the rectangle is 200

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Program 3-4// This program demonstrates how cin can read multiple values// of different data types.#include <iostream.h>

void main(void){

int whole;float fractional;char letter;

cout << "Enter an integer, a float, and a character: ";cin >> whole >> fractional >> letter;cout << "whole: " << whole << endl;cout << "fractional: " << fractional << endl;cout << "letter: " << letter << endl;

}

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Program Output

Enter an integer, a float, and a character: 4 5.7 b [Enter]

whole: 4 fractional: 5.7 letter: b

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Reading Strings

• cin can read strings as well as numbers.• Strings are stored in character arrays.char Company[12];

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Program 3-5#include <iostream.h>#include<string>

void main(void){

string name;cout << "What is your name? ";cin >> name;cout << "Good morning " << name << endl;

}

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Program Output

What is your name? Charlie [Enter]Good morning Charlie

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Program 3-6// This program demonstrates how cin can read a// string into a character array

#include <iostream.h>

void main(void){

char name[21];

cout << "What is your name? ";cin >> name;cout << "Good morning " << name << endl;

}

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Program Output

What is your name? Charlie [Enter]Good morning Charlie

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Program 3-7// This program reads two strings into two character arrays.#include <iostream.h>

void main(void){

char first[16], last[16];cout << "Enter your first and last names and I will\n";cout << "reverse them.\n"; cin >> first >> last;cout << last << ", " << first << endl;

}

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Program Output

Enter your first and last names and I willreverse them.Johnny Jones [Enter]Jones, Johnny

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Notes on strings:

• If a character array is intended to hold strings, it must be at least one character larger than the largest string that will be stored in it.

• The cin object will let the user enter a string larger than the array can hold. If this happens, the string will overflow the array’s boundaries and destroy other information in memory.

• If you wish the user to enter a string that has spaces in it, you cannot use this input method.

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3.2 Focus on Software Engineering: Mathematical Expressions

• C++ allows you to construct complex mathematical expressions using multiple operators and grouping symbols.

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Program 3-7// This program reads two strings into two character

arrays#include <iostream.h>

void main(void){

char first[16], last[16];

cout << Enter your first and last names and I will\n";cout << "reverse them.\n";cin >> first >> last;cout << last << ", " << first << endl;

}

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Program 3-7 Output with Example Input

Enter your first and last names and I will reverse them.Johnny Jones [Enter]Jones, Johnny

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Program 3-8// This program asks the user to enter the numerator// and denominator of a fraction and it displays the// decimal value#include <iostream.h>

void main(void){

float numerator, denominator;

cout << "This program shows the decimal value of ";cout << "a fraction.\n";

cout << “Enter the numerator: “;cin >> numerator;cout << “Enter the denominator: “;cin >> denominator;

cout << “The decimal value is “; cout << (numerator / denominator);}

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Program Output for Program 3-8 with Example Input

This program shows the decimal value of a fraction.

Enter the numerator: 3 [Enter]Enter the denominator: 6 [Enter]The decimal value is 0.1875

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Table 3-1 Precedence of Arithmetic Operators (Highest to Lowest)

(unary negation) -* / %+ -

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Table 3-2 Some Expressions

Expression Value 5 + 2 * 4 13 10 / 2 - 3 2 8 + 12 * 2 - 4 28 4 + 17 % 2 - 1 4 6 - 3 * 2 + 7 - 1 6

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Associativity

• If two operators sharing an operand have the same precedence, they work according to their associativity, either right to left or left to right

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Table 3-3 Associativity of Arithmetic OperatorsOperator Associativity(unary negation) - Right to left* / % Left to right+ - Left to right

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Converting Algebraic Expressions to Programming Statements

AlgebraicExpression

Operation C++ Equivalent

6B 6 times B 6 * B(3)(12) 3 times 12 3 * 124xy 4 times x times y 4 * x * y

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No Exponents Please!

