Chapter 7 Conditional Statements

Chapter 7

Conditional Statements

7.1 Conditional ExpressionsCondition – any expression that evaluates to

true/false value

<expr1> <relational_operator> <expr2>

• Relational operators are BINARY

X < 4 Y == a+b q+1 != s*2

7.1.1 Relational Operators

Relational Operator Description

== Equality (be sure to use two equal signs) *

!= Inequality

< Less than

> Greater than

<= Less than or equal to

>= Greater than or equal to

* Single equal sign (=) is an assignment / does NOT compare values


const int CONST_iEXP = 9;int iexp1 = 0, iexp2 = 5;float fexp = 9.0;char cexp = 'a';

bool result = true;

result = iexp1 == 0; // true

result = iexp2 >= iexp1; // true

result = iexp1 > CONST_iEXP; // false

result = fexp == CONST_iEXP; // true

result = cexp <= iexp1; // false

result = iexp1 != iexp2; // true

result = cexp == 'a'; // true


// ----- ILLEGAL OR MALFORMED CONDITIONS ----

result = 3 < X < Y;

// Malformed condition. What does it mean?// 3 < X and X < Y ???// 3 < X or X < Y ???

cexp == "a";

// Illegal. Attempting to compare a character// to a string literal.

7.1.2 Logical Operators

Logical operators - combine bool expressions

Operators:

or: x || y (binary operator)• false only when both operands are false

and: x && y (binary operator)• true only when both operands are true

not: !x (unary operator)• false when operand is true; otherwise true.

7.1.2 Logical OperatorsTruth table - displays Boolean results produced when the operator is applied to specified operands

Logical AND and OR truth table

Condition c1

Condition c2

c1 && c2 c1 || c2

true true true true

true false false true

false true false true

false false false false

7.1.2 Logical OperatorsLogical NOT truth table

Order of precedence ! (not)&& (and)|| (or)

Condition c !c

true false

false true


Misc Information:• Parentheses change the precedence

• Parentheses can help clarify complicated conditions

• Short-circuit evaluation - once the outcome of condition can be determined, evaluation ends


int iexp1 = 0, iexp2 = 5;float fexp = 9.0;char cexp = 'a';const int CONST_iEXP = 9;

bool result;result = iexp1 < iexp2 && fexp == 9.0;result = iexp1 > CONST_iEXP || fexp == 9.0; // trueresult = !(fexp == 9.0 || iexp1 > CONST_iEXP);// false

// Short-Circuit Evaluationresult = fexp == 9.0 || iexp1 > CONST_iEXP; // true

Various logical operators

7.2 The if Statement

Selects actions to be taken only when a specific condition is satisfied

Syntax:if ( <condition> )

<action>

Example:if ( N % 2 == 1)

cout << N << “ is ODD” << endl;

7.2 if (<condition>) <action><condition> - a valid expression that can be

interpreted as a TRUE/FALSE value• Relational expression (e.g., x <= 2)

• Logical expression ( x>5 && y != 2)

• Integer-valued expression: 0=false; otherwise true.

<action> - a valid C++ statement or block• Single statement terminated with semicolon

• Compound statement (block) enclosed in set braces { }if (Age > 21) if (Income > 24000)

Sell_Beer = true; {

SSTax = 0.05 * Income;

FedTax = 0.075 * Income;

}

7.2 The if Statementif (condition) statement;

// Example 1if ( test >= 80 && test < 90 ) cout << "You have earned a B" << endl;

// Example 2 – relational expression/compound statement.if ( test >= 90 ){ cout << "You have earned an A" << endl; cout << "Excellent work!" << endl;}

// Example 3 – logical expression/compound statement.if ( test >= 70 && test < 80 ){ cout << "You have earned a C" << endl;}

7.2.1 The else Statement

• Optional part of an if statement• Can’t stand alone• Must be associated with an open if

if ( <condition> )

<action 1> // Consequence (true-part)

else

<action 2> // Alternative (false-part)

