EECE 230 Chapter 4- Control Structures I, Spring 2012

Chapter 4: Control Structure 1 C++ Programming: Program Design Including Data Structures, Second Edition 2 Objectives In this chapter you will: • Learn about control structures • Examine relational and logical operators • Explore how to form and evaluate logical (Boolean) expressions • Discover how to use the selection control structures if, if...else, and switch in a program C++ Programming: Program Design Including Data Structures, Second Edition 3 Control Structures • A computer can proceed: − In sequence − Selectively (branch) - making a choice − Repetitively (iteratively) - looping • Some statements are executed only if certain conditions are met • A condition is represented by a logical (Boolean) expression that can be true or false • A condition is met if it evaluates to true C++ Programming: Program Design Including Data Structures, Second Edition 5 Relational Operators • Relational operators: − Allow comparisons − Require two operands (binary) − Return 1 if expression is true, 0 otherwise • Comparing values of different data types may produce unpredictable results − For example, 8 < '5' should not be done • Any nonzero value is treated as true C++ Programming: Program Design Including Data Structures, Second Edition 8 Comparing string Types • Relational operators can be applied to strings • Strings are compared character by character, starting with the first character • Comparison continues until either a mismatch is found or all characters are found equal • If two strings of different lengths are compared and the comparison is equal to the last character of the shorter string − The shorter string is less than the larger string C++ Programming: Program Design Including Data Structures, Second Edition 9 string Comparison Example • Suppose we have the following declarations: − string str1 = "Hello"; − string str2 = "Hi"; − string str3 = "Air"; − string str4 = "Bill"; C++ Programming: Program Design Including Data Structures, Second Edition 10 Logical (Boolean) Operators • Logical (Boolean) operators enable you to combine logical expressions • Three logical (Boolean) operators: − ! - not − && – and − || - or • Logical operators take logical values as operands and yield logical values as results • ! is unary; && and || are binary operators • Putting ! in front of a logical expression reverses its value C++ Programming: Program Design Including Data Structures, Second Edition 11 C++ Programming: Program Design Including Data Structures, Second Edition 12 Precedence of Operators • Relational and logical operators are evaluated from left to right • The associativity is left to right • Parentheses can override precedence C++ Programming: Program Design Including Data Structures, Second Edition 13 C++ Programming: Program Design Including Data Structures, Second Edition 14 One-Way (if) Selection • The syntax of one-way selection is: if(expression) statement • Statement is executed if the value of the expression is true • Statement is bypassed if the value is false; program goes to the next statement C++ Programming: Program Design Including Data Structures, Second Edition 15 C++ Programming: Program Design Including Data Structures, Second Edition 16 Two-Way (if…else) Selection • Two-way selection takes the form: if(expression) statement1 else statement2 • If expression is true, statement1 is executed otherwise statement2 is executed C++ Programming: Program Design Including Data Structures, Second Edition 18 Compound (Block of) Statement • Compound statement (block of statements): { statement1; statement2; . . . statementn; } • A compound statement is a single statement C++ Programming: Program Design Including Data Structures, Second Edition 19 Nested if • Nesting: one control statement in another • An else is associated with the most recent if that has not been paired with an else • For example: if(score >= 90) cout<<"The grade is A"<<endl; else if(score >= 80) cout<<"The grade is B"<<endl; else cout<<"The grade is F"<<endl; C++ Programming: Program Design Including Data Structures, Second Edition 20 Conditional Operator (?:) • Conditional operator (?:) takes three arguments (ternary) • Syntax for using the conditional operator: expression1 ? expression2 : expression3 • If expression1 is true, the result of the conditional expression is expression2. Otherwise, the result is expression3 C++ Programming: Program Design Including Data Structures, Second Edition 21 switch Structures • Switch structure: alternate to if-else • Switch expression is evaluated first • Value of the expression determines which corresponding action is taken • Expression is sometimes called the selector C++ Programming: Program Design Including Data Structures, Second Edition 22 switch Structures (continued) • Expression value can be only integral • Its value determines which statement is selected for execution • A particular case value should appear only once C++ Programming: Program Design Including Data Structures, Second Edition 24 switch Structures (continued) • One or more statements may follow a case label • Braces are not needed to turn multiple statements into a single compound statement • The break statement may or may not appear after each statement • switch, case, break, and default are reserved words C++ Programming: Program Design Including Data Structures, Second Edition 25 switch Statement Rules (continued) • When value of the expression is matched against a case value, − Statements execute until break statement is found or the end of switch structure is reached • If value of the expression does not match any of the case values − Statements following the default label execute If no default label and no match the entire switch statement is skipped • A break statement causes an immediate exit from the switch structure