1. Introduction to Computing Lecture 06: Repetition and Loop Statements (Part I)
Assist.Prof.Dr. Nükhet ÖZBEK
Ege University
Department of Electrical & Electronics Engineering

2. Topics Repetition in programs while statement do while statement
for statement

3. Repetition in programs
So far, the statements in the program body executes only once
However, sometimes it is necessary to repeat a process many times
Repetition steps in a program is called a loop

4. Repetition

5. Repetition int a1,a2,a3, … a50, sum; scanf(“%d”,&a1); scanf(“%d”,&a2);
sum = a1 + a2 + a3 + … + a50;
This is not feasible!

6. Repetition int a, sum=0; scanf(“%d”,&a); sum = sum + a;…
This is not feasible either!

7. The while statement Implements the repetition in an algorithm
Repeatedly executes a block of statements
Tests a condition (Boolean expression) at the start of each iteration
Terminates when condition becomes false (zero)

8. Example: addnum.c Read in numbers, add them, and
print their sum and average
set sum to 0
set count to 0
input totalNumbers
while (count < totalNumbers) {
input nextNum
add nextNum to sum
add 1 to count }
output "Sum was" sum
output "Mean was" sum/count

9. Example: addnum.c (cont)
Read in numbers, add them, and
print their sum and average
set sum to 0
set count to 0
input totalNumbers
while (count < totalNumbers) {
input nextNum
add nextNum to sum
add 1 to count }
output "Sum was" sum
output "Mean was" sum/count
Iteration Control
Initialize
Check condition
Update

10. Example: addnum.c (cont)
#include <stdio.h>
/**********************************\
Read in numbers and add them up
Print out the sum and the average
\**********************************/
int main() {
return 0; }
Read in numbers, add them, and
print their sum and average
set sum to 0
set count to 0
input totalNumbers
while (count < totalNumbers) {
input nextNum
add nextNum to sum
add 1 to count }
output "Sum was" sum
output "Mean was" sum/count

11. Example: addnum.c (cont)
#include <stdio.h>
/**********************************\
Read in numbers and add them up
Print out the sum and the average
\**********************************/
int main() {
float nextNum, sum = 0.0;
int count = 0, totalNumbers;
return 0; }
Read in numbers, add them, and
print their sum and average
set sum to 0
set count to 0
input totalNumbers
while (count < totalNumbers) {
input nextNum
add nextNum to sum
add 1 to count }
output "Sum was" sum
output "Mean was" sum/count
only the variables sum and count are initialized to 0

12. Example: addnum.c (cont)
#include <stdio.h>
/**********************************\
Read in numbers and add them up
Print out the sum and the average
\**********************************/
int main() {
float nextNum, sum = 0.0;
int count = 0, totalNumbers;
scanf("%d", &totalNumbers);
return 0; } .
Read in numbers, add them, and
print their sum and average
set sum to 0
set count to 0
input totalNumbers
while (count < totalNumbers) {
input nextNum
add nextNum to sum
add 1 to count }
output "Sum was" sum
output "Mean was" sum/count

13. Example: addnum.c (cont)
#include <stdio.h>
/**********************************\
Read in numbers and add them up
Print out the sum and the average
\**********************************/
int main() {
float nextNum, sum = 0.0;
int count = 0, totalNumbers;
scanf("%d", &totalNumbers);
while (count < totalNumbers) }
return 0;
Read in numbers, add them, and
print their sum and average
set sum to 0
set count to 0
input totalNumbers
while (count < totalNumbers) {
input nextNum
add nextNum to sum
add 1 to count }
output "Sum was" sum
output "Mean was" sum/count

14. Example: addnum.c (cont)
#include <stdio.h>
/**********************************\
Read in numbers and add them up
Print out the sum and the average
\**********************************/
int main() {
float nextNum, sum = 0.0;
int count = 0, totalNumbers;
scanf("%d", &totalNumbers);
while (count < totalNumbers)
scanf("%f", &nextNum);
sum += nextNum;
count++; }
return 0;
Read in numbers, add them, and
print their sum and average
set sum to 0
set count to 0
input totalNumbers
while (count < totalNumbers) {
input nextNum
add nextNum to sum
add 1 to count }
output "Sum was" sum
output "Mean was" sum/count
Same as: count = count + 1;
Decrement: count --;
Same as: sum = sum + nextNum;
Others: -=, *=, /=, etc.

