c++ programming, namiq sultan1 chapter 4 making decisions namiq sultan university of duhok...

C++ Programming, Namiq Sultan 1

Chapter 4 Making Decisions

Namiq Sultan

University of Duhok

Department of Electrical and Computer Engineerin

Reference: Starting Out with C++, Tony Gaddis, 2nd Ed.


4.1 Relational Operators

• Relational operators allow you to compare numeric values and determine if one is greater than, less than, equal to, or not equal to another.


Table 4-1

Relational Operators (in Order of Precedence)

Meaning

>

<

> =

< =

= =

!=

Greater than

Less than

Greater than or equal to

Less than or equal to

Equal to

Not equal to


Table 4-2

Expression What the Expression Means X > Y

X < Y

X > = Y

X < = Y

X = = Y

X != Y

Is X greater than Y ?

Is X less than Y ?

Is X greater than or equal to Y ?

Is X less than or equal to Y ?

Is X equal to Y ?

Is X not equal to Y ?


The Value of a Relationship

• Relational expressions are also know as a Boolean expression

• Warning! The equality operator is two equal signs together

==


Table 4-3 Assume x is 10 and y is 7

Expression Value

x < y False, because x is not less than y .

x > y True, because x is greater than y .

x > = y True, because x is greater than or equal to y .

x < = y False, because x is not less than or equal to y .

y != x True, because y is not equal to x .


Program 4-1

// This program displays the values of true and false states.

#include <iostream>

using namespace std;

int main()

{int trueValue, falseValue, x = 5, y = 10;

trueValue = x < y;falseValue = y== x;cout << "True is " << trueValue << endl;cout << "False is " << falseValue << endl;

}

Program OutputTrue is 1False is 0


4.2 The if Statement

• The if statement can cause other statements to execute only under certain conditions.


Program 4-2

// This program averages 3 test scores#include <iostream> using namespace std;

int main(void){

int score1, score2, score3;float average;

cout << "Enter 3 test scores and I will average them: ";cin >> score1 >> score2 >> score3;average = (score1 + score2 + score3) / 3.0;cout << "Your average is " << average << endl;if (average > 95)

cout << "Congratulations! That's a high score!\n";

}


Program Output with Example Input

Enter 3 test scores and I will average them: 80 90 70 [Enter]

Your average is 80

Program Output with Other Example Input

Enter 3 test scores and I will average them: 100 100 100 [Enter]

Your average is 100

Congratulations! That's a high score!


Table 4-5

Statements Outcome

if (H o u rs > 4 0 ) O v e rT im e = 1 ;

Assigns 1 to O v erT im e only when H o u rs is greater than 40

if (V a lu e > 3 2 ) co u t < < " In v a lid n u m b er \n " ;

Displays the message “Invalid number” only when V alu e is greater than 32

if (O v erT im e = = 1 ) P a y R a te * = 2 ;

Multiplies P ay R a te by 2 only when O v erT im e is equal to 1


Be Careful With Semicolonsif (expression)

statement;

• Notice that the semicolon comes after the statement that gets executed if the expression is true; the semicolon does NOT follow the expression

Expression

Statement(s)

TrueFalse


Program 4-3

// This program demonstrates how a misplaced semicolon// prematurely terminates an if statement.#include <iostream> using namespace std;

int main(){

int x = 0, y = 10;cout << “x is " << x << " and y is " << y << endl;if (x > y); // misplaced semicolon!

cout << “x is greater than y\n"; // Always executed

}

Program OutputX is 0 and Y is 10

X is greater than Y


Programming Style and the if Statement

• The conditionally executed statement should appear on the line after the if statement.

• The conditionally executed statement should be indented one “level” from the if statement.

• Note: Each time you press the tab key, you are indenting one level.


And Now Back to Truth

• When a relational expression is true, it has the value 1.• When a relational expression is false it has the value 0.• An expression that has the value 0 is considered false by

the if statement.• An expression that has any value other than 0 is considered

true by the if statement.


• Consider the following statement:

if (x = 2) // caution here!!!!

cout << “It is True!”;• This statement does not determine if x is equal to 2, it

assigns x the value 2, therefore, this expression will always be true because the value of the expression is 2, a non-zero value


Program 4-5

// This program averages 3 test scores. The if statement uses// the = operator, but the == operator was intended.#include <iostream> using namespace std;

int main(){

int score1, score2, score3;float average;

cout << "Enter 3 scores and I will average them:";cin >> score1 >> score2 >> score3;average = (score1 + score2 + score3) / 3.0;cout << "Your average is " << average << endl;if (average = 100) // Wrong

cout << "Congratulations! That's a high score!\n";

}



Enter your 3 test scores and I will average them: 80 90 70[Enter]

Your average is 80

Congratulations! That’s a perfect score!


