# Chapter 4: Making Decisions.

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Chapter 4: Making Decisions

Outline Relational Operations If statement Logical operators Switch If
If/else statement If/else if Logical operators Switch

Relational Operators Operators:
Relational operations allow you to compare numeric and char values and determine whether one is greater, less, equal to, or not equal to another. Operators: > Greater than < Less than >= Greater than or equal to <= Less than or equal to == Equal to != Not equal to

Relational Expressions
Boolean expressions – true or false Examples: 12 > 5 is true 7 <= 5 is false if x is 10, then x == 10 is true, x != 8 is true, and x == 8 is false

int x, y ; x = 4; y = 6; EXPRESSION VALUE x < y x + 2 < y x != y x + 3 >= y y == x y == x+2 y = x + 3

int x, y ; x = 4; y = 6; EXPRESSION VALUE x < y true x + 2 < y false x != y true x + 3 >= y true y == x false y == x true y = x

Relational Expressions
Can be assigned to a variable: result = x <= y; By default, assigns 0 for false, 1 for true Do not confuse = and == 0 is false; any other value is true

Like all C++ expressions,
relational expressions are evaluated to yield a numerical result. A condition that we would interpret as true evaluates to an integer value of 1; a false condition results in an integer value of 0. Samples: cout << (3 < 4) << endl; cout << (2.0 > 3.0); Results: 1

Outline Relational Operations If statement Logical operators Switch If
If/else statement If/else if Logical operators Switch

The if Statement Allows statements to be conditionally executed or skipped over Models the way we mentally evaluate situations: "If it is raining, take an umbrella." "If it is cold outside, wear a coat."

Flowchart for Evaluating a Decision

if statement Select whether or not to execute a statement (which can be a single statement or an entire block) without the else clause TRUE expression statement FALSE

Flowchart for Evaluating a Decision

The if Statement General Format:
if (expression) statement; If the expression is true, then statement is executed. If the expression is false, then statement is skipped.

Example: Check a Car’s Mileage
#include <iostream> int main ( void ) { const double LIMIT = ; //set car mileage limit double mileage = 0.0; //stores mileage entered by user cout << "Please enter the mileage recorded on the car: "; cin >> mileage; if (mileage > LIMIT) cout << "This car is over the limit."; cout << "\n\nEnd of program."; return 0; }

Example: Results Output from first run:
Please enter the mileage recorded on the car: This car is over the limit. End of program. Output from second run: Please enter the mileage recorded on the car:

(Program Continues)

Flowchart for Lines 21 and 22

if statement notes Do not place ; after (expression)
Place statement; on a separate line after (expression), indented: if (score > 90) grade = 'A'; Be careful testing floats and doubles for equality 0 is false; any other value is true

Expanding the if Statement
To execute more than one statement as part of an if statement, enclose them in { }: if (score > 90) { grade = 'A'; cout << "Good Job!\n"; } { } creates a block of code (like your main function)

Outline Relational Operations If statement Logical operators Switch If
If/else statement If/else if Logical operators Switch

if-else provides two-way selection between executing one of 2 clauses: the if clause or the else clause FALSE TRUE expression else clause if clause 24

The if/else Statement Provides two possible paths of execution
Performs one statement or block if the expression is true, otherwise performs another statement or block. General Format: if (expression) statement1; // or block else statement2; // or block

if-else Syntax if ( expression ) statementA; else statementB;
NOTE: StatementA and StatementB each can be a single statement, a null statement, or a block.

if/else – what happens To evaluate: if (expression) statement1; else
If the expression is true, then statement1 is executed and statement2 is skipped. If the expression is false, then statement1 is skipped and statement2 is executed.

Use of blocks recommended
if ( expression ) { } else “if clause” “else clause”

A compound statement consists of
individual statements enclosed within braces. Syntax: if(expression) { statement1; statement2; statement3; } else statement4; statement5; statement6;

Exercise: mail order Assign value .25 to discount_rate and assign value to ship_cost if purchase is over Otherwise, assign value .15 to discount_rate and assign value 5.00 to ship_cost Either way, calculate total_bill if(expression) { statement1; statement2; statement3; } else statement4; statement5; statement6;

These braces cannot be omitted
if ( purchase > ) { discount_rate = .25 ; ship_cost = ; } else discount_rate = .15 ; ship_cost = ; total_bill = purchase * (1.0 – discount_rate) + ship_cost ;

(Program Continues)

Outline Relational Operations If statement Logical operators Switch If
If/else statement If/else if Logical operators Switch

Used when only one condition can be true
if/else if format (if-else chain or extended if-else) Using nested if statements if ( Expression1 ) Statement1 else if ( Expression2 ) Statement2 . else if ( ExpressionN ) StatementN else Statement N+1 EXACTLY 1 block of these statements will be executed. Used when only one condition can be true