15. Example: addnum.c (cont)
#include <stdio.h>
/**********************************\
Read in numbers and add them up
Print out the sum and the average
\**********************************/
int main() {
float nextNum, sum = 0.0;
int count = 0, totalNumbers;
scanf("%d", &totalNumbers);
while (count < totalNumbers)
scanf("%f", &nextNum);
sum += nextNum;
count++; }
printf("Sum was %f\n",sum);
printf("Mean was %f\n",sum/count);
return 0;
Read in numbers, add them, and
print their sum and average
set sum to 0
set count to 0
input totalNumbers
while (count < totalNumbers) {
input nextNum
add nextNum to sum
add 1 to count }
output "Sum was" sum
output "Mean was" sum/count

16. Example: addnum.c (cont)
#include <stdio.h>
/**********************************\
Read in numbers and add them up
Print out the sum and the average
\**********************************/
int main() {
float nextNum, sum = 0.0;
int count = 0, totalNumbers;
scanf("%d", &totalNumbers);
while (count < totalNumbers)
scanf("%f", &nextNum);
sum += nextNum;
count++; }
printf("Sum was %f\n",sum);
printf("Mean was %f\n",sum/count);
return 0;
Read in numbers, add them, and
print their sum and average
set sum to 0
set count to 0
input totalNumbers
while (count < totalNumbers) {
input nextNum
add nextNum to sum
add 1 to count }
output "Sum was" sum
output "Mean was" sum/count

17. Example: addnum.c (cont)
#include <stdio.h>
/**********************************\
Read in numbers and add them up
Print out the sum and the average
\**********************************/
int main() {
float nextNum, sum = 0.0;
int count = 0, totalNumbers;
scanf("%d", &totalNumbers);
while (count < totalNumbers)
scanf("%f", &nextNum);
sum += nextNum;
count++; }
printf("Sum was %f\n",sum);
printf("Mean was %f\n",sum/count);
return 0;
Example: addnum.c (cont)
totalNumbers
count
nextNum
sum
????
0.0

18. Example: addnum.c (cont)
#include <stdio.h>
/**********************************\
Read in numbers and add them up
Print out the sum and the average
\**********************************/
int main() {
float nextNum, sum = 0.0;
int count = 0, totalNumbers;
scanf("%d", &totalNumbers);
while (count < totalNumbers)
scanf("%f", &nextNum);
sum += nextNum;
count++; }
printf("Sum was %f\n",sum);
printf("Mean was %f\n",sum/count);
return 0;
Example: addnum.c (cont)
totalNumbers
count
nextNum
sum
????
0.0 3

19. Example: addnum.c (cont)
#include <stdio.h>
/**********************************\
Read in numbers and add them up
Print out the sum and the average
\**********************************/
int main() {
float nextNum, sum = 0.0;
int count = 0, totalNumbers;
scanf("%d", &totalNumbers);
while (count < totalNumbers)
scanf("%f", &nextNum);
sum += nextNum;
count++; }
printf("Sum was %f\n",sum);
printf("Mean was %f\n",sum/count);
return 0;
Example: addnum.c (cont)
totalNumbers
count
nextNum
sum
????
0.0 3 4 1
4.0

20. Example: addnum.c (cont)
#include <stdio.h>
/**********************************\
Read in numbers and add them up
Print out the sum and the average
\**********************************/
int main() {
float nextNum, sum = 0.0;
int count = 0, totalNumbers;
scanf("%d", &totalNumbers);
while (count < totalNumbers)
scanf("%f", &nextNum);
sum += nextNum;
count++; }
printf("Sum was %f\n",sum);
printf("Mean was %f\n",sum/count);
return 0;
Example: addnum.c (cont)
totalNumbers
count
nextNum
sum
????
0.0 3 4 -1 1
4.0 2
3.0

21. Example: addnum.c (cont)
#include <stdio.h>
/**********************************\
Read in numbers and add them up
Print out the sum and the average
\**********************************/
int main() {
float nextNum, sum = 0.0;
int count = 0, totalNumbers;
scanf("%d", &totalNumbers);
while (count < totalNumbers)
scanf("%f", &nextNum);
sum += nextNum;
count++; }
printf("Sum was %f\n",sum);
printf("Mean was %f\n",sum/count);
return 0;
Example: addnum.c (cont)
totalNumbers
count
nextNum
sum
????
0.0 3 4 -1
6.2 1
4.0 2
3.0 3
9.2

