University:
Kentucky State University
Course:
COS 232| Programming I
Academic year:

2023
Views:

465

Pages:

29
Author:

Arielle Armstrong

Chapter 5: Control Structures II (Repetition) Objectives In this chapter, you will: • Learn about repetition (looping) control structures • Explore how to construct and use countcontrolled, sentinel-controlled, flag-controlled, and EOF-controlled repetition structures • Examine break and continue statements • Discover how to form and use nested control structures 2 Why Is Repetition Needed? • Repetition allows you to efficiently use variables • Can input, add, and average multiple numbers using a limited number of variables • For example, to add five numbers: − Declare a variable for each number, input the numbers and add the variables together − Create a loop that reads a number into a variable and adds it to a variable that contains the sum of the numbers 3 while Looping (Repetition) Structure • The general form of the while statement is: 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 4 while Looping (Repetition) Structure (continued) • Infinite loop: continues to execute endlessly − Avoided by including statements in loop body that assure exit condition is eventually false 5 while Looping (Repetition) Structure (continued) 6 Designing while Loops 7 Case 1: 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 8 Case 2: Sentinel-Controlled while Loops • Sentinel variable is tested in the condition and loop ends when sentinel is encountered 9 Case 3: Flag-Controlled while Loops • A flag-controlled while loop uses a bool variable to control the loop • The flag-controlled while loop takes the form: 10 More on Expressions in while Statements • The expression in a while statement can be complex − For example: while ((noOfGuesses < 5) && (!isGuessed)) { … } 11 for Looping (Repetition) Structure • The general form of the for statement is: • The initial statement, loop condition, and update statement are called for loop control statements − initial statement usually initializes a variable (called the for loop control, or for indexed, variable) • In C++, for is a reserved word 12 for Looping (Repetition) Structure (continued) 13 for Looping (Repetition) Structure (continued) 14 for Looping (Repetition) Structure (continued) • C++ allows you to use fractional values for loop control variables of the double type − Results may differ • The following is a semantic error: • The following is a legal for loop: for (;;) cout << "Hello" << endl; 15 for Looping (Repetition) Structure (continued) 16 do…while Looping (Repetition) Structure • General form of a do...while: • The statement executes first, and then the expression is evaluated • To avoid an infinite loop, body must contain a statement that makes the expression false • The statement can be simple or compound • Loop always iterates at least once 17 do…while Looping (Repetition) Structure (continued) 18 do…while Looping (Repetition) Structure (continued) 19 Divisibility Test by 3 and 9 21 Choosing the Right Looping Structure • All three loops have their place in C++ − If you know or can determine in advance the number of repetitions needed, the for loop is the correct choice − If you do not know and cannot determine in advance the number of repetitions needed, and it could be zero, use a while loop − If you do not know and cannot determine in advance the number of repetitions needed, and it is at least one, use a do...while loop 22 break and continue Statements • break and continue alter the flow of control • break statement is used for two purposes: − To exit early from a loop • Can eliminate the use of certain (flag) variables − To skip the remainder of the switch structure • After the break statement executes, the program continues with the first statement after the structure 23 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 24 Nested Control Structures • To create the following pattern: * ** *** **** ***** • We can use the following code: for (i = 1; i <= 5 ; i++) { for (j = 1; j <= i; j++) cout << "*"; cout << endl; } 25 Nested Control Structures (continued) • What is the result if we replace the first for statement with the following? for (i = 5; i >= 1; i--) • Answer: ***** **** *** ** * 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 pretest loops • do...while loop is called a posttest loop • while and for may not execute at all, but do...while always executes at least once 27 Summary (continued) • while: expression is the decision maker, and the statement is the body of the loop • A while loop can be: − Counter-controlled − Sentinel-controlled − EOF-controlled • In the Windows console environment, the end-of-file marker is entered using Ctrl+z 28 Summary (continued) • for loop: simplifies the writing of a countercontrolled while loop − Putting a semicolon at the end of the for loop is a semantic error • 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 29

Chapter 5 Control Structures II (Repetition)

Related Documents

Recommended Documents

Description

Related Documents

Free up your schedule!

Take 5 seconds to unlock