4.3 Flags

• A flag is a variable, usually a boolean or an integer, that signals when a condition exists.

• If your compiler does not support the bool data type, use int instead.


Program 4-6

// This program averages 3 test scores. It uses the variable highScore as a flag.#include <iostream> using namespace std;

int main( ){

int score1, score2, score3;float average;bool highScore = false;

cout << "Enter your 3 test scores and I will average them: ";cin >> score1 >> score2 >> score3;average = (score1 + score2 + score3) / 3.0;if (average > 95)

highScore = true; // Set the flag variablecout << "Your average is " << average << endl;if (highScore)

cout << "Congratulations! That's a high score!\n";\}


Program 4-6


Enter your 3 test scores and I will average them: 100 100 100 [Enter]

Your average is 100.0

Congratulations! That's a high score!


4.4 Expanding the if Statement

• The if statement can conditionally execute a block of statement enclosed in braces.

if (expression){ statement; statement; // Place as many statements here as necessary.}


Program 4-7

// This program averages 3 test scores.

// It uses the variable highScore as a flag.

#include <iostream>


int main(void)

{

int score1, score2, score3;

float average;

bool highScore = false;

cout << "Enter 3 test scores and I will average them: ";

cin >> score1 >> score2 >> score3;

average = (score1 + score2 + score3) / 3.0;

if (average > 95)

highScore = true; // Set the flag variable


cout << "Your average is " << average << endl;

if (highScore)

{

cout << "Congratulations!\n";

cout << "That's a high score.\n";

cout << "You deserve a pat on the back!\n";

}

}





Congratulations!

That's a high score.

You deserve a pat on the back!

Program Output with Different Example Input




Don’t Forget the Braces!

• If you intend to execute a block of statements with an if statement, don’t forget the braces.

• Without the braces, the if statement only executes the very next statement.


4.5 The if/else Statement• The if/else statement will execute one group of statements if the

expression is true, or another group of statements if the expression is false.if (expression)

statement or block of statements;

else


expression

Statement(s) Statement(s)

TrueFalse


Program 4-9

// This program uses the modulus operator to determine// if a number is odd or even. If the number is evenly divided// by 2, it is an even number. A remainder indicates it is odd.#include <iostream> using namespace std;

int main(){

int number;

cout << "Enter an integer and I will tell you if it\n";cout << "is odd or even. ";cin >> number;if (number % 2 == 0)

cout << number << " is even.\n";else

cout << number << " is odd.\n";

}



Enter an integer and I will tell you if itis odd or even. 17 [Enter]

17 is odd.


Program 4-10

// This program asks the user for two numbers, num1 and num2.// num1 is divided by num2 and the result is displayed.// Before the division operation, however, num2 is tested// for the value 0. If it contains 0, the division does not take place.#include <iostream> using namespace std;

int main(){

float num1, num2, quotient;

cout << "Enter a number: ";cin >> num1;cout << "Enter another number: ";cin >> num2;


if (num2 == 0){

cout << "Division by zero is not possible.\n";cout << "Please run the program again and enter\n";cout << "a number besides zero.\n";

}else{

quotient = num1 / num2;cout << "The quotient of " << num1 << " divided by ";cout << num2 << " is " << quotient << ".\n";

}

}


Program Output

(When the user enters 0 for num2)

Enter a number: 10 [Enter]

Enter another number: 0 [Enter]

Division by zero is not possible.

Please run the program again and enter

a number besides zero.


4.6 The if/else if Construct

• The if/else if statement is a chain of if statements. They perform their tests, one after the other, until one of them is found to be true.

if (expression)


else if (expression)


//

// put as many else it’s as needed here

//

else if (expression)



Program 4-11

// This program uses an if/else if statement to assign a// letter grade (A,B,C,D,or F) to a numeric test score.#include <iostream> using namespace std;

int main(){

int testScore;char grade;

cout << "Enter your numeric test score and I will\n";cout << "tell you the letter grade you earned: ";cin >> testScore;


if (testScore < 60)grade = 'F';

else if (testScore < 70)grade = 'D';

else if (testScore < 80)grade = 'C';

else if (testScore < 90)grade = 'B';

else if (testScore <= 100)grade = 'A';

cout << "Your grade is " << grade << ".\n";

}

exprn Statement(s)

exprn Statement(s)

exprn Statement(s)

exprn Statement(s)

Statement(s)

T

T

T

T

F

F

F

F



Enter your test score and I willtell you the letter grade you earned: 88 [Enter]

Your grade is B.