Example Marital Status Input Code Married M Single S Divorce D
Widowed W

Example int main ( void ) {
char marital_status; //marital status code entered by user //Prompt user for marital status code cout << "Enter a marital code: " << endl; cin >> marital_status; //Displays marital status message if (marital_status == 'M') cout <<"Individual is married." << endl; else if (marital_status == 'S') cout <<"Individual is single." << endl; else if (marital_status == 'D') cout <<"Individual is divorced." << endl; else if (marital_status == 'W') cout <<"Individual is widowed." << endl; else cout << "An invalid code was entered." << endl; //used as error message return 0; }

Output (Test all possible paths):
Enter a marital code: D Individual is divorced. S Individual is single. M Individual is married. W Individual is widowed. m An invalid code was entered. //results in an error message

Exercise: extended if statement (if-else chain):
Calculate the monthly income of a salesperson by using the following commission schedule: Monthly Sales Income Greater than or equal to \$50,000 \$375 plus 16% of sales Less than \$50,000 but greater than or equal to \$40,000 \$350 plus 14% sales Less than \$40,000 but greater than or equal to \$30,000 \$325 plus 12% sales Less than \$30,000 but greater than or equal to \$20,000 \$300 plus 9% sales Less than \$20,000 but greater than or equal to \$10,000 \$250 plus 5% of sales Less than \$10,000 \$200 plus 3% of sales

//Calculates salesperson's income
if(monthly_sales >= ) income = * monthly_sales; else if(monthly_sales >= ) income = * monthly_sales; else if(monthly_sales >= ) income = * monthly_sales; else if(monthly_sales >= ) income = * monthly_sales; else if(monthly_sales >= ) income = * monthly_sales; else income = * monthly_sales;

int main ( void ) { //stores salesperson’s name, monthly sales and calculated income double monthly_sales = 0.0, income = 0.0; string salesperson_name; //Prompts user for salesperson's name & sales cout << "Please enter the saleperson's name: "; getline(cin, salesperson_name); cout << "Enter the value of monthly sales: " << endl; cin >> monthly_sales; //Calculates salesperson's income if (monthly_sales >= ) income = * monthly_sales; else if (monthly_sales >= ) income = * monthly_sales; else if (monthly_sales >= ) income = * monthly_sales; else if (monthly_sales >= ) income = * monthly_sales; else if (monthly_sales >= ) income = * monthly_sales; else income = * monthly_sales; //Displays salesperson's name & income cout << setprecision(2) << fixed <<showpoint; cout << salesperson_name << " has earned a total monthly income of \$" << income << endl << " based on monthly sales of \$" << monthly_sales; return 0; }

Outline Relational Operations If statement Logical operators Switch If
If/else statement If/else if Logical operators Switch

Boolean Expressions Simple boolean expression use relational and/or logical operators. 6 Relational (or comparison) Operators < <= > >= == != 3 Logical Operators ! && ||

Logical Operators Used to create relational expressions from other relational expressions Operators, meaning, and explanation: && AND New relational expression is true if both expressions are true || OR New relational expression is true if either expression is true ! NOT Reverses the value of an expression – true expression becomes false, and false becomes true

LOGICAL EXPRESSION MEANING DESCRIPTION ! p NOT p ! p is false if p is true ! p is true if p is false p && q p AND q p && q is true if both p and q are true. It is false otherwise. p || q p OR q p || q is true if either p or q or both are true It is false otherwise.

Truth Table – Logical “AND”
X Y X && Y TRUE FALSE

Truth Table – Logical “AND”
X Y X && Y TRUE FALSE

Truth Table – Logical “OR”
X Y X || Y TRUE FALSE

Truth Table – Logical “OR”
X Y X || Y TRUE FALSE

Truth Table – Logical “not”
X !X TRUE FALSE

Truth Table – Logical “not”
X !X TRUE FALSE If the Boolean expression is true, the combined expression is false. If the Boolean expression is false, the combined expression is true.

Operator Meaning Associativity
! NOT Right * / % Mult, Div, Modulus Left + , Add, Sub Left < Less than Left <= Less than or equal to Left > Greater than Left >= Greater than or equal to Left == Is equal to Left != Is not equal to Left && AND Left || OR Left = Assignment Right

Logical Operators - examples
int x = 12, y = 5, z = -4; (x > y) && (y > z) (x > y) && (z > y) (x <= z) || (y == z) (x <= z) || (y != z) !(x >= z)

Logical Operators - examples
int x = 12, y = 5, z = -4; (x > y) && (y > z) true (x > y) && (z > y) false (x <= z) || (y == z) (x <= z) || (y != z) !(x >= z)

Write an expression for each
tax_rate is over 25% and income is less than \$20000 (tax_rate > 0.25) && (income < 20000) temperature is less than or equal to 75 or humidity is less than 70% (temperature <= 75) || (humidity < .70) age is over 21 and age is less than 60 (age > 21) && (age < 60) age is 21 or 22 (age == 21) || (age == 22)