• C++ does not have an exponent operator.• Use the pow() library function to raise a

number to a power.• Will need #include <math.h> for pow()

function.area = pow(4,2) // will store 42 in area

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Program 3-9// This program calculates the area of a circle.// The formula for the area of a circle is Pi times// the radius squared. Pi is 3.14159.#include <iostream.h>#include <math.h> // needed for the pow function

void main(void){

double area, radius;cout << "This program calculates the area of a circle.\n";cout << "What is the radius of the circle? ";cin >> radius;area = 3.14159 * pow(radius,2);cout << "The area is " << area;

}

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Program 3-9 Output With Example Input

This program calculates the area of a circle.What is the radius of the circle? 10[Enter]The area is 314.159

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3.3 When you Mix Apples and Oranges: Type Coercion

• When an operator’s operands are of different data types, C++ will automatically convert them to the same data type.

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Type Coercion Rules:

• Rule 1: Chars, shorts, and unsigned shorts are automatically promoted to int.

• Rule 2: When an operator works with two values of different data types, the lower-ranking value is promoted to the type of the higher-ranking value.

• Rule 3: When the final value of an expression is assigned to a variable, it will be converted to the data type of the variable.

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3.4 Overflow and Underflow

• When a variable is assigned a value that is too large or too small in range for that variable’s data type, the variable overflows or underflows.

• Overflow - when a variable is assigned a number that is too large for its data type

• Underflow - when a variable is assigned a number that is too small for its data type

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Program 3-10// This program demonstrates integer underflow and// overflow#include <iostream.h>

void main(void){

short testVar = 32767;cout << testVar << endl;testVar = testVar + 1;cout << testVar << endl;testVar = testVar - 1;cout << testVar << endl;

}

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Program Output

32767-3276832767

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Program 3-11// This program can be used to see how your system// handles floating point overflow and underflow.


void main(void){

float test;test = 2.0e38 * 1000; // Should overflow testcout << test << endl;test = 2.0e-38 / 2.0e38;cout << test << endl;

}

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3.5 The Typecast Operator

• The typecast operator allows you to perform manual data type conversion.

Val = int(number); //If number is a floating //point variable, it will be //truncated to an integer and //stored in the variable Val

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void main(void){

int months, books;float perMonth;cout << "How many books do you plan to read? ";cin >> books;cout << "How many months will it take you to read them? ";cin >> months;perMonth = float(books) / months;cout << "That is " << perMonth << " books per month.\n";

}

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Program Output

How many books do you plan to read? 30 [Enter]

How many months will it take you to read them? 7 [Enter]

That is 4.285714 books per month.

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Typecast Warnings

• In Program 3-11, the following statement would still have resulted in integer division:

perMonth = float(books / months);

• Because the division is performed first and the result is cast to a float.

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Program 3-13// This program uses a typecast operator to print // a character from a number. #include <iostream.h>

void main(void){

int number = 65;cout << number << endl;cout << char(number) << endl;

}

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Program Output

65A

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3.6 The Power of Constants

• Constants may be given names that symbolically represent them in a program.

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Program 3-14// This program calculates the area of a circle.#include <iostream.h>#include <math.h>

void main(void){

const float pi = 3.14159;double area, radius;cout << "This program calculates the area of a circle.\n";cout << "What is the radius of the circle? ";cin >> radius;area = pi * pow(radius,2);cout << "The area is " << area;

}

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The #define Directive• The older C-style method of creating named

constants is with the #define directive, although it is preferable to use the const modifier.

#define PI 3.14159

• is roughly the same asconst float PI=3.14159;

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Program 3-15

#include <iostream.h>#include <math.h> // needed for pow function#define PI 3.14159

void main(void){

double area, radius;cout << "This program calculates the area of a circle.\n";cout << "What is the radius of the circle? ";cin >> radius;area = PI * pow(radius, 2);cout << "The area is " << area;

}

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3.7 Multiple Assignment and Combined Assignment

• Multiple assignment means to assign the same value to several variables with one statement.