Program 4-12

// This program uses independent if/else statements to assign a// letter grade (A, B, C, D, or F) to a numeric test score.// Do you think it will work?#include <iostream> using namespace std;

int main( ){

int testScore;char grade;

cout << "Enter your test score and I will tell you\n";cout << "the letter grade you earned: ";cin >> testScore;


if (testScore < 60)grade = 'F';

if (testScore < 70)grade = 'D';

if (testScore < 80)grade = 'C';

if (testScore < 90)grade = 'B';

if (testScore <= 100)grade = 'A';

cout << "Your grade is " << grade << ".\n";

}

exprn Statement(s)

exprn Statement(s)

exprn Statement(s)

exprn Statement(s)

Statement(s)

T

T

T

T

F

F

F

F



Enter your test score and I will tell youthe letter grade you earned: 40 [Enter]

Your grade is A.


Program 4-13//This program uses an if/else if statement to//assign a letter grade ( A, B, C, D, or F )//to a numeric test score.#include <iostream> using namespace std;

int main(){

int testScore;

cout << "Enter your test score and I will tell you\n";cout << "the letter grade you earned: ";cin >> testScore;if (testScore < 60){

cout << "Your grade is F.\n";cout << "This is a failing grade. Better see your ";cout << "instructor.\n";

}


else if (testScore < 70) {

cout << "Your grade is D.\n"; cout << "This is below average. You should get "; cout << "tutoring.\n";

}else if (testScore < 80)

{ cout << "Your grade is C.\n"; cout << "This is average.\n";

} else if (testScore < 90) {

cout << "Your grade is B.\n"; cout << "This is an above average grade.\n";

}



Enter your test score and I will tell youthe letter grade you earned: 94 [Enter]

Your grade is A.

This is a superior grade. Good work!

else if (testScore <= 100) {

cout << "Your grade is A.\n";cout << "This is a superior grade. Good work!\n";

}}


4.7 Using a Trailing else

• A trailing else, placed at the end of an if/else if statement, provides default action when none of the if’s have true expressions


Program 4-14

// This program uses an if/else if statement to assign a// letter grade (A, B, C, D, or F) to a numeric test score.// A trailing else has been added to catch test scores > 100.

#include <iostream> using namespace std;

int main(){

int testScore;

cout << "Enter your test score and I will tell you\n";cout << "the letter grade you earned: ";cin >> testScore;


if (testScore < 60){

cout << "Your grade is F.\n";cout << "This is a failing grade. Better see your ";cout << "instructor.\n";

}else if (testScore < 70){

cout << "Your grade is D.\n";cout << "This is below average. You should get ";cout << "tutoring.\n";

}


else if (testScore < 80){

cout << "Your grade is C.\n";cout << "This is average.\n";

}else if (testScore < 90){

cout << "Your grade is B.\n";cout << "This is an above average grade.\n";

}


else if (testScore <= 100){

cout << "Your grade is A.\n";cout << "This is a superior grade. Good work!\n";

}else // Default action{

cout << testScore << " is an invalid score.\n";cout << "Please enter scores no greater than 100.\n";

}

}



Enter your test score and I will tell you

the letter grade you earned: 104 [Enter]

104 is an invalid score.

Please enter scores no greater than 100.


4.9 Nested if Statements

• A nested if statement is an if statement in the conditionally-executed code of another if statement.


Program 4-16

// This program demonstrates the nested if statement.#include <iostream> using namespace std;

int main(){

char employed, recentGrad;

cout << "Answer the following questions\n";cout << "with either Y for Yes or ";cout << "N for No.\n";cout << "Are you employed? ";cin >> employed;cout << "Have you graduated from college ";cout << "in the past two years? ";cin >> recentGrad;


if (employed == 'Y'){

if (recentGrad == 'Y') // Nested if{

cout << "You qualify for the special ";cout << "interest rate.\n";

}}

}



Answer the following questions

with either Y for Yes or N for No.

Are you employed? Y[Enter]

Have you graduated from college in the past two years? Y[Enter]

You qualify for the special interest rate.


Program Output with Other Example Input

Answer the following questions

with either Y for Yes or N for No.

Are you employed? Y[Enter]

Have you graduated from college in the past two years? N[Enter]


4.10 Logical Operators

• Logical operators connect two or more relational expressions into one, or reverse the logic of an expression.


