EECE 230 Chapter 2- Control Structure II, Spring 2012

Chapter 2: Control Structure II C++ Programming: Program Design Including Data Structures, Second Edition 2 Objectives In this chapter you will: • Learn about repetition (looping) control structures • Explore how to construct and use count- controlled, sentinel-controlled, flag-controlled, and EOF-controlled repetition structures • Examine break and continue statements • Discover how to form and use nested control structures C++ Programming: Program Design Including Data Structures, Second Edition 3 The while Loop • The general form of the while statement is: while(expression) statement • while is a reserved word • Statement can be simple or compound • Expression acts as a decision maker and is usually a logical expression • Statement is called the body of the loop • The parentheses are part of the syntax C++ Programming: Program Design Including Data Structures, Second Edition 4 The while Loop (continued) • Expression provides an entry condition • Statement executes if the expression initially evaluates to true • Loop condition is then reevaluated • Statement continues to execute until the expression is no longer true C++ Programming: Program Design Including Data Structures, Second Edition 6 The while Loop (continued) • Infinite loop: continues to execute endlessly • Can be avoided by including statements in the loop body that assure exit condition will eventually be false C++ Programming: Program Design Including Data Structures, Second Edition 7 Counter-Controlled while Loops • If you know exactly how many pieces of data need to be read, the while loop becomes a counter-controlled loop • The syntax is: counter = 0; while(counter < N) { . counter++; . } C++ Programming: Program Design Including Data Structures, Second Edition 8 Sentinel-Controlled while Loops • Sentinel variable is tested in the condition and loop ends when sentinel is encountered • The syntax is: cin>>variable; while(variable != sentinel) { . cin>> variable; . } C++ Programming: Program Design Including Data Structures, Second Edition 9 EOF-Controlled while Loops • Use an EOF (End Of File)-controlled while loop • The logical value returned by cin can determine if the program has ended input or received wrong input (false value ) • The syntax is: cin >> variable; while (cin) { . cin >> variable; . } C++ Programming: Program Design Including Data Structures, Second Edition 10 The eof Function • The function eof can determine the end of file status • eof is a member of data type istream • The syntax for the function eof is: istreamVar.eof() where istreamVar is an input stream variable, such as cin C++ Programming: Program Design Including Data Structures, Second Edition 11 The for Loop • The general form of the for statement is: for(initial statement; loop condition; update statement) statement • The initial statement, loop condition, and update statement are called for loop control statements C++ Programming: Program Design Including Data Structures, Second Edition 13 The for Loop (continued) • The for loop executes as follows: − initial statement executes − loop condition is evaluated • If loop condition evaluates to true • Execute for loop statement • Execute update statement • Repeat previous step until the loop condition evaluates to false • initial statement initializes a variable C++ Programming: Program Design Including Data Structures, Second Edition 14 The for Loop (continued) • initial statement in the for loop is the first to be executed and is executed only once • If the loop condition is initially false, the loop body does not execute • The update expression changes the value of the loop control variable which eventually sets the value of the loop condition to false • The for loop executes indefinitely if the loop condition is always true C++ Programming: Program Design Including Data Structures, Second Edition 15 The for Loop (continued) • Fractional values can be used for loop control variables • A semicolon at the end of the for statement is a semantic error − In this case, the action of the for loop is empty • If the loop condition is omitted − It is assumed to be true C++ Programming: Program Design Including Data Structures, Second Edition 16 The for Loop (continued) • In a for statement, all three statements (initial statement, loop condition, and update statement) can be omitted • The following is a legal for loop: for(;;) cout<<"Hello"<<endl; C++ Programming: Program Design Including Data Structures, Second Edition 17 The do…while Loop • The general form of a do...while statement is: do statement while(expression); • The statement executes first, and then the expression is evaluated • If the expression evaluates to true, the statement executes again • As long as the expression in a do...while statement is true, the statement executes C++ Programming: Program Design Including Data Structures, Second Edition 18 The do…while Loop (continued) • To avoid an infinite loop, the loop body must contain a statement that makes the expression false • The statement can be simple or compound • If compound, it must be in braces • do...while loop has an exit condition and always iterates at least once (unlike for and while) C++ Programming: Program Design Including Data Structures, Second Edition 20 Break & Continue Statements • break and continue alter the flow of control • When the break statement executes in a repetition structure, it immediately exits • The break statement, in a switch structure, provides an immediate exit • The break statement can be used in while, for, and do...while loops C++ Programming: Program Design Including Data Structures, Second Edition 21 Break & Continue Statements (continued) • The break statement is used for two purposes: 1. To exit early from a loop 2. To skip the remainder of the switch structure • After the break statement executes, the program continues with the first statement after the structure • The use of a break statement in a loop can eliminate the use of certain (flag) variables C++ Programming: Program Design Including Data Structures, Second Edition 22 Break & Continue Statements (continued) • continue is used in while, for, and do-while structures • When executed in a loop − It skips remaining statements and proceeds with the next iteration of the loop C++ Programming: Program Design Including Data Structures, Second Edition 23 Break & Continue Statements (continued) • In a while and do-while structure − Expression (loop-continue test) is evaluated immediately after the continue statement • In a for structure, the update statement is executed after the continue statement − Then the loop condition executes C++ Programming: Program Design Including Data Structures, Second Edition 24 Nested Control Structures • Suppose we want to create the following pattern * ** *** **** ***** C++ Programming: Program Design Including Data Structures, Second Edition 25 Nested Control Structures (continued) Suppose we want to create the following patterns: ***** **** *** ** * * ** *** **** ***** C++ Programming: Program Design Including Data Structures, Second Edition 26 Summary • C++ has three looping (repetition) structures: while, for, and do…while • while, for, and do are reserved words • while and for loops are called pre-test loops • do...while loop is called a post-test loop • while and for may not execute at all, but do...while always executes at least once C++ Programming: Program Design Including Data Structures, Second Edition 27 Summary • while: expression is the decision maker, and the statement is the body of the loop • In a counter-controlled while loop, − Initialize counter before loop − Body must contain a statement that changes the value of the counter variable • A sentinel-controlled while loop uses a sentinel to control the while loop • An EOF-controlled while loop executes until the program detects the end-of-file marker C++ Programming: Program Design Including Data Structures, Second Edition 28 Summary • for loop: simplifies the writing of a count- controlled while loop • Executing a break statement in the body of a loop immediately terminates the loop • Executing a continue statement in the body of a loop skips to the next iteration • After a continue statement executes in a for loop, the update statement is the next statement executed