1. Week 4 – Repetition Structures / Loops
EKT 150 SEM 1 10/11

2. Outline Introduction While loops Do-while loops For loops Nested loops
Three types of while loops
Do-while loops
For loops
Nested loops
EKT 120 SEM II 09/10

3. Outline Introduction While loops Do-while loops For loops Nested loops
Three types of while loops
Do-while loops
For loops
Nested loops

4. Why Need Loops ?
Suppose we want to add five numbers and find the average.
From what you have learned so far, you could proceed as follows
scanf(“%d %d %d %d %d”, &num1, &num2, &num3, &num4,&num5);
sum = num1 + num2 + num3 + num4 + num5;
average = sum / 5;
If 100 numbers, 1000 numbers?

5. Repetition (Loop)‏ Used to control the flow of a program
Loops are basically repetitions or iterations used to repeat a segment of code
Three statements can be used to implement loops in C
while statement
do-while statement
for statement
while and for statements are similar in implementation, but have different syntax
The do-while statement is different - the following code is executed at least once

6. The while loop structure
The general form of the while statement is:
while (expression)‏
statement;
To avoid an infinite loop, make sure that the loop’s body contains statement (s) that assure that the exit condition i.e. the expression in the while statement will eventually be false.
while loop repeated until condition becomes false

7. Flowchart of a while statement

8. The while loop structure-cont
There are basically three types of while loops:
Counter-controlled while loop
Sentinel-controlled while loop
Flag-controlled while loop

9. Counter-Controlled while Loops
Definite repetition: know how many times loop will execute
Control variable used to count repetitions
Example:
int counter = 1; // declare and initialize while ( counter <= 10 ) // test condition {
printf( "%d\n", counter );
++counter; // update }
Requirement:
1. Declare and initialize control variable value (or loop counter)‏
2. A condition that tests for the final value of the control variable (i.e., whether looping should continue)‏
3. Update control variable (incr/decr)‏

10. Counter-Controlled while Loops
Another example:
int product = 2;
while ( product <= 1000 ) product = 2 * product;
declare and initialize
test condition
increment
product <= 1000
product = 2 * product
true
false

11. Example: A class of ten students took a quiz
Example: A class of ten students took a quiz. The grades (integers in the range 0 to 100) for this quiz are available to you. Determine the class average on the quiz
Pseudocode:
Set total to zero Set grade counter to one
While grade counter is less than or equal to ten Input the next grade Add the grade into the total Add one to the grade counter
Set the class average to the total divided by ten Print the class average

12. 1. Initialize Variables 2. Execute Loop 3. Output results
1 /* Fig. 3.6: fig03_06.c
2 Class average program with
3 counter-controlled repetition */
4 #include <stdio.h> 5
6 int main()‏
7 {
8 int counter, grade, total, average; 9
10 /* initialization phase */
11 total = 0;
12 counter = 1; 13
14 /* processing phase */
15 while ( counter <= 10 ) {
printf( "Enter grade: " );
scanf( "%d", &grade );
total = total + grade;
counter = counter + 1;
20 } 21
22 /* termination phase */
23 average = total / 10;
24 printf( "Class average is %d\n", average ); 25
26 return 0; /* indicate program ended successfully */
27 }
1. Initialize Variables
2. Execute Loop
3. Output results
OUTPUT
Enter grade: 98
Enter grade: 76
Enter grade: 71
Enter grade: 87
Enter grade: 83
Enter grade: 90
Enter grade: 57
Enter grade: 79
Enter grade: 82
Enter grade: 94
Class average is 81

13. Sentinel-Controlled while Loops
Indefinite repetition
Used when number of repetitions not known and loop needs to input value repeatedly for each iteration
Sentinel value indicates "end of data“

14. Sentinel-Controlled while Loops-cont
Requirement:
1. Read control variable value before enter loop
2. A condition that tests control variable’s validity (i.e., whether looping should continue)‏
3. Read again control variable before end of loop
Example:
int number, count, sum;
printf(“To stop enter Enter positive numbers : " );
scanf(“%d”, &number);
while (number != -999)‏ {
sum = sum + number; //find sum of numbers entered
count++; //count how many numbers entered }
printf(“\nThe sum of %d numbers is %d“, count, sum);
Sentinel value

15. Sentinel-Controlled while Loops-cont
Another example:
char choice;
printf(“Continue?y-yes n-no :”);
scanf(“%c”, &choice);
while ( choice == ‘y’)‏
{ printf(“\nEnter grade point:”);
scanf(“%f”, &grade_pt);
if(grade_pt > 2.0)‏
printf(“\nPass); }
read control variable
test condition
read again

