1. Lecture 4 - Loops
UniMAP EKT120 Sem 1 08/09

2. Outline Introduction While loops Do-while loops For loops Nested loops
Three types of while loops
Do-while loops
For loops
Nested loops
UniMAP EKT120 Sem 1 08/09

3. 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?
UniMAP EKT120 Sem 1 08/09

4. 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
UniMAP EKT120 Sem 1 08/09

5. The while loop structure
The general form of the while statement is:
while (expression)
statement;
The example of pseudocode statement for while statement: While there are more items on my shopping list
Purchase next item
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
UniMAP EKT120 Sem 1 08/09

6. Flowchart of a while statement
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7. 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
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8. 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)
UniMAP EKT120 Sem 1 08/09

9. 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
UniMAP EKT120 Sem 1 08/09

10. 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“
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11. Sentinel-Controlled while Loops-cont
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);
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
Sentinel value
UniMAP EKT120 Sem 1 08/09

12. 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
UniMAP EKT120 Sem 1 08/09

13. Flag-Controlled while Loops
Uses a boolean variable to control the loop.
Loop exit when expression is evaluated to false.
Set to false
bool found = false;
while (!found)
{ printf(“Enter number between 1 and 200:”);
scanf(%d”, &guess);
if ((guess>= 88) &&(guess <=128))
{found = true;
printf(“\nBINGO!”);} }
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
test condition
decision
structure
Set to true
UniMAP EKT120 Sem 1 08/09

14. Example (while statement)
(i) Fix any error you may find:
while (c < 5) {
product * = c;
++c;
(ii) What is wrong with the statements which is supposed to calculate the sum of integers from 1 to 100?
while (z > = 0) {
sum + = z;
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15. Example (while statement)
What does the following program print?
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16. Example (while statement)
Write a program that calculates the sum of integer from 1 to 10. Use the while statement.
UniMAP EKT120 Sem 1 08/09

17. UniMAP EKT120 Sem 1 08/09

18. The do-while Repetition 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 );
UniMAP EKT120 Sem 1 08/09

19. Flowchart of a do-while structure
true
false
action(s)
condition
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20. 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’)
count-controlled
sentinel-controlled
UniMAP EKT120 Sem 1 08/09

21. 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 “, i);
i = i + 5;
} while (i <= 20);
The output is:
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22. The do-while Looping Structure (Example)
Example: Consider the following two loops
printf("%d “, i);
printf("%d “, i);
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23. The do-while Looping Structure (Example)
In (a), the while loop, produces nothing
In (b) the do...while loop, outputs the number 11
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24. 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
UniMAP EKT120 Sem 1 08/09

25. The for Repetition Structure
Example: For loops and while loops are equivalent:
expression1; while (expression2 ) { statement; expression3; }
for ( expression1; expression2 ; expression3)
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26. The for Repetition Structure
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 );
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27. Flowchart of a for statement
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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
UniMAP EKT120 Sem 1 08/09

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’);
UniMAP EKT120 Sem 1 08/09

30. Example (for statement)
What is the program output for the following:
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31. Example (for statement)
Write C codes for the following:
Sum the odd integers between 1 and 99 using for statement. Assume the integer variable and sum have been defined.
Write for statement that prints the following values in sequence: a) b)
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32. Example (for statement)
What is the program output if number 3 and 4 are
entered by user?
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33. Example (for statement)
(i) Write a program that calculates and prints the sum of the even integers from 2 to 30
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34. Example (for statement)
(ii) Write a program that reads five integers. For each integer read, your program should print a line containing the amount of symbol *.
UniMAP EKT120 Sem 1 08/09

35. End Lecture 4 - Loops
Q & A!
UniMAP EKT120 Sem 1 08/09