1. Chapter 2.1
Repetition

2. A First Book of ANSI C, Fourth Edition
Introduction
A section of code that is repeated is called a loop:
because after the last statement in the code is executed,
the program branches, or loops, back to the first statement
and starts another repetition through the code
Each repetition is also called an iteration or pass through the loop
A First Book of ANSI C, Fourth Edition

3. A First Book of ANSI C, Fourth Edition
Basic Loop Structures
Constructing a repeating section of code requires that four elements be present:
Repetition statement
while statement
for statement
do-while statement
Condition
A statement that initially sets the condition being tested
A statement within the repeating section of code that alters the condition so that it eventually becomes false
A First Book of ANSI C, Fourth Edition

4. Pretest and Posttest Loops
In a pretest loop , the condition is checked at the beginning of each iteration.
In a post-test loop, the condition is checked at the end of each iteration.
A First Book of ANSI C, Fourth Edition

5. Counter-Controlled Loop
the condition is used to keep track of the number of repetitions
Also known as a fixed-count loop
int count=0;
while (count < 5) {
printf("Hello\n");
count = count + 1; }
printf("Finished\n");
Hello
Finished
Press any key to continue
Number of repetitions is known when entering the loop
A First Book of ANSI C, Fourth Edition

6. Condition-Controlled Loop
the tested condition does not depend on a count being achieved, but rather on a specific value being encountered
int mark;
printf("Enter a mark: ");
scanf("%d", &mark);
while (mark<0 || mark>100) {
printf("Invalid! Enter a new mark: "); }
printf("Finished\n");
Enter a mark: -12
Invalid! Enter a new mark: -65
Invalid! Enter a new mark: 1000
Invalid! Enter a new mark: 200
Invalid! Enter a new mark: 56
Finished
Press any key to continue
Number of repetitions is not known when entering the loop
A First Book of ANSI C, Fourth Edition

7. Sentinel-Controlled Loop
a special input value indicates that there is no more data
int num;
while (num != -1) {
printf("Enter a number: ");
scanf("%d", &num); }
printf("Finished\n");
Enter a number: 12
Enter a number: 23
Enter a number: 20
Enter a number: -1
Finished
Press any key to continue
Number of repetitions is not known when entering the loop
A First Book of ANSI C, Fourth Edition

8. A First Book of ANSI C, Fourth Edition
The while Statement
The general form of the while statement is
while (expression)
statement;
The transfer of control back to the start of a while statement to reevaluate the expression is known as a program loop
The following is a valid but infinite loop:
int count=1;
while (count <= 10)
printf("%d ",count);
A First Book of ANSI C, Fourth Edition

9. The while Statement (cont.)
A First Book of ANSI C, Fourth Edition

10. The while Statement (cont.)
Output is:
Output is:
A First Book of ANSI C, Fourth Edition

11. Computing Sums and Averages Using a while Loop
A First Book of ANSI C, Fourth Edition

12. Computing Sums and Averages Using a while Loop (cont.)
A First Book of ANSI C, Fourth Edition

13. Computing Sums and Averages Using a while Loop (cont.)
A First Book of ANSI C, Fourth Edition

14. Computing Sums and Averages Using a while Loop (cont.)
Ensures that any previous value present in the storage locations assigned to the variable total is overwritten and the total starts at a correct value
Accumulating statement
Calculating an average
A First Book of ANSI C, Fourth Edition

15. A First Book of ANSI C, Fourth Edition
Sentinels
A program, such as Program 5.7, can be made much more general by removing the restriction that exactly four numbers are to be entered
The user enters a value for how many numbers will be averaged
You can use a sentinel (a data value used to signal either the start or end of a data series)
The sentinel values must be selected so as not to conflict with legitimate data values
A First Book of ANSI C, Fourth Edition

16. Sentinels (continued)
A First Book of ANSI C, Fourth Edition

17. Sentinels (continued)
One useful sentinel in C is the named constant EOF (End Of File)
The actual value of EOF is compiler-dependent, but it is always assigned a code that is not used by any other character
EOF is defined in stdio.h
A First Book of ANSI C, Fourth Edition