• Means: “otherwise”, “catch-all”, “none of the above”

7.2.1 The else Statement

else <action> // consequence.• no condition or expression associated with it

• relies on the value of the condition associated with the previous if (or chain of ifs)

• executes action(s) only if the previous condition (or chain of ifs) is false

• if more than one statement, the action must be enclosed in curly braces

7.2.1 The if-else Statement

if ( grade >= 60 ) pass = true;else{ pass = false; cout << "Hope you do better next time" << endl;}

else Example

7.2.1 Multiple alternative if

The alternative of an if is another decision:

if ( avg >= 90 ) cout << “Grade = A" << endl;else if ( avg >= 80 ) cout << "B" << endl;

-------- NOTE indentation.

if ( avg >= 90 ) cout << “Grade = A" << endl;else if ( avg >= 80 ) cout << "B" << endl;

7.2.1 Selecting from multiple alternatives

Only ONE action is selected from a chain of decisions.

if ( Avg >= 90 ) Grade = ‘A’;

else if ( Avg >= 80 ) Grade = ‘B’;else if ( Avg >= 70) Grade = ‘C’;else if ( Avg >= 60) Grade = ‘D’;else // Optional Grade = ‘F’;

NOTE: The construct terminates when a choice is made.

QUESTIONS: # of decisions needed to assign A? C? F?

7.2.1 Sequence of if statementsInefficient: how many decisions for A, C, F?

if ( avg >= 90 ) cout << "A" << endl;

if ( avg >= 80 && avg < 90 ) cout << "B" << endl;

if ( avg >= 70 && avg < 80 ) cout << "C" << endl;

if ( avg >= 60 && avg < 70 ) cout << "D" << endl;

if ( avg < 60 ) cout << "F" << endl;

7.2.1 Multiple-alternative chain of if

Mutual exclusion: result can be only one of the choices

if ( avg >= 90 )

cout << "A" << endl;

else if ( avg >= 80 )

cout << "B" << endl;

else if ( avg >= 70 ) cout << "C" << endl;

else if ( avg >= 60 )

cout << "D" << endl;

else

cout << "F" << endl;

90

80

70

60

7.2.1 Multiple alternative if chainStart If Statement

1st Condition>= 90

Action 1Display “A”

2nd Condition>= 80

3rd Condition>= 70

Action 2Display “B”

Action 3Display “C”

Action ElseDisplay “F”

End If Statement

True

True

True

False

False

False

4th Condition>= 60

False

Action 4Display “D”

True

Notes:1.Each condition is tried

until one is satisfied (i.e., value is true)

2.The matching consequence is executed

3.The decision process terminates the

7.2.1 The Nested if constructNested the consequence is itself a decision.

if ( gpa >= 3.75 )

if ( credits > 25 )

if ( money < 30000 )

{ scholarship = 5000;

cout << "Way to go!" << endl;

}

else

scholarship = 2000;

else

scholarship = 1000;

else

{ scholarship = 0;

cout << "You're on your own." << endl;

}

7.3 Variable ScopeScope of a variable – determines:

• What code can access or change the variable

• How long the variable exists or lives

7.3 Variable ScopeBelow, var_a and var_b defined within the

scope of the block• Both accessible within the block where defined• Final line generates an error message - var_b

is not defined

{ int var_a = 5, var_b = 10; var_a++; cout << "var_a: " << var_a << endl;}cout << "var_b: " << var_b; // Error: undeclared

// identifier var_b

7.3 Variable Scope

Local scope – variables or constants declared within braces

7.3 Variable Scope

Constant PI and variable global_area - physically declared outside of function - placed at the global level

#include <iostream>using std::cout;using std::endl;#include <cmath> // Needed for powconst float PI = 3.141592F; // global scopefloat global_area = 0; // global scopeint main(){ float radius = 5; // local scope global_area = static_cast<float>( PI * pow( radius, 2 ) ); cout << global_area << " sq. in." << endl; return 0;}// Output78.5398 sq. in.

