Chapter 5 - Notes Control Structures II (Repetition)

Transcription

1 Chapter 5 - Notes Control Structures II (Repetition) I. The while Looping ( Repetition ) Structure A. Allows you to repeat a set of instructions until certain conditions are met. B. Syntax and Use: while ( expression ) statement ; i. while is a reserved word. ii. expression is any logical expression that evaluates out to a true or false. a. Provides an entry condition. If the expression evaluates to true, the statement is executed and then the expression is re-evaluated. b. Each time the expression evaluates to true, the statement is repeated until the expression becomes false. c. When false, the program jumps past the statement and continues on to the first statement following the loop. d. An infinite loop is a loop whose expression statement is always true. iii. iv. statement is any legal C++ statement. ( including another while loop, or a compound statement ) Example Program int i ; i = 0 ; while ( i <= 20 ) Page 1

2 cout << i << " " ; i = i + 5 ; system("pause") ; return 0 ; v. Notice: in the above program, the control variable i, must be initialized to some value before it can be evaluated in the boolean expression. vi. vii. viii. Also notice that within the loop somewhere, the control variable value is increased so that it becomes closer to an exit condition. If the control variable is not changed within the loop, the dreaded infinite loop will result. Notice that if the order of the statements inside the above while loop changes, the output changes. int i ; i = 0 ; while ( i <= 20 ) i = i + 5 ; cout << i << " " ; system("pause") ; return 0 ; Page 2

3 C. Counter-Controlled while Loops 1. Used when the programmer knows in advance how many times the program needs to go through a loop. 2. Example Program: using namespace std ; int num, counter ; cout << "How many times would you like to go through the loop: " ; counter = 1 ; while ( counter <= num ) cout << "Loop #" << counter << endl ; counter++ ; cout << endl ; system("pause") ; return 0 ; How many times would you like to go through the loop: 125 Loop #1 Loop #2 Loop #3 : : Loop #123 Loop #124 Loop #125 Page 3

4 3. Notice that the user can determine from the keyboard how many times the loop should execute with the above example. Or, if the programmer wants, the amount of loops to execute can be hard-coded with an assignment statement: num = 125 ;... instead of prompting the user. D. Sentinel-Controlled while Loops 1. The number of times that a loop must be passed through in your program may not always be known. One of the ways that a loop knows when to exit is with a sentinel value. 2. Sentinel: A value outside the domain of the values being processed by the loop. Example: If your loop is processing positive integers, a sentinel value would be some negative value. 3. The sentinel value indicates the end of the values that you want to process and causes the boolean expression in your while loop to evaluate to false. 4. Example Program: int sum, count, num ; sum = 0 ; count = 0 ; cout << "Please enter any amount of positive integers " ; cout << "and then a negative integer to indicate the end. " ; while ( num >= 0 ) sum += num ; count++ ; cout << "The average of your numbers is " << sum / count ; Page 4

5 cout << endl << endl ; system("pause") ; return 0 ; Please enter any amount of positive integers and then a negative integer to indi cate the end The average of your numbers is 56 E. Flag-Controlled while Loops 1. Uses a boolean variable to control when to exit a loop. 2. Often used with a conditional statement inside the loop to switch a flag from true to false or false to true. 3. Used for searching through lists for a particular value. 4. Example Program: bool found ; int num ; found = false ; while (!found ) cout << "I'm thinking of a number from 1 to 20, what is it: " ; cout << endl ; if ( num == 14 ) Page 5

6 else cout << "Correct, it was 14!!" << endl << endl; found = true ; cout << "Sorry, it's not " << num << ". Try again." << endl << endl; system("pause") ; return 0 ; I'm thinking of a number from 1 to 20, what is it: 5 Sorry, it's not 5. Try again. I'm thinking of a number from 1 to 20, what is it: 18 Sorry, it's not 18. Try again. I'm thinking of a number from 1 to 20, what is it: 14 Correct, it was 14!! F. EOF-Controlled while Loop Two Methods 1. The use of the input stream variable ( cin ): i. Example: cin >> somevariable ; while ( cin ) statement1 ; statement2 ; : : cin >> somevariable ; Page 6

7 ii. The cin input stream variable returns a value of true or false. a. If the program has reached the end of the input data, the input stream variable returns the logical value false. b. If the program reads any faulty data and goes into the fail state, the input stream variable returns the value false. c. In any other case, the input stream variable returns the value true. 2. The use of the ( eof ) Function: i. Syntax: istreamvariable.eof ( ) a. Where istream Variable is a user declared identifier. b. The function returns a value of true, if the program has read past the end of the input file. c. The function returns false if it has not read past the end of the input file. ii. Example of use : ifstream infile ; char ch1 ; infile.open("somefilename.dat") ; infile.get( ch1 ) ; while (! infile.eof ( ) ) cout << ch1 ; infile.get ( ch1 ) ; II. The for Looping ( Repetition ) Structure A. A specialized form of the while Loop that makes it a little easier to implement count-controlled loops. Page 7

