Announcements HW1 grades are announced at SUCourse You may see Belal Amro at his office hour for homework grades at FENS 2014 on Wednesday 10:40-12:30 New (2 nd ) homework is assigned last week will be due next week 26 th October Wednesday 19:00 About functions and if-else statements Common Submission Mistakes In HW1, some students submitted empty/wrong files Make sure that you send the cpp file; otherwise we cannot grade rar format is not allowed for compression; please use zip Do not use blanks, Turkish characters, special symbols in the filenames Only English alphabet letters, digits and underscore are allowed Please submit the required files only, not the entire project folder General rules about homeworks Use of global variables (variables defined outside of functions) prohibited No abrupt program termination in the middle of the program. Modularity and code duplication are important Code duplication must avoided

Summary of what we learned last week Functions – Divide and conquer Functions with Parameters Local variables vs Parameters Scope

Textbook Sections 4.2, 4.3, 4.4 and 4.7 Conditional Statements (if–else)

Conditional Statements So far statements of our programs execute sequentially one after another. What happens when we want to execute a statement depending on a condition? e.g. If it snows, announce that the schools are on vacation e.g. If there is enough money in the bank account, give the money we want to execute one statement when a condition holds and another statement when a condition does not hold? e.g. If dollar is high, sell dollar. Otherwise, buy dollar. we want to select from many statements according to one or more criteria (selection). e.g. If dollar is high and euro is low, sell dollar and buy euro. If dollar is low and euro is high, sell euro and buy dollar. If both of them are high, sell both and buy YTL. You achieve conditional execution with if-else statements

Syntax if ( ) { ;... ; } else { ;... ; } If condition is TRUE then statement_true_1 … statement_true_N are executed, if condition is FALSE statement_false_1 … statement_false_N are executed. if ( ) { ;... ; } else and statement_false’s are optional if condition is FALSE then nothing will be executed and execution continues with the next statement in the program must be in parantheses ( )

Another Syntax (without { }) if ( ) ; else ; if ( ) ; Can be used when there is only one statement Not suggested (we will see why)

Flow diagram of if-else test condition true statements true next statement false statements

Example (not in the book) Write a program that inputs two integer numbers and displays the maximum one. Two solutions using if and else together using only if (no else)

Solution 1 – with_if_else.cpp int main () { int num1, num2, max; cout << "Enter two numbers: "; cin >> num1 >> num2; if (num1 > num2) // check if first number is larger than the second one { max = num1; // if so maximum is the first number } else { max = num2; // otherwise maximum is the second number } cout << "maximum of these two numbers is: " << max << endl; return 0; }

Solution 2 – with_if.cpp (no else) int main () { int num1, num2, max; cout << "Enter two numbers: "; cin >> num1 >> num2; max = num1; // default assignment - maximum is the first number if (num2 > max) // check if second number is larger than the first one { max = num2; // if so update the maximum, if not do nothing } cout << "maximum of these two numbers is: " << max << endl; return 0; }

Boolean type and expressions The condition in an if statement must be a Boolean expression (named for George Boole) Values are true or false bool is a built-in type like int, double int degrees; bool isHot = false; cout << "enter temperature: “; cin >> degrees; if (degrees > 35) { isHot = true; } Boolean values have numeric equivalents false is 0, true is any nonzero value if (3 * 4 – 8) cout << "hello"; else cout << "bye" ; prints hello boolean output yields 0 (for false) or 1 (true) cout << (4 < 5); prints 1

< less than number < 5 <= less than or equal number <= 0 > greater than num1 > num2 >= greater than or equal num1 >= num2 == equality check num1 == 0 != inequality check num1 != num2 Relational Operators Relational operators are used to compare values: They take two operands operands can be literals, variables or expressions Used for many types numeric comparisons string comparisons (lexicographical, i.e. alphabetical) boolean comparisons (false is less than true)