18. Sentinels (continued)
A First Book of ANSI C, Fourth Edition

19. The break and continue Statements
A break forces an immediate exit from while, switch, for, and do-while statements only
Sample output 1:
int count=1;
float num;
while(count <= 10) {
printf("Enter a number: ");
scanf("%f", &num);
if (num > 76)
printf("You lose!");
break; /* break out of the loop */ }
else
printf("Keep on truckin!");
count++;
/* break jumps to here */
Enter a number: 1
Keep on truckin!Enter a number: 2
Keep on truckin!Enter a number: 20
Keep on truckin!Enter a number: 100
You lose!Press any key to continue
Sample output 2:
Enter a number: 1
Keep on truckin!Enter a number: 2
Keep on truckin!Enter a number: 3
Keep on truckin!Enter a number: 4
Keep on truckin!Enter a number: 5
Keep on truckin!Enter a number: 6
Keep on truckin!Enter a number: 7
Keep on truckin!Enter a number: 8
Keep on truckin!Enter a number: 9
Keep on truckin!Enter a number: 10
Keep on truckin!Press any key to continue
A First Book of ANSI C, Fourth Edition

20. The break and continue Statements
The continue applies to loops only; when a continue statement is encountered in a loop, the next iteration of the loop begins immediately
Sample output 1:
int count=0;
float grade, total=0;
while (count < 5) {
printf("Enter a grade: ");
scanf("%f", &grade);
if(grade < 0 || grade > 100)
continue;
total = total + grade;
count = count + 1; }
printf("Total is %.2f\n", total);
Enter a grade: 10
Enter a grade: 20
Enter a grade: 30
Enter a grade: 40
Enter a grade: 50
Total is
Press any key to continue
Sample output 2:
Enter a grade: 10
Enter a grade: 30
Enter a grade: -10
Enter a grade: 1000
Enter a grade: 20
Total is 80.00
Press any key to continue
A First Book of ANSI C, Fourth Edition

21. A First Book of ANSI C, Fourth Edition
The Null Statement
A semicolon with nothing preceding it is also a valid statement, called the null statement ;
Use the null statement where a statement is syntactically required, but no action is needed
Null statements typically are used either with while or for statements
A First Book of ANSI C, Fourth Edition

22. A First Book of ANSI C, Fourth Edition
The for Statement
The for statement combines all four elements required to easily produce a loop on the same line
This statement does not require that any of the items in parentheses be present or that they actually be used for initializing or altering the values in the expression statements
However, the two semicolons must be present
for ( ; count <= 20;) is valid
Omitting tested expression results in infinite loop
for (initializing list; tested expression; altering list)
statement;
A First Book of ANSI C, Fourth Edition

23. The for Statement (continued)
A First Book of ANSI C, Fourth Edition

24. The for Statement (continued)
Output is:
A First Book of ANSI C, Fourth Edition

25. The for Statement (continued)
A First Book of ANSI C, Fourth Edition

26. The for Statement (continued)
A First Book of ANSI C, Fourth Edition

27. The for Statement (continued)
Comma-separated list
A First Book of ANSI C, Fourth Edition

28. Computing Sums and Averages Using a for Loop
A First Book of ANSI C, Fourth Edition

29. Case Studies: Loop Programming Techniques
Technique 1: Selection within a loop
Technique 2: Input data validation
Technique 3: Interactive loop control
Technique 4: Evaluating equations
A First Book of ANSI C, Fourth Edition

30. Technique 1: Selection within a Loop
A First Book of ANSI C, Fourth Edition

31. Technique 2: Input Data Validation
Same code used
in lines 6-7!
A First Book of ANSI C, Fourth Edition

32. Technique 2: Input Data Validation (cont.)
A First Book of ANSI C, Fourth Edition

33. Technique 3: Interactive Loop Control
A First Book of ANSI C, Fourth Edition

34. Technique 4: Evaluating Equations
A First Book of ANSI C, Fourth Edition

35. Technique 4: Evaluating Equations (cont.)
A First Book of ANSI C, Fourth Edition

36. A First Book of ANSI C, Fourth Edition
Nested Loops
A First Book of ANSI C, Fourth Edition

37. Nested Loops (continued)
Sample run:
i is now 1
j = 1 j = 2 j = 3 j = 4
i is now 2
i is now 3
i is now 4
i is now 5
A First Book of ANSI C, Fourth Edition

38. Nested Loops (continued)
A First Book of ANSI C, Fourth Edition

39. The do-while Statement
A First Book of ANSI C, Fourth Edition

40. The do-while Statement (cont.)
The general form of the do statement is do
statement;
while (expression);
do-while is a posttest loop
One type of application is ideally suited for a posttest loop: input data validation application do {
printf("\nEnter an ID number: ");
scanf("%f", &idNum);
} while (idNum < 1000 || idNum > 1999);
A First Book of ANSI C, Fourth Edition