Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. Chapter 4: Making Decisions 1

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. Displaying true/false bool test = true; std::cout << std::boolalpha << test << '\n'; std::cout << std::noboolalpha << test << '\n'; Output true 1 2

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. 4.1 Relational Operators 3

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. Relational Operators Used to compare numbers to determine relative order Operators: > Greater than < Less than >= Greater than or equal to <= Less than or equal to == Equal to != Not equal to 4

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. Relational Expressions Boolean expressions – true or false Examples: 12 > 5 is true 7 <= 5 is false if xyx is 10, then 10 == xyz is true, 8 != xyz is true, and 8 == xyz is false 5

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. Relational Expressions Can be assigned to a variable: result = x1 <= y1; Assigns 0 for false, 1 for true Do not confuse = and == The expression x1 = 10 is always true Use 10 = x1 which will cause a syntax error 6

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. 4.2 The if Statement 7

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. 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." 8

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. Flowchart for Evaluating a Decision 9

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. Flowchart for Evaluating a Decision 10

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. The if Statement General Format: if (expression) statement; Where expression evaluates to true or false And statement is a single C++ statement 11

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. The if Statement – What Happens To evaluate: if (expression) statement; If the expression is true, then statement is executed. If the expression is false, then statement is skipped. 12

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. if Statement in Program 4-2 Continued… 13

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. if Statement in Program

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. Flowchart for Program 4-2 Lines 21 and 22 15

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. 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 float s and double s for equality The value 0 is evaluated as false ; any other value is true 16

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. 4.3 Expanding the if Statement 17

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. Expanding the if Statement To execute more than one statement as part of an if statement, enclose them in { } braces: if (score > 90) { grade = 'A'; std::cout << "Good Job!\n"; } { } creates a block of code 18

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. 4.4 The if/else Statement 19

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. 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. 20

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. The if/else statement General Format: if (expression) statement1; // or block else statement2; // or block The if statement is green The else statement is red 21

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. if/else – What Happens To evaluate: if (expression) statement1; // block1 else statement2; // block2 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. 22

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. The if/else statement and Modulus Operator in Program

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. Flowchart for Program 4-8 Lines 14 through 18 24

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. Testing the Divisor in Program 4-9 Continued… 25

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. Testing the Divisor in Program

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. 4.5 Nested if Statements 27

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. Nested if Statements An if statement that is nested inside another if statement Nested if statements can be used to test more than one condition 28

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. Flowchart for a Nested if Statement 29

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. Nested if Statements From Program

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. Nested if Statements Another example, from Program

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. Use Proper Indentation! This if and else go together 32

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. 4.6 The if/else if Statement 33

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. The if/else if Statement Tests a series of conditions until one is found to be true Often simpler than using nested if/else statements Can be used to model thought processes such as: "If it is raining, take an umbrella, else, if it is windy, take a hat, else, take sunglasses” 34

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. if/else if Format if (expression1) statement1; // or block else if (expression2) statement2; // or block.. // other else ifs. else if (expressionn) statementn; // or block 35

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. The if/else if Statement in Program

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. Using a Trailing else to Catch Errors in Program 4-14 The trailing else clause is optional, but it is best used to catch errors. This trailing else catches invalid test scores 37

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. 4.7 Flags 38

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. Flags Variable that signals a condition Usually implemented as a bool variable Can also be an int eger The value 0 is considered false Any nonzero value is considered true As with other variables in functions, must be assigned an initial value before it is used 39

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. 4.8 Logical Operators 40

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. Logical Operators Used to create relational expressions from other relational expressions Operators, meaning, and explanation: && ANDNew relational expression is true if both expressions are true || ORNew relational expression is true if either expression is true ! NOTReverses the value of an expression – true expression becomes false, and false becomes true 41

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. Logical Operators – Examples int x1 = 12, y1 = 5, z1 = -4; (x1 > y1) && (y1 > z1)true (x1 > y1) && (z1 > y1)false (x1 <= z1) || (y1 == z1)false (x1 <= z1) || (y1 != z1)true !(x1 >= z1)false 42

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. The logical && operator in Program

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. The logical || Operator in Program

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. The logical ! Operator in Program

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. Logical Operator-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) 46

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. 4.9 Checking Numeric Ranges with Logical Operators 47

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. Checking Numeric Ranges with Logical Operators Used to test to see if a value falls inside a range: if (grade >= 0 && grade <= 100) std::cout << "Valid grade"; Can also test to see if value falls outside of range: if (grade = 100) std::cout << "Invalid grade"; Cannot use mathematical notation: if (0 <= grade <= 100) //doesn ’ t work! Evaluated as if ((0 <= grade) <= 100) And is always true. The 0 <= grade evaluates to true or false (1 or 0) either of which is <=