Examples Let’s look at some examples with literals to better see the boolean results. 23 >= 45false 49.0 == 7*7 true 34-3 != 30+1false string s1= "elma", s2= "armut", s3= "Elma"; s1 < s2 false s3 < s1 true Why s3 < s2 is true? ‘E’ has a smaller code than ‘a’ Uppercase letters have smaller codes than lowercase letters

Logical operators Boolean expressions can be combined using logical operators: AND, OR, NOT In C++ we use && || ! respectively ABA || BA && B true falsetruefalse true false A! A truefalse true

Example Range check: between 0 and 100 (includes 0 and 100), or not? If so, display a message saying that the number is in the range. If not, the message should say “out of the range”. Solution 1: using logical AND operator if (num >= 0 && num <= 100) cout << "number in the range"; else cout << "number is out of range"; Solution 2: using logical AND and NOT operators if ( ! (num >= 0 && num <= 100) ) cout << "number is out of range"; else cout << "number is in the range"; Solution 3: using logical OR operator if (num 100) cout << "number is out of range"; else cout << "number is in the range";

De Morgan’s Rules (Section 4.7) Compare solution 2 and 3 two conditions are equivalent ( ! (num >= 0 && num <= 100) ) ( num 100 ) De Morgan’s Rules (assume a and b are two boolean expressions) ! (a && b) = !a || !b ! (a || b) = !a && !b De Morgan’a Rules can be generalized to several expressions (e.g. 4 boolean expressions case) ! (a && b && c && d) = !a || !b || !c || !d ! (a || b || c || d) = !a && !b && !c && !d

Operator Precedence - Revisited Upper operator groups have precedence OperatorExplanationAssociativity + - ! plus and minus signs, logical NOTright-to-left * / % multiplication, division and modulusleft-to-right + - addition, subtractionleft-to-right > stream insertion and extractionleft-to-right >= inequality comparison operatorsleft-to-right == != equal, not equal comparisonleft-to-right && logical andleft-to-right || logical orleft-to-right = += -= *= /= %= assignment operatorsright-to-left

Operator Precedence Examples cout << num1 < year; syntax error (very cryptic) the problem is that << has precedence over < does not compile as intended Solution: cout << (num1 < year); Advice: use parenthesized expressions in cout What about (0 <= num <= 100) for range check? not a syntax error but that expression does not make a range check. It is always true. Why? What is the value of !12+5&&32/35 ? result is 0

Nested if statements if/else statements are inside other if/else statements Method to select from multiple choices Example: input a numeric grade and display messages according to its value low average good otherwise invalid grade

Nested if Example Example: input a numeric grade and display messages according to its value low average good otherwise invalid grade Several solutions exist (not in the book) First solution: if’s are after if’s see if_after_if.cpp Second solution: if’s are after else’s see if_after_else.cpp or if_after_else2.cpp

if ( ) { if ( ) { if ( ) { ; } else { ; } else { ; } else { ; } Nested if-else Syntax 1 if condition_1 is TRUE then check condition_2 if condition_2 is TRUE then check condition_3 if condition_3 is TRUE statements_alltrue are executed, if condition_3 is FALSE statements_true_1and2 are executed, if condition_2 is FALSE statements_true_1_only are executed, if condition_1 is FALSE statements_1_false are executed,

if ( ) { ;... ; } else if ( ) { ;... ; } else if ( ) { ;... ; }... else { ;... ; } Nested if-else Syntax 2 if condition_1 is TRUE then statement_1true_1 … statement_1true_N are executed, if condition_1 is FALSE then check condition_2 and if condition_2 is TRUE then statement_2true_1 … statement_2true_N are executed, if condition_2 is FALSE then check condition_3 and if condition_3 is TRUE then Statement_3true_1 … statement_3true_N are executed, … if condition_(N-1) is FALSE then statement_allfalse_1 … statement_allfalse_N are executed.