22. Syntax for while statement
while (loop repetition condition)
statement; OR {
statement1;
statement2; …
statementn; }

23. Common Mistakes in while -- “one liners”
while (num < minimum)
scanf(“%d”, &num);
printf(“Number must be greater than %d.\n”, minimum);
printf(“Please try again.\n”);
while (num < minimum) {
scanf(“%d”, &num); }
printf(“Number must be greater than %d.\n”, minimum);
printf(“Please try again.\n”);

24. Common Mistakes in while -- “one liners” (cont)
while (num < minimum)
scanf(“%d”, &num);
printf(“Number must be greater than %d.\n”, minimum);
printf(“Please try again.\n”);
while (num < minimum) {
scanf(“%d”, &num);
printf(“Number must be greater than %d.\n”, minimum);
printf(“Please try again.\n”); }

25. Common Mistakes in while -- extra semi-colon;
while (num < minimum); {
scanf(“%d”, &num);
printf(“Number must be greater than %d.\n”, minimum);
printf(“Please try again.\n”); }
Marks the end of the while-block -- usual cause of infinite loops

26. Common Mistakes in while
Loop control variable is not updated. “Infinite loop”
while (count < totalNumbers) {
scanf("%f", &nextNum);
sum += nextNum; }
while (count < totalNumbers) {
scanf("%f", &nextNum);
sum += nextNum;
count++; }

27. do … while Implements a post-tested loop do { printf(“Main menu:\n”);
printf(“1. Account balance.\n”);
printf(“2. Withdraw cash.\n”);
printf(“3. Transfer funds.\n\n”);
printf(“9. Exit\n\n”);
printf(“Please enter your choice: “);
scanf(“%d”, &choice);
/* Insert selection instructions here. */
} while ( choice != 9 );

28. do … while (cont) do { printf(“Main menu:\n”);
printf(“1. Account balance.\n”);
printf(“2. Withdraw cash.\n”);
printf(“3. Transfer funds.\n\n”);
printf(“9. Exit\n\n”);
printf(“Please enter your choice: “);
scanf(“%d”, &choice);
/* Insert selection instructions here. */
} while ( choice != 9 );

29. Syntax for do … while statement{
statement1;
statement2; …
statementn;
} while (loop repetition condition);

30. Notes on do … while
do … while is executed at least one

31. The for statement
Form of pre-tested loop which allows for initialization and iteration control
Syntax:
for (initialization; condition; update ) {
instruction block }
Careful! A semi-colon here marks the end of the instruction block!

32. Example: addfor.c Read in numbers, add them, and
print the sum and the average
set sum to 0
set count to 0
input totalNumbers
while (count < totalNumbers) {
input nextNum
add nextNum to sum
add 1 to count }
output "Sum was" sum
output "Mean was" sum/count

33. Example: addfor.c (cont)
#include <stdio.h>
/**********************************\
Read in numbers and add them up
Print out the sum and the average
\**********************************/
int main() {
float nextNum, sum = 0.0;
int count, totalNumbers;
scanf("%d", &totalNumbers);
for ( count=0;
count < totalNumbers;
count++ )
scanf("%f", &nextNum);
sum += nextNum; }
printf("Sum was %f\n",sum);
printf("Mean was %f\n",sum/count);
return 0;
Example: addfor.c (cont)
Read in numbers, add them, and
print the sum and the average
set sum to 0
set count to 0
input totalNumbers
while (count < totalNumbers) {
input nextNum
add nextNum to sum
add 1 to count }
output "Sum was" sum
output "Mean was" sum/count

34. Initialize Example: addfor.c (cont) Read in numbers, add them, and
#include <stdio.h>
/**********************************\
Read in numbers and add them up
Print out the sum and the average
\**********************************/
int main() {
float nextNum, sum = 0.0;
int count, totalNumbers;
scanf("%d", &totalNumbers);
for ( count=0;
count < totalNumbers;
count++ )
scanf("%f", &nextNum);
sum += nextNum; }
printf("Sum was %f\n",sum);
printf("Mean was %f\n",sum/count);
return 0;
Read in numbers, add them, and
print the sum and the average
set sum to 0
set count to 0
input totalNumbers
while (count < totalNumbers) {
input nextNum
add nextNum to sum
add 1 to count }
output "Sum was" sum
output "Mean was" sum/count
Initialize