Table 4-6

Operator Meaning Effect

& & AND Connects two expressions into one. Bothexpressions must be true for the overallexpression to be true.

|| OR Connects two expressions into one. One orboth expressions must be true for the overallexpression to be true. It is only necessary forone to be true, and it does not matter which.

! NOT The ! operator reverses the “truth” of anexpression. It makes a true expression false,and a false expression true.


Table 4-7

Expression 1 Expression 2 Expression 1 && Expression 2

True

False

False

True

False

True

False

True

False (0)

False (0)

False (0)

True (1)


Program 4-18

// This program demonstrates the && logical operator.#include <iostream> using namespace std;

int main(){ char employed, recentGrad;

cout << "Answer the following questions\n"; cout << "with either Y for Yes or "; cout << "N for No.\n"; cout << "Are you employed? "; cin >> employed;


cout << "Have you graduated from college "; cout << "in the past two years? "; cin >> recentGrad; if (employed == 'Y‘ && recentGrad == 'Y') // && Operator { cout << "You qualify for the special "; cout << "interest rate.\n"; } else { cout << "You must be employed and have \n"; cout << "graduated from college in the\n"; cout << "past two years to qualify.\n"; }

}



Answer the following questionswith either Y for Yes orN for No.Are you employed? Y[Enter]Have you graduated from college in the past two years? N[Enter]You must be employed and havegraduated from college in thepast two years to qualify.


Table 4-8

Expression 1 Expression 2 Expression 1 || Expression 2

True False True (1)

False True True (1)

False False False (0)

True True True (1)


Table 4-9

Expression !(Expression)

True False (0)

False True (1)


Program 4-20//This program asks the user for his annual income and

//the number of years he has been employed at his current job.

//The ! operator reverses the logic of the expression in the if/else statement.

#include <iostream>


int main(){

float income; int years;cout << "What is your annual income? ";cin >> income;cout << "How many years have you worked at " << "your current job? ";cin >> years;if (!(income >= 35000 || years > 5)) // Uses the ! Logical operator{

cout << "You must earn at least $35,000 or have\n";cout << "been employed for more than 5 years.\n";

}else

cout << "You qualify.\n";

}


Precedence of Logical Operators

!

&&

||


4.11 Checking Numeric Ranges With Logical Operators

• Logical operators are effective for determining if a number is in or out of a range.


4.15 The Conditional Operator

• You can use the conditional operator to create short expressions that work like if/else statements

expression ? result if true : result if false;

X < 0 ? Y = 1 0 : Z = 2 0 ;


Program 4-29// This program calculates a consultant's charges at $50 per hour, for a minimum of

// 5 hours. ?: operator adjusts hours to 5 if less than 5 hours were worked.

#include <iostream>


int main()

{

const float payRate = 50.0;

float hours, charges;

cout << "How many hours were worked? ";

cin >> hours;

hours = hours < 5 ? 5 : hours;

charges = payRate * hours;

cout << "The charges are $" << charges << endl;

}



How many hours were worked? 10 [Enter]

The charges are $500.00


How many hours were worked? 2 [Enter]

The charges are $250.00


4.16 The switch Statement

• The switch statement lets the value of a variable or expression determine where the program will branch to.


Program 4-31

// The switch statement in this program tells the user

// something he or she already knows: what they just entered!

#include <iostream>


int main(void)

{

char choice;

cout << "Enter A, B, or C: ";

cin >> choice;


switch (choice)

{

case 'A': cout << "You entered A.\n";

break;

case 'B’: cout << "You entered B.\n";

break;

case 'C’: cout << "You entered C.\n";

break;

default: cout << "You did not enter A, B, or C!\n";

}

}



Enter A, B, or C: B [Enter]

You entered B.

Program Output with Different Example Input

Enter a A, B, or C: F [Enter]

You did not enter A, B, or C!


Program 4-34

// The switch statement in this program uses the "fallthrough"

// feature to catch both upper and lowercase letters entered

// by the user.

#include <iostream>


int main()

{

char feedGrade;

cout << "Our dog food is available in three grades:\n";

cout << "A, B, and C. Which do you want pricing for? ";

cin >> feedGrade;


switch(feedGrade){

case 'a':case 'A': cout << "30 cents per pound.\n"; break;case 'b':case 'B': cout << "20 cents per pound.\n"; break;case 'c':case 'C': cout << "15 cents per pound.\n"; break;default: cout << "That is an invalid choice.\n";

}}

c++ programming, namiq sultan1 chapter 4 making decisions namiq sultan university of duhok...

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