Chapter 4 Logical Expressions & If-Else

2 Overview  More on Data Type bool u Using Relational & Logical Operators to Construct & Evaluate Logical Expressions u If-Else Statements u If Statements u Nested If Statements for Multi-way Branching u Testing a C++ Program

3 Boolean expressions u Boolean expressions (expressions of type bool) are constructed by using relational and logical operators 6 Relational Operators >= == != 3 Logical Operators !&&||

4 Relational operators u Relational operators and variables of any type are combined to form Boolean expressions : Left-side ExpressionOpRight-side Expression volume>mass/density B*B * A*C>0.0 abs(number)==35 initial!='Q'

5 Evaluating relational expressions int x = 4, y = 6; EXPRESSIONVALUE x < ytrue x + 2 < yfalse x != ytrue x + 3 >= ytrue y == xfalse y == x+2true y = x + 37 (not Boolean)

6 Assignment expressions u In C++, it is possible to use the assignment operator inside a larger expression  The assignment sub-expression returns the value and type of the variable after assignment #include //what is the output? using namespace std; int main(){ int i=0; cout << (i = int( )) << endl; cout << i << endl; //what is the output? return 0;}

7 Comparing Strings u Two objects of type string (or a string object and a string literal) can be compared using the relational operators  A character-by-character comparison is made using the ASCII code of each character u If all the characters are equal, then the 2 strings are equal  Otherwise, the string with the first character of smaller ASCII value is the “lesser” string

8 string myState; string yourState; myState = “Texas”; yourState = “Maryland”; EXPRESSIONVALUE myState == yourState false myState > yourState true myState == “Texas”true myState < “texas”true Comparison Operator Examples

9 OperatorMeaning Associativity !, +, -NOT, unary +, unary -Right *, /, %Multiplication, Division, Modulus Left +, -Addition, SubtractionLeft <Less thanLeft <=Less than or equal toLeft >Greater thanLeft >=Greater than or equal toLeft ==, != Is equal to, Is not equal to Left &&logical ANDLeft ||logical OR Left = AssignmentRight

10 LOGICAL EXPRESSION MEANINGDESCRIPTION ! 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 qp || q is false if both p and q are false. It is true otherwise.

11 int age; bool isSenior, hasFever; float temperature; age = 20; temperature = 102.0; isSenior =(age >= 55);//isSenior is false hasFever =(temperature > 98.6); // hasFever is true EXPRESSIONVALUE isSenior && hasFever false isSenior || hasFever true !isSeniortrue !hasFeverfalse Logical Expression Examples

12 What is the value? int age, height; age = 25; height = 70; EXPRESSION VALUE ! (age < 10) ? ! (height > 60) ?

13 “Short-Circuit” Evaluation u C++ uses short circuit evaluation of logical expressions  This means logical expressions are evaluated left to right and evaluation stops as soon as the final truth value can be determined

14 Short-Circuit Example int age, height; age = 25; height = 70; EXPRESSION (age > 50) && (height > 60) //false u Evaluation can stop now because result of && is only true when both sides are true u It is already determined that the expression will be false

15 What happens? int age, height; age = 25; height = 70; EXPRESSION ! (height > 60) || (age > 50) D oes this part need to be evaluated?

16 Write an expression for each u taxRate is over 25% and income is less than $20000 u temperature is less than or equal to 75 or humidity is less than 70% u age is over 21 and age is less than 60 u age is 21 or 22

17 Some Answers (taxRate >.25) && (income < 20000) (temperature <= 75) || (humidity <.70) (age > 21) && (age < 60) (age == 21) || (age == 22)

18 Short-Circuit Benefits u one Boolean expression can be placed first to “guard” a potentially unsafe operation in a second Boolean expression u time is saved in evaluation of complex expressions using operators || and &&

19 Short Circuit Example int number; bool test; float x; test = (number != 0) && (x < 1/number); is evaluated first and has value false u The entire expression will have a value of false u Due to short-circuiting the sub-expression to the right of && is not evaluated  This avoids a possible division-by-zero error

20 WARNING about Expressions in C++ u “Boolean expression” means an expression whose value is true or false u an expression is any valid combination of operators and operands u each expression has a value u this can lead to UNEXPECTED RESULTS u construct your expressions CAREFULLY u use of parentheses is encouraged u otherwise, use the precedence chart to determine order

21 Comparing float (or double) Values u do not compare float values for equality, compare them for near-equality. float myNumber; float yourNumber; cin >> myNumber; cin >> yourNumber; test = (fabs(myNumber - yourNumber) < ); //NOT test = (myNumber == yourNumber);

22 Control Structures u Control structures alter the order in which statements in a program are executed  By default, each statement is executed exactly once  Statements are executed sequentially u There are 2 general types of control structures:  Selection, also called branching  Repetition, also called looping

23 C++ Control Structures u Selection if if... else switch u Repetition for loop while loop do... while loop

24 Control Structures use logical expressions which may include: 6 Relational Operators >= == != 3 Logical Operators !&&||

25 What can go wrong here? float average; float total; int howMany;. average = total/howMany;

26 Improved Version float average, float total; int howMany; if (howMany > 0) { average = total/howMany; cout << average; } else cout << “No prices were entered”;

27 If-Else Syntax if ( Expression ) StatementA else StatementB u NOTE: StatementA and StatementB each can be a single statement, a null statement, or a block.