Example The extended if statement should be as follows to allow the user to enter either uppercase or lowercase marital codes: //Displays marital status message if (marital_status == 'M' || marital_status == 'm') cout <<"Individual is married." << endl; else if (marital_status == 'S' || marital_status == 's') cout <<"Individual is single." << endl; else if (marital_status == 'D' || marital_status == 'd') cout <<"Individual is divorced." << endl; else if (marital_status == 'W' || marital_status == 'w') cout <<"Individual is widowed." << endl; else cout << "An invalid code was entered." << endl;

Logical Operators - notes
! has highest precedence, followed by &&, then || If the value of an expression can be determined by evaluating just the sub-expression on left side of a logical operator, then the sub-expression on the right side will not be evaluated (short circuit evaluation) 3 logical operators order of precedence ! && ||

As with all expressions, parentheses can be used to alter the
assigned operator priority and to improve the readability of relational expressions. By evaluating the expressions within the parentheses first, we find that the following compound condition is evaluated as shown: (6 * 3 == 36 / 2) || (13 < 3 * 3 + 4) && !(6 – 2 < 5) (18 == 18) || ( 13 < ) && !( 4 < 5 ) 1 || (13 < 13) && !1 1 || && 0 1 || 1

Nested if statements An if-else statement can contain simple or compound statements. Any valid C++ statement can be used, including another if-else statement. Thus, one or more if-else statements can be included within either part of an if-else statement. for statement1 in the if statement if ( hours < 9 ) statement1; else cout << “pop”; results in the nested if statement { cout << “snap”; if ( hours > 6 ) cout << “snap extra”; }

because in their absence, C++ associates an else with the closest
The braces around the inner one-way if are essential, because in their absence, C++ associates an else with the closest unpaired if. Thus, without the braces, the foregoing statement is equivalent to if ( hours < 9 ) if ( hours > 6 ) cout << “snap”; else cout << “pop”; Here, the else is paired with the inner if, which destroys the meaning of the original if-else statement. Note also that the indentation is irrelevant as far as the computer is concerned. Whether the indentation exists or not, the statement is compiled by associating the last else with the closest unpaired if, unless braces are used to alter the default pairing. The process of nesting if statements can be extended indefinitely, so the cout << “snap”; statement could itself be replaced by either a complete if-else statement or another one-way if statement.

Outline Relational Operations If statement Logical operators Switch If
If/else statement If/else if Logical operators Switch

Used when more than one condition can be true
Multi-alternative Selection is also called multi-way branching, and can be accomplished by one of two methods: Using sequential if statements if ( Expression1 ) Statement1; if ( Expression2 ) Statement2; . if ( ExpressionN ) StatementN; One or more of these statements may be executed. Used when more than one condition can be true

Slide 4- 63

switch Statement Is a selection control structure for multi-way branching. SYNTAX switch ( IntegralExpression ) { case constant1 : statement(s); // optional break; case constant2 : . default : // optional statement(s); // optional }

Control in switch statement
control branches to the statement following the case label that matches the value of IntegralExpression. Control proceeds through all remaining statements, including the default, unless redirected with break if no case label matches the value of IntegralExpression, control branches to the default label, if present--otherwise control passes to the statement following the entire switch statement forgetting to use break can cause logical errors because after a branch is taken, control proceeds sequentially until either break or the end of the switch statement occurs

The switch Statement Used to select among statements from several alternatives In some cases, can be used instead of if/else if statements

switch statement format
switch (expression) //integer or character { // start of compound statement case exp1: // terminate with a colon statement1; break; case exp2: // terminate with a colon statement2; ... case expn: // terminate with a colon statementn; default: // terminate with a colon statementn+1; } //end of switch & compound statement

switch statement requirements
1) expression must be an integer variable or an expression that evaluates to an integer value exp1 through expn must be constant integer expressions or literals, and must be unique in the switch statement default is optional but highly recommended

switch statement – how it works
1) expression is evaluated The value of expression is compared against exp1 through expn. If expression matches value expi, the program branches to the statement following expi and continues to the end of the switch If no matching value is found, the program branches to the statement after default:

break statement Used to exit a switch statement
Useful to execute a single case statement without executing the statements following it If it is left out, the program "falls through" the remaining statements in the switch statement Used to stop execution in the current block

Slide 4- 72

Multiple case values switch (number) { case 1: cout << "Have a Good Morning" << endl; break; case 2: cout << "Have a Happy Day" << endl; case 3: case 4: case 5: cout << "Have a Nice Evening" << endl; } We can use multiple case values to refer to the same set of statements; the default label is optional

Character data types in “Switch”
switch (choice) { case 'a': case 'e': case 'i': case 'o': case 'u': cout << "\nThe character in choice is a vowel." << endl; break; default: cout << "\nThe character in choice is not a vowel." << endl; break; //this break is optional } // end of switch statement Because character data types are always converted to integers in an expression, a switch statement can also be used to "switch" on the basis of the value of a character expression