1. Chapter 7 Conditional Statements

2. 7.1 Conditional Expressions
Conditions
Compare the values of variables, constants and literals using one or more relational operators
Binary operators
A variable, constant, or literal must appear on each side of the operator
The comparison determines whether the expression is true or false
Remember: A constant is similar to a variable except the value can’t change. A literal is a number, character or string without a name associated with it.

3. 7.1.1 Relational Operators – Options
Description ==
Equality (be sure to use two equal signs) !=
Inequality
<
Less than
>
Greater than
<=
Less than or equal to
>=
Greater than or equal to
Important
Single equal sign (=) is an assignment
Double equal sign (==) tests for equality
Remember: A single equal sign (=) is an assignment and a double equal sign (==) represents a test for equality.

4. 7.1.1 Relational Operators – Example
const int CONST_INT_EXP = 9; int int_exp1 = 0, int_exp2 = 5; float float_exp = 9.0; char char_exp = 'a'; bool result; result = int_exp1 == 0; // true result = int_exp2 >= int_exp1; // true result = int_exp1 > CONST_INT_EXP; // false result = float_exp == CONST_INT_EXP; // true result = char_exp <= int_exp1; // false, remember ASCII values? result = int_exp1 != int_exp2; // true result = char_exp == 'a'; // true // ILLEGAL OR MALFORMED CONDITIONS // Malformed condition. May or may not compile depending on compiler used. Even if the compiler // will compile this condition, it should NEVER be written in this manner. result = int_exp1 < int_exp2 < float_exp; // Illegal. Attempting to compare a character to a string literal. result = char_exp == "a";
Remember: The ASCII table shows the association between a character and a number. The number is how the character is stored in memory and therefore is used in the comparison. See Appendix B for an ASCII chart.

5. 7.1.2 Logical Operators – Options
Combine multiple relational operators into a larger composite condition
Operators
|| (OR) - binary operator
False value only if the conditions on each side of the operator are false
&& (AND) - binary operator
Results in a true value only if the condition on both sides of the operator are true
! (NOT) - unary operator
Reverses logic of the single condition

6. 7.1.2 Logical Operators – Truth Table
Displays Boolean results produced when the operator is applied to specified operands
Condition 1
Condition 2
&& Result
|| Result
true
false
Condition
! Result
true
false

7. 7.1.2 Logical Operators – Order of Precedence
&& operator evaluated before the || operator
The ! operator
Highest level of precedence of all logical operators and is higher than the relational operators
Parentheses change the precedence
Parentheses can help clarify complicated conditions
Short-circuit evaluation
Once the outcome of condition can be determined, evaluation ends

8. 7.1.2 Logical Operators – Example
int int_exp1 = 0, int_exp2 = 5; float float_exp = 9.0; char char_exp = 'a'; const int CONST_INT_EXP = 9; bool result; result = int_exp1 < int_exp2 && float_exp == 9.0; // true result = int_exp1 > CONST_INT_EXP || float_exp == 9.0; // true result = !(float_exp == 9.0 || int_exp1 > CONST_INT_EXP); // false // Short-Circuit Evaluation result = float_exp == 9.0 || int_exp1 > CONST_INT_EXP; // true

9. 7.2 The if Statement – Syntax
Uses conditions to determine a specific action
if ( <condition> )
<action>
<condition>
Any valid expression, either built from relational and logical operators or from evaluation of a single variable
Zero is false while any non-zero value is considered true
Any valid C++ statement
Multiple statements must be enclosed in curly braces { }

10. 7.2 The if Statement – Example
// Example 1 if ( test >= 80 && test < 90 ) cout << "You have earned a B" << endl; // Example 2 if ( test >= 90 ) { // Start of the action block cout << "You have earned an A" << endl; cout << "Excellent work!" << endl; } // End of the action block // Example 3 if ( test >= 70 && test < 80 ) { // Start of the action block cout << "You have earned a C" << endl;

11. 7.2.1 The else Statement – Syntax
Optional part of if statement
Can’t stand alone and must be associated with an if
No condition or expression associated with it
Relies on results of the condition associated with the if
Executes action(s) only if the condition is false
Action can contain one or more statements
If more than one statement, the action must be enclosed in curly braces
if ( <condition> )
<action 1>
else
<action 2>

12. 7.2.1 The else Statement – Example
if ( grade >= 60 ) pass = true; else { pass = false; cout << "Hope you do better next time" << endl; }

13. 7.2.2 The else if Statement – Nested if
Embedding another if in action block of the else
if ( avg >= 90 )
cout << "A" << endl;
else
if ( avg >= 80 )
cout << "B" << endl;
Nested if indentation can cause the code to become difficult to read

14. 7.2.2 The else if Statement – Syntax
else if syntax
if ( <condition 1> )
<action 1>
else if ( <condition 2> )
<action 2>
else if ( <condition 3> )
<action 3>
. . .
else // Optional
<last action>
Remember: The else is always optional and must be placed at the end of the if statement.