A = B = C = D = 12;Store1 = Store2 = Store3 = BegInv;

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Program 3-16// The program tracks the inventory of three widget stores// that opened at the same time. Each store started with the// same number of widgets in inventory. By subtracting the// number of widgets each store has sold from its inventory,// the current inventory can be calculated.


void main(void){

int begInv, sold, store1, store2, store3;

cout << “One week ago, 3 new widget stores opened\n";cout << “at the same time with the same beginning\n";cout << “inventory. What was the beginning inventory? ";cin >> begInv;store1 = store2 = store3 = begInv;

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Program 3-16 Continued

cout << "How many widgets has store 1 sold? ";cin >> sold;store1 = store1 – sold; //Subtract sold from store1

cout << "How many widgets has store 2 sold? ";cin >> sold;store2 = store2 – sold; //Subtract sold from store2cout << "How many widgets has store 3 sold? ";cin >> sold;store3 = store3 – sold; //Subtract sold from store 3cout << "Store1: " << store1 << endl;cout << "Store2: " << store2 << endl;cout << "Store3: " << store3 << endl;

}

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Program 3-16 Output with Example InputOne week ago, 3 new widget stores openedat the same time with the same beginninginventory. What was the beginning inventory?

100 [Enter]How many widgets has store 1 sold? 25 [Enter]How many widgets has store 2 sold? 15 [Enter]How many widgets has store 3 sold? 45 [Enter]

The current inventory of each store:

Store 1: 75Store 2: 85Store 3: 55

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Table 3-8

Statement What It Does Value of xafter theStatement

x = x + 4 ; Adds 4 to x 10

x = x - 3 ; Subtracts 3 from x 3

x = x * 1 0 ; Multiplies x by 10 60

x = x / 2 ; Divides x by 2 3

x = x % 4 Makes x the remainder of x / 4 2

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Table 3-9

Operator Example Usage Equivalent To+= x += 5; x = x + 5;-= y -= 2; y = y - 2;*= z += 10; z = z + 10;/= a /= b; a = a / b;

%= c %= 3; c = c % 3;

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Program 3-17// The program tracks the inventory of three widget stores// that opened at the same time. Each store started with the// same number of widgets in inventory. By subtracting the// number of widgets each store has sold from its inventory,// the current inventory can be calculated.


void main(void){

int begInv, sold, store1, store2, store3;

cout << “One week ago, 3 new widget stores opened\n";cout << “at the same time with the same beginning\n";cout << “inventory. What was the beginning inventory? ";cin >> begInv;store1 = store2 = store3 = begInv;

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Program 3-17 Continued

cout << "How many widgets has store 1 sold? ";cin >> sold;store1 -= sold; //Subtract sold from store1

cout << "How many widgets has store 2 sold? ";cin >> sold;store2 -= sold; //Subtract sold from store2cout << "How many widgets has store 3 sold? ";cin >> sold;store3 -= sold; //Subtract sold from store 3cout << "Store1: " << store1 << endl;cout << "Store2: " << store2 << endl;cout << "Store3: " << store3 << endl;

}

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Program 3-17 Output with Example InputOne week ago, 3 new widget stores openedat the same time with the same beginninginventory. What was the beginning inventory?

100 [Enter]How many widgets has store 1 sold? 25 [Enter]How many widgets has store 2 sold? 15 [Enter]How many widgets has store 3 sold? 45 [Enter]

The current inventory of each store:

Store 1: 75Store 2: 85Store 3: 55

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3.8 Formatting Output

• The cout object provides ways to format data as it is being displayed. This affects the way data appears on the screen.

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Program 3-18#include<iostream.h>

void main(void){

int num1 = 2897, num2 = 5,num3 = 837, num4 = 34, num5 = 7, num6 = 1623, num7 = 390, num8 = 3456, num9 = 12;

// Display the first row of numberscout << num1 << " "; cout << num2 << " "; cout << num3 << endl;// Display the second row of numberscout << num4 << " "; cout << num5 << " "; cout << num6 << endl;// Display the third row of numberscout << num7 << " ";cout << num8 << " "; cout << num9 << endl;

}

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Program Output

2897 5 83734 7 1623390 3456 12

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Program 3-19// This program displays three rows of numbers. #include <iostream.h>#include <iomanip.h> // for setw function

void main(void){

int num1 = 2897, num2 = 5, num3 = 837, num4 = 34, num5 = 7, num6 = 1623, num7 = 390, num8 = 3456, num9 = 12;

// Display the first row of numberscout << setw(4) << num1 << " ";cout << setw(4) << num2 << " ";cout << setw(4) << num3 << endl;

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Program continues

// Display the second row of numberscout << setw(4) << num4 << " ";cout << setw(4) << num5 << " ";cout << setw(4) << num6 << endl;// Display the third row of numberscout << setw(4) << num7 << " ";cout << setw(4) << num8 << " ";cout << setw(4) << num9 << endl;