16. 1. Initialize Variables 2. Get user input 2.1 Perform Loop
1 /* Fig. 3.8: fig03_08.c
2 Class average program with
3 sentinel-controlled repetition */
4 #include <stdio.h> 5
6 int main()‏
7 {
8 float average,total; /* new data type */
9 int counter, grade,; 10
11 /* initialization phase */
12 total = 0;
13 counter = 0; 14
15 /* processing phase */
16 printf( "Enter grade, -1 to end: " );
17 scanf( "%d", &grade ); 18
19 while ( grade != -1 ) {
total = total + grade;
counter = counter + 1;
printf( "Enter grade, -1 to end: " );
scanf( "%d", &grade );
24 }
1. Initialize Variables
2. Get user input
2.1 Perform Loop

17. 3. Calculate Average 3.1 Print Results Program Output25
26 /* termination phase */
27 if ( counter != 0 ) {
average = total / counter;
printf( "Class average is %.2f", average );
30 }
31 else
printf( "No grades were entered\n" ); 33
34 return 0; /* indicate program ended successfully */
35 }
3. Calculate Average
3.1 Print Results
Program Output
Enter grade, -1 to end: 75
Enter grade, -1 to end: 94
Enter grade, -1 to end: 97
Enter grade, -1 to end: 88
Enter grade, -1 to end: 70
Enter grade, -1 to end: 64
Enter grade, -1 to end: 83
Enter grade, -1 to end: 89
Enter grade, -1 to end: -1
Class average is 82.50

18. Flag-Controlled while Loops
Uses a boolean variable to control the loop.
Loop exit when expression is evaluated to false.
Set to false
Requirement:
1. Set control variable to false before loop
2. A condition that tests control variable. If expr evaluated to true, loop continue
3. A decision structure to test value validity
4. Set control variable to true to indicate found
bool found = false;
while (!found)
{ printf(“Enter number between 1 and 200:”);
scanf(%d”, &guess);
if ((guess>= 88) &&(guess <=128))‏
{found = true;
printf(“\nBINGO!”);} }
test condition
Decision
structure
Set to true

19. The do-while Repetition Structure
Similar to the while structure
Condition for repetition tested after the body of the loop is performed
All actions are performed at least once
Expression can be written as count-controlled or sentinel-controlled
Format:
do {
statement;
} while ( condition );

20. Flowchart of a do-while structure
true
false
action(s)‏
condition

21. The do-while Repetition Structure
Example : prints the integers from 1 to 10
int counter = 1;
do {
printf( "%d ", counter );
} while (++counter <= 10);
Another example:
do{
printf(“\nEnter grade point:”);
scanf(“%f”, &grade_pt);
if(grade_pt > 2.0)‏
printf(“\nPass);
printf(“Continue?y-yes n-no :”);
scanf(“%c”, &choice);
}while ( choice == ‘y’)‏
counter-controlled
sentinel-controlled

22. The do-while Repetition Structure
To avoid an infinite loop, make sure that the loop body contains a statement that ultimately makes the expression false and assures that it exits
Another example:
int i = 0; do {
printf(“%d\t”,i);
i = i + 5;
} while (i <= 20);
The output is:

23. The do-while Looping Structure (Example)‏
Example: Consider the following two loops a)
i=11;
while(i<=10); {
printf(“%d”,i);
i=i+5; } b)
i=11; do {
printf(“%d”,i);
i=i+5;
} while(i<=10);

24. The do-while Looping Structure (Example)‏
In (a), the while loop, produces nothing
In (b) the do...while loop, outputs the number 11

25. The for Repetition Structure
Format when using for loops
for ( initialization; loopContinuationTest; increment statement)‏
Use for loops when already know how many times to repeat
Example:
for(counter = 1; counter <= 10;counter++ )‏
printf( "%d\n", counter );
Prints the integers from one to ten

26. The for Repetition Structure
For loops can usually be rewritten as while loops:
initialization; while ( loopContinuationTest ) { statement; increment statement; }
Initialization and increment
Can be comma-separated lists
Example:
for (int i = 0, j = 0; j + i <= 10; j++, i++)‏
printf( "%d\n", j + i );

27. Flow of a for statement

28. Nested loop Loop within a loop Inner loop is performed first E.g.
for(i=1;i<4;i++){
for(j=1;j<5;j++)‏
printf(“%d”, j);
printf(“\n”);}
Output
1234

29. Nested loop Another example:
Program find and print avg of three test scores, then asks whether want to continue
do{
for(count=0; count <3;count++)‏
{ printf(“\nEnter test marks:”);
scanf(“%d”, marks);
total=total+marks; }
avg=total/count;
printf(“\nAverage:%5.2f”, avg);
printf(“\nContinue?”);
scanf(“%c”, &choice);
}while(choice == ‘y’);

30. End Week 4- Loops
Q & A!