Short-circuit Evaluation Some subexpressions in Boolean expressions are not evaluated if the entire expression’s value is already known using the subexpression evaluated so far. Rule: Evaluate the first (leftmost) boolean subexpression. If its value is enough to judge about the value of the entire expression, then stop there. Otherwise continue evaluation towards right. if (count != 0 && scores/count < 60) { cout << "low average" << endl; } In this example, if the value of count is zero, then first subexpression becomes false and the second one is not evaluated. In this way, we avoid “division by zero” error (that would cause to crash the execution of the program) Alternative method to avoid division by zero without using short-circuit evaluation: if (count != 0) { if (scores/count < 60) { cout << "low average warning" << endl; }

Dangling Else Problem if ( x % 2 == 0) if ( x < 0 ) cout << x << " is an even, negative number" << endl; else cout << x << " is an odd number" << endl; What does it display for x=4? The problem is that it displays “odd number” message for positive even numbers and zero. Reason is that, although indentation says the reverse, else belongs to second (inner) if else belongs to the most recent if Solution: use braces (see next slide)

Solution to Dangling Else Problem if ( x % 2 == 0) { if ( x < 0 ) cout << x << " is an even, negative number"<< endl; } else { cout << x << " is an odd number" << endl; } Now else belongs to the first if if – else matching rule Each else belongs to the nearest if for which there is no else and in the same compound block

Functions Overview Functions are sequence of statements with its own local variables supports modularity, reduces code duplication Data transfer between function to be called and caller function by means of parameters currently one-way from caller function into function to be called Later we will see how to return data back to the caller function

Function Prototype (from 2.6) Functions definition has two parts function heading name, parameters, return type function body (local variables and statements within curly brackets) void display (string name) { cout << “Hello ” << name << endl; } Like variables, a function must be declared before its first call Problem of function declaration order You cannot call a function before you declare it SOLUTION: You may define function prototypes (a copy of the heading) at the beginning without function declarations

Function Prototype – Example Problem What is the problem below (program order.cpp) ? void Hi (string name) { cout << "Hi " << name << endl; Greetings(); } void Greetings() { cout << "Things are happening inside this computer" << endl; } int main() { Hi("Fred"); return 0; } Greetings() is called in Hi() but it is declared afterwards

Function Prototype – Solution Add function prototypes to the beginning (order2.cpp) #include #include using namespace std; void Hi(string); void Greetings(); void Hi (string name) { cout << "Hi " << name << endl; Greetings(); } void Greetings() { cout << "Things are happening inside this computer" << endl; } int main() { Hi("Fred"); return 0; } Prototypes Function Declarations

Function Prototypes *** Do not forget semicolon after the prototype definition *** no semicolon after the parameters in normal definition Sometimes prototypes are not necessary if the order of function calls allows but it is good programming practice to have them Parameter names are not needed in prototypes but it is OK if you have the parameter names In #included files we have the functions’ prototypes only implementations of function bodies are in libraries or in other cpp files they are linked together

You can use enum type when you want to define all the possible values for your type type for CardSuit type for colors Type definition syntax enum TypeName {list of literals separated by comma}; Type definition example enum CardSuit {spade, heart, diamond, club}; You can define variables of enum types, you can use them as parameters and return types throughout the cpp file An enum type must be defined at global level (after using namespace std; and before the function declarations) Enumerated Types ( Section 9.3.4)

Enum types Example use enum CardSuit {spade, heart, diamond, club}; CardSuit c; c = club; Each constant of an enum type has an associated integer code starting from 0 spade is 0, heart is 1, diamond is 2, club is 3 Displaying an enum type variable actually displays its integer code cout << c; // displays 3 Cannot assign an integer to an enum variable c = 2; // illegal c = CardSuit(2); // legal, c becomes diamond Cannot input into an enum cin >> c; // invalid Can use comparison operators (, = operators compare codes) if (c <= heart) cout << "hi" << endl;