}

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Program Output

2897 5 837 34 7 1623 390 3456 12

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Program 3-20//This program demonstrates the setw manipulator being//used with values of various data types.#include <iostream.h>#include <iomanip.h>

void main(void){

int intValue = 3928;float floatValue = 91.5;char cStringValue[] = "John J. Smith";cout << "(" << setw(5) << intValue << ")" << endl;cout << "(" << setw(8) << floatValue << ")" << endl;cout << "(" << setw(16) << cStringValue << ")" << endl;

}

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Program Output

( 3928)( 91.5)( John J. Smith)

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Precision

• Floating point values may be rounded to a number of significant digits, or precision, which is the total number of digits that appear before and after the decimal point.

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Program 3-21// This program demonstrates how setprecision rounds a// floating point value#include <iostream.h>#include <iomanip.h>

void main(void){

float quotient, number1 = 132.364, number2 = 26.91;

quotient = number1 / number2;cout << quotient << endl;cout << setprecision(5) << quotient << endl;cout << setprecision(4) << quotient << endl;cout << setprecision(3) << quotient << endl;cout << setprecision(2) << quotient << endl;cout << setprecision(1) << quotient << endl;

}

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Program Output

4.918774.91884.9194.924.95

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Table 3-11

Number Manipulator Value Displayed

28.92786 setprecision(3) 28.9

21 setprecision(5) 21

109.5 setprecision(4) 109.5

34.28596 setprecision(2) 34

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Program 3-22//This program asks for sales figures for 3 days.

The// total sales is calculated and displayed in a

table#include <iostream.h>#include <iomanip.h> void main(void){

float day1, day2, day3, total;

cout << "Enter the sales for day 1: ";cin >> day1;cout << "Enter the sales for day 2: ";cin >> day2;

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Program Continues

cout << "Enter the sales for day 3: ";cin >> day3;total = day1 + day2 + day3;cout << "\nSales Figures\n";cout << "-------------\n";cout << setprecision(5);cout << “Day 1: " << setw(8) << day1 << endl;cout << “Day 2: " << setw(8) << day2 << endl;cout << “Day 3: " << setw(8) << day3 << endl;

cout << “Total: " << setw(8) << total << endl;}

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Program Output

Enter the sales for day 1: 321.57 [Enter]Enter the sales for day 2: 269.62 [Enter]Enter the sales for day 3: 307.77 [Enter] Sales Figures-------------Day 1: 321.57Day 2: 269.62Day 3: 307.77Total: 898.96

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Program 3-23//This program asks for sales figures for 3 days. The // total sales is calculated and displayed in a table.#include <iostream.h>#include <iomanip.h>

void main(void){

float day1, day2, day3, total;cout << "Enter the sales for day 1: ";cin >> day1;cout << "Enter the sales for day 2: ";cin >> day2;cout << "Enter the sales for day 3: ";cin >> day3;total = day1 + day2 + day3;

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Program Continues

cout << "\nSales Figures\n";cout << "------\n";cout << setprecision(2) << setiosflags(ios::fixed);cout << “Day 1: " << setw(8) << day1 << endl;cout << “Day 2: " << setw(8) << day2 << endl;cout << “Day 3: " << setw(8) << day3 << endl;cout << "Total: " << setw(8) << total << endl;

}

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Program Output

Enter the sales for day 1: 1321.87 [Enter]Enter the sales for day 2: 1869.26 [Enter]Enter the sales for day 3: 1403.77 [Enter]

Sales Figures-------------Day 1: 1321.87Day 2: 1869.26Day 3: 1403.77Total: 4594.9

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IOS Flags

• setiosflags manipulator can be used to format output in a variety of ways, depending on which flag is specified.