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved Menus 49

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. Menus Menu-driven program: program execution controlled by user selecting from a list of actions Menu: list of choices on the screen Menus can be implemented using if/else if statements 50

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. Menu-Driven Program Organization Display list of numbered or lettered choices for actions Prompt user to make selection Test user selection in expression if a match, then execute code for action if not, then go on to next expression 51

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved Validating User Input 52

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. Validating User Input Input validation: inspecting input data to determine whether it is acceptable Bad output will be produced from bad input Can perform various tests: Range Reasonableness Valid menu choice Divide by zero 53

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. Input Validation in Program

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved Comparing Characters and Strings 55

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. Comparing Characters Characters are compared using their ASCII values 'A' < 'B' The ASCII value of 'A' (65) is less than the ASCII value of 'B'(66) '1' < '2' The ASCII value of '1' (49) is less than the ASCI value of '2' (50) Lowercase letters have higher ASCII codes than uppercase letters, so 'a' > 'Z' 56

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. Relational Operators Compare Characters in Program

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. Comparing string Objects Like characters, strings are compared using their ASCII values std::string name1 = "Mary"; std::string name2 = "Mark"; name1 > name2 // true name1 <= name2 // false name1 != name2 // true name1 < "Mary Jane" // true The characters in each string must match before they are equal 58

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. Relational Operators Compare Strings in Program

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved The Conditional Operator 60

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. The Conditional Operator Only ternary operator (3 operands) Can use to create short if/else statements Format: expr ? expr : expr; x1 < 0 ? y1 = 10 : z1 = 20; First Expression: Expression to be tested 2nd Expression: Executes if first expression is true 3rd Expression: Executes if the first expression is false 61

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. The Conditional Operator The value of a conditional expression is The value of the second expression if the first expression is true The value of the third expression if the first expression is false Parentheses () may be needed in an expression due to precedence of conditional operator 62

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. The Conditional Operator in Program

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved The switch Statement 64

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. The switch Statement Used to select among statements from several alternatives In some cases, can be used instead of if/else if statements 65

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. switch Statement Format switch (expression) // integer-type variable { case val1: // integer-type const or literal statement1; case val2: statement2;... case valn: statementn; default: statementn+1; } Integer-type includes char, int, short, long 66

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. The switch Statement in Program

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. switch Statement Requirements 1) expression must be an integer variable or an expression that evaluates to an integer value 2)val1 through valn must be constant or literal integer expressions, and must be unique within the switch statement 3) default is optional but recommended 68

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. switch Statement-How it Works 1) expression is evaluated 2)The value of expression is compared against val1 through valn. 3)If expression matches value vali, the program branches to the statement following vali and continues to the end of the switch 4)If no matching value is found, the program branches to the statement after default: 69

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. break Statement Used to exit a switch statement If it is left out, the program "falls through" the remaining statements associated with the remaining case statements in the switch statement Usually used as the last statement associated with a case. 70

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. break and default statements in Program 4-25 Continued… 71

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. break and default statements in Program

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. Using switch in Menu Systems switch statement is a natural choice for menu-driven program: display the menu get the user's menu selection use variable containing user input as expression in switch statement use menu choices as val s in case statements 73

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved More About Blocks and Scope 74

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. More About Blocks and Scope A block of code is a group of statements between a set of braces { }. It is good programming style to line up the open and close brace and to indent the code between the braces A block of code may be nested in another block. Scope of a variable is the block in which it is defined, from the point of definition to the end of the block Usually defined at beginning of a block Must be defined before first use 75

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. Inner Block Variable Definition in Program

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. Variables with the Same Name Variables defined inside { } have local or block scope You can define a variable inside a block that is within another block with the same name as a variable in the outer block. When in inner block, the outer definition is not available Not a good programming style 77

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. Two Variables with the Same Name in Program

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. Chapter 4 Review if (expression) statement; if (expression) { // block of statements } if (expression) statement1; // or block1 else statement2; // or block2 if (expression1) statement1; // or block1 else if (expression2) statement2; // or block2 else statement3; // or block3 Relational operators = != == Logical Operators && || ! Conditional Operator exp1 ? exp2 : exp3 switch/case break/default Block/Scope { } BRACES 79

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. End of Chapter 4 80

Copyright © 2015, 2012, 2009 Pearson Education, Inc., Publishing as Addison-Wesley All rights reserved. In-Class Lab Write a program that asks the user for this months sales. Using the following chart, write an if/else if statement that assigns 0.10, 0.15, or 0.20 to rate, depending on the value in sales. Calculate and display the commission in dollars and cents Sales Commission Rate Under $10,000 10% $10,000 thru $15,000 15% Over $15,000 20% 81