15. 7.2.2 The else if Statement – Inefficient if
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;
How many conditions are evaluated if avg equals 90? 8
How many conditions are evaluated if avg equals 60?

16. 7.2.2 The else if Statement – Efficient if
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;
How many conditions are evaluated if avg equals 90? 1
How many conditions are evaluated if avg equals 60? 4

17. 7.2.2 The else if Statement – Flowchart

18. 7.2.2 The else if Statement – Nested Control Statements
Has another control statement in its action block
if ( gpa >= 3.75 )
if ( credits > 25 )
if ( money < ) {
scholarship = 5000;
cout << "Way to go!" << endl; }
else
scholarship = 2000;
scholarship = 1000;
scholarship = 0;
cout << "You're on your own." << endl;

19. 7.3 Variable Scope – Definition
Scope of a variable
Determines what code can access or change the variable
Determines how long the variable exists or lives
Local scope
Variables or constants declared within braces
Global scope
Variables or constants declared outside of any enclosing braces, usually above main
Automatically initialized to 0
Avoid global variables

20. 7.3 Variable Scope – Example
Below, 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; }
// Error: undeclared identifier var_b
cout << "var_b: " << var_b;

21. 7.3 Variable Scope – Global Identifiers
PI and variable global_area are physically declared outside of function, placed at the global level
#include <iostream>
using std::cout;
using std::endl;
#include <cmath> // Needed for pow
const float PI = F; // global scope
float global_area = 0; // global scope
int main ( ) {
float radius = 5; // local scope
global_area = static_cast<float> (PI * pow ( radius, 2 ));
cout << global_area << " sq. in." << endl;
return 0; }
// Output
sq. in.

22. 7.4 The switch Statement – Definition
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

23. 7.4 The switch Statement – Syntax
switch( <variable> ) { // Required case <literal or const 1>: <action 1> break; case <literal or const 2>: <action 2> ... 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

24. 7.4 The switch Statement – 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

25. 7.4 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

26. 7.4 The switch Statement – Example with Literals
int menu_item = 0; switch ( menu_item ) { case 1: cout << "You have chosen option 1." << endl; break; case 2: cout << "You have chosen option 2." << endl; case 3: cout << "You have chosen option 3." << endl; default: cout << "Invalid menu option." << endl; }

27. 7.4 The switch Statement – Example with Constants
const short GREEN = 0; const short YELLOW = 1; const short RED = 2; short light_color = GREEN; switch ( light_color ) { case GREEN: cout << "Go!" << endl; break; case YELLOW: // Let fall through case RED: cout << "Stop!"; cout << "Proceed when light is green." << endl; default: cout << "Power is out!" << endl; }

28. 7.4 The switch Statement – Example with Characters
char letter_grade; cout << "Enter letter grade: "; cin >> letter_grade; switch ( letter_grade ) { case 'A': cout << "Excellent!\n"; break; case 'B': cout << "Above average.\n";
case 'C': cout << "Average.\n"; break; case 'D': cout << "Below average.\n"; case 'F': cout << "Failed!\n"; default: cout << "Invalid grade.\n"; }
Remember: It is only possible to use a switch statement with integers or single characters.

29. 7.4 The switch Statement – Common Use
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:
Remember: You must include the entire body of the switch statement within braces.

30. 7.5 Conditional Operator – Definition
Considered a ternary operator, meaning it has three operand
Syntax
<condition> ? <true expression> : <false expression>
One of the expressions is returned based upon the evaluation of the condition

31. 7.5 Conditional Operator – Example
int a = 5, b = 0; int larger = a > b ? a : b; cout << larger << endl; // Output 5 Equivalent code
int a = 5, b = 0; int larger; if ( a > b ) larger = a; else larger = b; cout << larger << endl;

32. 7.5 Conditional Operator – More Complex Example
cout << (hour < 10 ? "0" : "") << hour << ":" << (minute < 10 ? "0" : "") << minute << ":" << (second < 10 ? "0" : "") << second << endl; // Output 09:10:05
Empty quotes above tell cout to print nothing if the condition is false (i.e. hour is 10 or greater)

33. 7.7 C The Differences – Boolean
Older versions of C did not have a Boolean data type
There wasn’t a predefined true or false
All relational operators return either a zero for false or a non-zero value, usually one, for true
The C99 version of the ANSI Standard includes a Boolean data type
#define BOOL int #define FALSE 0 #define TRUE !FALSE int main ( void ) { BOOL done = FALSE; return 0; }
C programmers often made their own data type similar to what is show to the left