• setiosflags(ios::fixed) will cause all subsequent numbers to be printed in fixed point notation

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Program 3-24// This program asks for sales figures for 3 days. The // total sales is calculated and displayed in a table. #include <iostream.h>#include <iomanip.h>

void main(void){

float day1, day2, day3, total;cout << "Enter the sales for day 1: ";cin >> day1;cout << "Enter the sales for day 2: ";cin >> day2;

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Program continues

cout << "Enter the sales for day 3: ";cin >> day3;total = day1 + day2 + day3;cout << "\nSales Figures\n";cout << "-------------\n";cout << setprecision(2)

<< setiosflags(ios::fixed | ios::showpoint);cout << “Day 1: " << setw(8) << day1 << endl;cout << “Day 2: " << setw(8) << day2 << endl;cout << “Day 3: " << setw(8) << day3 << endl;cout << "Total: " << setw(8) << total << endl;

}

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Program Output


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Table 3-12Format Flag Descriptionios::left Causes the field to be left-justified with padding spaces printed to

the right.ios::right Causes the field to be right-justified with padding spaces printed to

the left.ios::fixed Causes all subsequent numbers to be displayed in fixed point

notation.ios::scientific Causes all subsequent numbers to be displayed in scientific notation.ios::dec Causes all subsequent integers to be displayed in decimal format.

ios::hex Causes all subsequent integers to be displayed in hexadecimalformat.

ios::oct Causes all subsequent integers to be displayed in octal format.ios::showpoint Forces the decimal point and trailing zeroes to be displayed.ios::showpos Causes a + sign to be displayed in front of positive numbers.ios::uppercase Causes the E in scientific notation numbers, and the X in

hexadecimal numbers to be displayed in uppercase.

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Formatting Output With Member Functionscout.width(5); //calls the width member // function, same as // setw(5)

cout.precision(2); //sets precision to // 2 significant // digits

// The next statement works like // setiosflags(ios::fixed)cout.setf(ios::fixed);

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Program 3-25// This program asks for sales figures for 3 days. The // total sales is calculated and displayed in a table. #include <iostream.h>#include <iomanip.h> void main(void){


cout << "Enter the sales for day 1: ";cin >> day1;cout << "Enter the sales for day 2: ";cin >> day2;cout << "Enter the sales for day 3: ";cin >> day3;

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Program continues

total = day1 + day2 + day3; cout.precision(2); cout.setf(ios::fixed | ios::showpoint);

cout << "\nSales Figures\n";cout << "-------------\n";cout << “Day 1: ";cout.width(8);cout << day1 << endl;cout << “Day 2: ";cout.width(8);cout << day2 << endl;

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Program continues

cout << “Day 3: ";cout.width(8);cout << day3 << endl;cout << "Total: ";cout.width(8);cout << total << endl;

}

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Program Output


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Program 3-26// This program asks for sales figures for 3 days. The // total sales is calculated and displayed in a table.#include <iostream.h>#include <iomanip.h>

void main(void){


cout << "Enter the sales for day 1: ";cin >> day1;cout << "Enter the sales for day 2: ";cin >> day2;cout << "Enter the sales for day 3: ";cin >> day3;

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Program continues

total = day1 + day2 + day3;cout.precision(2);cout.setf(ios::fixed | ios::showpoint);cout << "\nSales Figures\n";cout << "-------------\n";cout << “Day 1: " << setw(8) << day1 << endl;cout << “Day 2: " << setw(8) << day2 << endl;cout << “Day 3: " << setw(8) << day3 << endl;cout << "Total: " << setw(8) << total << endl;return 0;

}

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Program Output


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Table 3-13

Member Function Descriptioncout.width Sets the display field width.cout.precision Sets the precision of floating point numbers.cout.setf Sets the specified format flags.cout.unsetf Disables, or turns off, the specified format

flags.

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3.9 Formatted Input

• The cin object provides ways of controlling string and character input.

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Program 3-27//This program uses setw with the cin object. #include <iostream.h>#include <iomanip.h>

void main(void){ char word[5]; cout << "Enter a word: "; cin >> setw(5) >> word; cout << "You entered " << word << endl;}

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Program 3-28//This program uses cin's width member function. #include <iostream.h>#include <iomanip.h>

void main(void){ char word[5]; cout << "Enter a word: "; cin.width(5); cin >> word; cout << "You entered " << word << endl;}

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Program Output for Programs 3-27 and 3-28

Enter a word: Eureka [Enter]You entered Eure

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Important points about the way cin handles field widths:

• The field width only pertains to the very next item entered by the user.

• cin stops reading input when it encounters a whitespace character. Whitespace characters include the [Enter] key, space, and tab.