8 1. The for loop is used primarily when the number of times a loop must be executed is known in advance. B. Syntax for ( initial statement ; loop condition ; update statement ) statement ; Example: for ( int i = 0 ; i < 10 ; i++ ) statement ; 1. NOTE: There are three main parts to the for Loop inside the parenthesis that are called the for Loop control statements. i. The initial statement: A control variable is initialized to some starting value. The control variable can be declared either before the for Loop in a separate statement, or in the initial statement. ii. The loop condition: Controls how many times the loop will be executed based on the comparison of the control variable to some expression. iii. The update statement: The update statement increments or decrements your control variable by the value you specify, making sure that as you increment or decrement, you are getting closer to a false condition. initial statement loop condition update statement for ( int i = 0 ; i < 10 ; i++ ) statement ; 2. Examples of for Loops i. The first for Loop declares and initializes the control variable in the initial statement. The loop is designed to print out the numbers from 0 to 10 on a single line, with a space between them. Page 8

9 for ( int num = 0; num <= 10; num++ ) cout << num << " " ; cout << endl << endl; system("pause"); return 0; ii. The second for Loop declares the control variable before the loop and initializes the control variable in the initial statement. int num; for ( num = 0; num <= 10; num++ ) cout << num << " " ; cout << endl << endl; system("pause"); return 0; Page 9

10 iii. The next for Loop declares and initializes the control variable in the initial statement. The loop is designed to print out every tenth number from 0 to 100 on a single line, with a space between them. for ( int num = 0; num <= 100; num = num + 10 ) cout << num << " " ; cout << endl << endl; system("pause"); return 0; iv. The final for Loop declares and initializes the control variable in the initial statement. The loop is designed to print out every tenth number from 100 down to 0 on a single line, with a space between them. for ( int num = 100; num >= 0; num = num - 10 ) cout << num << " " ; cout << endl << endl; system("pause"); return 0; Page 10

11 III. The do...while Looping ( Repetition ) Structure A. The main point to remember with this loop is that it will always execute at least once, whereas the regular while loop may not execute at all, depending on the conditional statement. ( Note that do is a reserved word.) B. Syntax: d o statement while ( expression ) ; OR do statement 1 ; statement 2 ; : statement n ; while ( expression ) ; C. A do..while Loop is often used to make sure the user enters the proper value. Example: int num; do cout << " Please enter a number between 1 and 10: "; cin >> num; cout << endl; while ( num < 1 num > 10 ) ; cout << "Your number is " << num << endl << endl; system("pause"); return 0; Page 11

12 Please enter a number between 1 and 10: 89 Please enter a number between 1 and 10: -5 Please enter a number between 1 and 10: 7 Your number is 7 IV. Break And Continue Statements A. Break Statement: (2 main uses) 1. To skip the remainder of the switch statement. 2. To exit early from a loop; i. The use of a break statement in a loop can eliminate the use of certain flag variables. ii. Example: sum = 0 ; while ( cin ) if ( num < 0 ) // if the number is negative, terminate loop cout << "Negative number found in data" << endl ; break ; sum = sum + num ; Page 12

13 B. Continue Statement: 1. Used in the while, for, and do...while loop structures. 2. When the continue statement is executed in a loop, it skips the remaining statements in the loop and proceeds with the next iteration of the loop. 3. Example: sum = 0 ; while ( cin ) if ( num < 0 ) // if the number is negative, terminate loop cout << "Negative number found in data" << endl ; continue ; sum = sum + num ; 4. Notice that if you want to continue with the loop, you must reprime the loop with a cin statement to put a value in the variable num. 5. In a for Loop, the continue statement would skip to the increment or decrement statement, then to the conditional expression. With the while and do...while Loops, you must reprime (update) the control variable before you call continue and the conditional expression is checked. V. Nested Control Structures A. Like if...else statements, loops can also be nested. In fact, if statements can be nested inside loops and loops can be nested inside if statements. The nesting of control structures can give your program more power and flexibility. Page 13

14 B. There are too many possibilities for me to list in these notes, but I will give a couple of popular examples on the next page. 1. Example of if statement nested inside a while loop: int sum, count; sum = 0; count = 1; while ( count <= 100 ) if ( count % 2 == 0 ) sum = sum + count ; count++ ; cout << "The sum of even numbers up to 100 is: " << sum << endl << endl; system("pause"); return 0; The sum of even numbers up to 100 is: The following is an example of nested for Loops that produce the following design: * ** *** **** ***** Page 14

15 i. Note that the variable row in the outer for Loop controls the number of rows of the display, and the variable col in the inside for Loop controls how many columns are displayed and what is displayed. for ( int row = 1; row <= 5; row++ ) for ( int col = 1; col <= row ; col++ ) cout << '*'; cout << endl; cout << endl; system("pause"); return 0; * ** *** **** ***** Page 15