7.3 Variable Scope

Any code within the file can access PI or global_area

#include <iostream>using std::cout;using std::endl;#include <cmath> // Needed for powconst float PI = 3.141592F; // global scopefloat global_area = 0; // global scopeint main(){ float radius = 5; // local scope global_area = static_cast<float>(PI * pow(radius, 2)); cout << global_area << " sq. in." << endl; return 0;}// Output78.5398 sq. in.

7.3 Variable Scope

Global variables - automatically initialized to 0Avoid global variables (i.e., global_area)

#include <iostream>using std::cout;using std::endl;#include <cmath> // Needed for powconst float PI = 3.141592F; // global scopefloat global_area = 0; // global scopeint main(){ float radius = 5; // local scope global_area = static_cast<float>(PI * pow(radius, 2)); cout << global_area << " sq. in." << endl; return 0;}// Output78.5398 sq. in.

7.4 The switch Statement

switch statement - another form of conditional statement• Also called a selection statement

• Checks only for equality and only for one variable


Works well for checking a variable for limited set of values• Only works with ordinal data types

• Ordinal data types - can be translated into an integer to provide a finite, known, number set

• Examples include int, bool, char, and long

7.4 The switch StatementGeneral form of the switch statement:

switch( <variable> ){ // Required

case <literal or const 1>: <action 1> break;case <literal or const 2>: <action 2> break;

...default: // Optional <default action>

}// Required

When first line is encountered, value of the variable determined

Execution jumps to the case which corresponds to the value of the variable being examined

Execution continues until either a break statement is encountered or to the end of switch


break statement - stops execution of the control structure prematurely

• Stops multiple case statements from being executed

• Many believe poor programming to use outside the context of the switch statement


default statement - executed if value of the variable doesn’t match any of previous cases• Type of catch all or “case else”

• Technically can use the default case in any position

• Should physically be the last one in the switch statement


int menu_item = 0;...switch ( menu_item ){ case 1: // Using literal values cout << "You have chosen option 1." << endl; break; case 2: cout << "You have chosen option 2." << endl; break; case 3: cout << "You have chosen option 3." << endl; break; default: cout << "Invalid menu option." << endl;}


const short GREEN = 0;const short YELLOW = 1;const short RED = 2;short light_color = GREEN; switch ( light_color ){ case GREEN: // Using constants cout << "Go!" << endl; break; case YELLOW: // Let fall through case RED: cout << "Stop!"; cout << "Proceed when light is green." << endl; break; default: cout << "Stop!"; cout << "Power is out!" << endl;}


char letter_grade;cout << "Enter letter grade: ";cin >> letter_grade;switch ( letter_grade ){ case 'A': // Using character literal values cout << "Excellent!" << endl; break; case 'B': cout << "Above average." << endl; break; case 'C': cout << "Average." << endl; break; case 'D': cout << "Below average." << endl; break; case 'F': cout << "Failed!" << endl; break; default: cout << "Invalid letter grade." << endl;}


One of the most common uses of switch statement is in menu driven programs

Student Grade Program - Main Menu -

1. Enter name 2. Enter test scores 3. Display test scores 9. Exit

Please enter your choice from the list above:

7.5 Conditional Operator

Conditional operator - considered a ternary operator, meaning it has three operands

Syntax:<condition> ? <true expression> : <false expression>


One of the expressions is returned based upon the evaluation of the condition

int a = 5, b = 0;

int larger = a > b ? a : b;

cout << larger << endl;// Output5


Equivalent if statement to code on previous page

int a = 5, b = 0;int larger;

if ( a > b ) larger = a;else larger = b;


More challenging conditional operator example

short hour = 9, minute = 10, second = 5;

cout << (hour < 10 ? "0" : "") << hour << ":" << (minute < 10 ? "0" : "") << minute << ":" << (second < 10 ? "0" : "") << second << endl;

// Output09:10:05

Empty quotes above tell cout to print nothing if the condition is false (i.e. hour is 10 or greater)

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Chapter 7 Conditional Statements