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Reading a “Line” of Input

cin.getline(line, 20);

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Program 3-29//This program demonstrates cin's getline member function.#include <iostream.h>#include <iomanip.h>

void main(void){

char sentence[81];

cout << "Enter a sentence: ";cin.getline(sentence, 81);cout << "You entered " << sentence << endl;

}

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Program Output

Enter a sentence: To be, or not to be, that is the question. [Enter]You entered To be, or not to be, that is the question.

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void main(void){

char ch;

cout << "Type a character and press Enter: ";cin >> ch;cout << "You entered " << ch << endl;

}

Program Output with Example InputType a character and press Enter: A [Enter] You entered A

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void main(void){

char ch;

cout << "This program has paused. Press enter to continue.";cin.get(ch);cout << "Thank you!" << endl;

}

Program OutputThis program has paused. Press Enter to continue.

[Enter]Thank you!

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void main(void){

char ch;

cout << "Type a character and press Enter: ";cin.get(ch);cout << "You entered " << ch << endl;cout << "Its ASCII code is " << int(ch) << endl;

}

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Program Output

Type a character and press Enter: [Enter] You entered Its ASCII code is 10

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Mixing cin >> and cin.get

• Mixing cin.get with cin >> can cause an annoying and hard-to-find problem.

• Pressing the [Enter] key after inputting a number will cause the newline character to be stored in the keyboard buffer. To avoid this, use cin.ignore.

• cin.ignore(20,’\n’); will skip the next 20 chars in the input buffer or until a newline is encountered, whichever comes first

• cin.ignore(); will skip the very next character in the input buffer

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3.10 Focus on Object-Oriented Programming:More About Object- Oriented Programming• A member function is a procedure, written in C++

code, that is part of an object. A member function causes the object it is a member of to perform an action

• In this chapter, we have used width, precision, setf, and unsetf for the cout object

• In this chapter we have used width, getline, get, and ignore for the cin object

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3.11 More Mathematical Library Functions

• The C++ runtime library provides several functions for performing complex mathematical operations.

• In this chapter we have used width, precision, setf, and unsetf for the cout object

• In this chapter we have used width, getline, get, and ignore for the cin object

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Table 3-14ab s Example Usage:

y = a b s (x ); Description Returns the absolute value of the argument. The

argument and the return value are integers. c o s Example Usage:

y = c o s (x ); Description Returns the cosine of the argument. The argument

should be an angle expressed in radians. The return type and the argument are doubles.

e x p Example Usage: y = e x p (x );

Description Computes the exponential function of the argument, which is x . The return type and the argument are doubles.

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Table 3-14 continuedfm o d Example Usage:

y = fm o d (x , z ); Description Returns, as a double, the remainder of the first argument

divided by the second argument. Works like the modulus operator, but the arguments are doubles. (The modulus operator only works with integers.) Take care not to pass zero as the second argument. Doing so would cause division by zero.

lo g Example Usage: y = lo g (x );

Description Returns the natural logarithm of the argument. The return type and the argument are doubles.

lo g 1 0 Example Usage: y = lo g 1 0 (x );

Description Returns the base-10 logarithm of the argument. The return type and the argument are doubles.

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Table 3-14 continueds in Example Usage:

y = s in (x ); Description Returns the sine of the argument. The argument should be an angle

expressed in radians. The return type and the argument are doubles. sq rt Example Usage:

y = sq rt(x ); Description Returns the square root of the argument. The return type and

argument are doubles.

ta n Example Usage: y = ta n (x ); Description Returns the tangent of the argument. The argument should be an

angle expressed in radians. The return type and the argument are doubles.

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Program 3-33// This program asks for the lengths of the 2 sides of a right // triangle. The length of the hypotenuse is then calculated// and displayed.#include <iostream.h>#include <math.h> // For sqrt and pow

void main(void){

float a, b, c;

cout << "Enter the length of side A: ";cin >> a;cout << "Enter the length of side B: ";cin >> b;c = sqrt(pow(a, 2.0) + pow(b, 2.0));cout.precision(2);cout << "The length of the hypotenuse is ";cout << c << endl;

}

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Program Output

Enter the length of side A: 5.0 [Enter]Enter the length of side B: 12.0 [Enter]The length of the hypotenuse is 13

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Random numbers

rand() (from the cstdlib library)

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Program 3-34// This program demonstrates random numbers.#include <iostream.h>#include <stdlib.h>using namespace std;

void main(void){

unsigned seed;cout << "Enter a seed value: ";cin >> seed;srand(seed);cout << rand() << endl;cout << rand() << endl;cout << rand() << endl;

}

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Program Output

Enter a seed value: 517313203621622Program Output with Other Example InputEnter a seed value: 165540296639920

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3.13 Optional Section : Introduction to Simple File Input and Output

• What is a File? A file is a collection on information, usually stored on a computer’s disk. Information can be saved to files and then later reused.