28 if... else Provides Two-way Selection u between executing one of 2 clauses  the if clause or the else clause TRUE FALSE if clauseelse clause expression

29 Use of blocks recommended if ( condition ) {//begin block //one or more statements go here }//end block else {//begin block //one or more statements go here }//end block

30 int carDoors, driverAge; float premium, monthlyPayment; if ((carDoors == 4)&&(driverAge > 24)) { premium = ; cout << "LOW RISK "; } else { premium = ; cout << "HIGH RISK"; } monthlyPayment = premium / ; If-Else with blocks

31 if ((carDoors == 4) && (driverAge > 24)) premium = ; cout << "LOW RISK"; else premium = ; cout << "HIGH RISK"; monthlyPayment = premium / ; u COMPILE ERROR OCCURS  The “if clause” is the statement following the if What happens without braces?

32 Braces define blocks of code u Braces can only be omitted when each clause is a single statement if ( lastInitial <= 45 ) volume = 1; else volume = 2; cout << "Look it up in volume # " << volume << "of NYC phone book";

33 If-Then-Else for a mail order u Assign value.25 to discountRate and assign value to shipCost if purchase is over u Otherwise, assign value.15 to discountRate and assign value 5.00 to shipCost u Either way, calculate totalBill

34 These braces cannot be omitted if (purchase > ) { discountRate = 0.25; shipCost = 10.00; } else { discountRate =.15; shipCost = 5.00; } totalBill = purchase *(1- discountRate) + shipCost;

35 If statement is a selection u of whether or not to execute a statement  which can be a single statement or a block TRUE FALSE statement expression

36 Terminating your program int number; cout << "Enter a non-zero number"; cin >> number; if (number == 0) { cout << "Bad input. Program terminated"; return 1; } // otherwise continue processing

37 if (number == 0) if (! number) {.... } u Each expression is only true when number has value 0. These are equivalent. Why?

38 Write If or If-Else for each u If taxCode is 7, increase price by adding taxRate times price to it u If code has value 1, read values for income and taxRate from cin, and calculate and display taxDue as their product u If A is strictly between 0 and 5, set B equal to 1/A, otherwise set B equal to A

39 Possible Answers if (taxCode == 7) price = price + taxRate * price; if ( code == 1) { cin >> income >> taxRate; taxDue = income * taxRate; cout << taxDue; } if (( A > 0)&&(A < 5)) B = 1/A; else B = A;

40 What output? and Why? int age; age = 30; if ( age < 18 ) cout << “Do you drive?”; cout << “Too young to vote”;

41 What output? and Why? int code; code = 0; if ( ! code ) cout << “Yesterday”; else cout << “Tomorrow”;

42 What output? and Why? int number; number = 0; if (number = 0) cout << "Zero value"; else cout << "Non-zero value";

43 Chained If u The if clause & else clause of an if...else statement can contain any kind of statement  including another if…else statement u This is called multi-way branching

44 Chained if Statements if (Expression1) Statement_1 else if (Expression2) Statement_2... else if (ExpressionN) Statement_N else Statement_N_plus_1 EXACTLY 1 of these statements will be executed.

45 Chained if Statements u Each Expression is evaluated in sequence, until some Expression is found that is true u Only the specific Statement following that particular true Expression is executed u If no Expression is true, the Statement following the final else is executed u The final else and final Statement are optional  If omitted, when no Expression is true no Statement is executed

46 Multi-way Branching if (creditsEarned >= 90) cout << "SENIOR STATUS"; else if (creditsEarned >= 60) cout << "JUNIOR STATUS"; else if (creditsEarned >= 30) cout << "SOPHOMORE STATUS"; else cout << "FRESHMAN STATUS";

47 Writing Chained if Statements u Display one word to describe the int value of number as “Positive”, “Negative”, or “Zero” u Your city classifies a pollution index  less than 35 as “Pleasant”  35 through 60 as “Unpleasant”  and above 60 as “Health Hazard” u Display the correct description of the pollution index value

48 One Answer if (number > 0) cout << "Positive"; else if (number < 0) cout << "Negative"; else cout << "Zero";

49 Other Answer if ( index < 35 ) cout << “Pleasant”; else if ( index <= 60 ) cout << “Unpleasant”; else cout << “Health Hazard”;

50 Write a void Function… u …called DisplayMessage (which you can call from main) to describe the pollution index value it receives as an argument u Write a driver program to test this function u Your city describes a pollution index  less than 35 as “Pleasant”  35 through 60 as “Unpleasant”  above 60 as “Health Hazard”

51 void DisplayMessage(int index) { if (index < 35) cout << "Pleasant"; else if (index <= 60) cout << "Unpleasant"; else cout << "Health Hazard"; }

52 The Driver Program #include using namespace std; void DisplayMessage (int); //prototype int main () { int pollutionIndex; //declare variable cout << "Enter air pollution index"; cin >> pollutionIndex; DisplayMessage(pollutionIndex); //call return 0; }

53 Nested Decisions more u Sometimes you will need more than two parts. if/else (can get very messy— careful indenting helps)  Use nested if/else statements (can get very messy— careful indenting helps) if(isHungry(Lisa) && isHungry(Ed)) { if(isHere(Boss) && isHungry(Boss)) { eat(FOOD_FANCY); shiftAppts(12.0,pm,2.0,pm); eat(FOOD_FANCY); shiftAppts(12.0,pm,2.0,pm); } else { if(isHere(Jim) && isHere(Lori)) else { if(isHere(Jim) && isHere(Lori)) { eat(FOOD_CAFETERIA); { eat(FOOD_CAFETERIA); } else { if(getTime() > MID_DAY) { eat(FOOD_BROWNBAG); else { if(getTime() > MID_DAY) { eat(FOOD_BROWNBAG); } else cancelLunch(); } } else cancelLunch(); } }}