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The Process of Using a File

• Using a file in a program is a simple three-step process• The file must be opened. If the file does not yet

exits, opening it means creating it.• Information is then saved to the file, read from

the file, or both.• When the program is finished using the file, the

file must be closed.

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Figure 3-8

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Figure 3-9

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Setting Up a Program for File Input/Output

• Before file I/O can be performed, a C++ program must be set up properly.

• File access requires the inclusion of the fstream.h header file.

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Table 3-16

File Type Default Open Mode ofstream The file is opened for output only. Information may be written

to the file, but not read from the file.

ifstream The file is opened for input only. Information may be read from the file, but not written to it.

fstream The file is opened for input and output. Information may be written to the file or read from it.

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Opening a File

• Before data can be written to or read from a file, the file must be opened.

ifstream inputFile;inputFile.open(“customer.dat”);

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Closing a File

• A file should be closed when a program is finished using it.

outputFile.close();

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Writing Information to a File

• The stream insertion operator (<<) may be used to write information to a file.

outputFile << “I love C++ programming !”

outputFile << “Price: “ << price;

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Program 3-35// This program uses the << operator to write information to a// file.#include <iostream.h>#include <fstream.h>

void main(void){

ofstream outputFile;

outputFile.open("demofile.txt");cout << "Now writing information to the file.\n";

// Write 4 great names to the fileoutputFile << "Bach\n";outputFile << "Beethoven\n";outputFile << "Mozart\n";outputFile << "Schubert\n";

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Program 3-35 (continued)// Close the fileoutputFile.close();cout << "Done.\n";

}

Program Screen Output

Now writing information to the file.Done.

Output to File demofile.txt

BachBeethovenMozartSchubert

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Reading Information from a File

• The stream extraction operator (>>) may be used to read information from a file.

inFile >> name;

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Program 3-36// This program uses the >> operator to read information from a// file.#include <iostream.h>#include <fstream.h>

void main(void){

ifstream inFile;char name[81];

inFile.open("demofile.txt");cout << "Reading information from the file.\n\n";

// Now read name 1 from the fileinFile >> name;cout << name << endl;// Now read name 2 from the fileinFile >> name;cout << name << endl

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Program 3-36 (continued)// Now read name 3 from the fileinFile >> name;cout << name << endl;// Now read name 4 from the fileinFile >> name;cout << name << endl;

// Close the fileinFile.close();cout << "\nDone.\n";

}

Program Screen OutputReading information to the file.

BachBeethovenMozartSchubert

Done.

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Program 3-37// This program uses the >> operator to read information from a// file.#include <iostream.h>#include <fstream.h>

void main(void){

ifstream inFile;int length, width, area;

inFile.open("dimensions.txt");cout << "Reading dimensions of 5 rectangles from the file.\n\n";

// Process rectangle 1inFile >> length;inFile >> width;area = length * width;cout << "Area of rectangle 1: " << area << endl;


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Program 3-37 (continued)// Process rectangle 3inFile >> length;inFile >> width;area = length * width;cout << "Area of rectangle 3: " << area << endl;



// Close the fileinFile.close();cout << "\nDone.\n";

}

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Program 3-37Before Program 3-37 is executed, the file dimensions.txt must be created with a text editor (such as Windows Notepad). Here is an example of the file's contents:

10 25 718 96 208 3

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Program 3-37The program's output is shown below.

Program OutputReading dimensions of 5 rectangles from the file.

Area of rectangle 1: 20Area of rectangle 2: 35Area of rectangle 3: 162Area of rectangle 4: 120Area of rectangle 5: 24

Done.

Documents

Chapter 3. Expressions and Interactivity