35. Check condition Example: addfor.c (cont)
#include <stdio.h>
/**********************************\
Read in numbers and add them up
Print out the sum and the average
\**********************************/
int main() {
float nextNum, sum = 0.0;
int count, totalNumbers;
scanf("%d", &totalNumbers);
for ( count=0;
count < totalNumbers;
count++ )
scanf("%f", &nextNum);
sum += nextNum; }
printf("Sum was %f\n",sum);
printf("Mean was %f\n",sum/count);
return 0;
Read in numbers, add them, and
print the sum and the average
set sum to 0
set count to 0
input totalNumbers
while (count < totalNumbers) {
input nextNum
add nextNum to sum
add 1 to count }
output "Sum was" sum
output "Mean was" sum/count
Check condition

36. The Update is performed AFTER the body of the loop
#include <stdio.h>
/**********************************\
Read in numbers and add them up
Print out the sum and the average
\**********************************/
int main() {
float nextNum, sum = 0.0;
int count, totalNumbers;
scanf("%d", &totalNumbers);
for ( count=0;
count < totalNumbers;
count++ )
scanf("%f", &nextNum);
sum += nextNum; }
printf("Sum was %f\n",sum);
printf("Mean was %f\n",sum/count);
return 0;
Example: addfor.c (cont)
Read in numbers, add them, and
print the sum and the average
set sum to 0
set count to 0
input totalNumbers
while (count < totalNumbers) {
input nextNum
add nextNum to sum
add 1 to count }
output "Sum was" sum
output "Mean was" sum/count
Update (aka Increment Step)
IMPORTANT!!
The Update is performed AFTER
the body of the loop

37. Example: addfor.c (cont)
#include <stdio.h>
/**********************************\
Read in numbers and add them up
Print out the sum and the average
\**********************************/
int main() {
float nextNum, sum = 0.0;
int count, totalNumbers;
scanf("%d", &totalNumbers);
for ( count=0;
count < totalNumbers;
count++ )
scanf("%f", &nextNum);
sum += nextNum; }
printf("Sum was %f\n",sum);
printf("Mean was %f\n",sum/count);
return 0;
Example: addfor.c (cont)
Read in numbers, add them, and
print the sum and the average
set sum to 0
set count to 0
input totalNumbers
while (count < totalNumbers) {
input nextNum
add nextNum to sum
add 1 to count }
output "Sum was" sum
output "Mean was" sum/count

38. Notes on for
What is the value of x when the following statement is complete?
for (x = 0; x < 100; x++) ;
What is the value of ctr when the following statement is complete?
for (ctr = 2; ctr < 10; ctr += 3) ;

39. Notes on for and while
An equivalent for statement for a while statement can be written and vice versa

40. while and for #include <stdio.h> int main() {
float nextNum, sum = 0.0;
int count, totalNumbers;
scanf("%d", &totalNumbers);
count = 0;
while (count < totalNumbers)
scanf("%f", &nextNum);
sum += nextNum;
count++; }
printf("Sum was %f\n",sum);
printf("Mean was %f\n",
sum/count);
return 0;
#include <stdio.h>
int main() {
float nextNum, sum = 0.0;
int count, totalNumbers;
scanf("%d", &totalNumbers);
for ( count=0;
count < totalNumbers;
count++ )
scanf("%f", &nextNum);
sum += nextNum; }
printf("Sum was %f\n",sum);
printf("Mean was %f\n",
sum/count);
return 0;

41. while and for (cont) Check condition Initialize
#include <stdio.h>
int main() {
float nextNum, sum = 0.0;
int count, totalNumbers;
scanf("%d", &totalNumbers);
count = 0;
while (count < totalNumbers)
scanf("%f", &nextNum);
sum += nextNum;
count++; }
printf("Sum was %f\n",sum);
printf("Mean was %f\n",
sum/count);
return 0;
#include <stdio.h>
int main() {
float nextNum, sum = 0.0;
int count, totalNumbers;
scanf("%d", &totalNumbers);
for ( count=0;
count < totalNumbers;
count++ )
scanf("%f", &nextNum);
sum += nextNum; }
printf("Sum was %f\n",sum);
printf("Mean was %f\n",
sum/count);
return 0;
Check condition
Initialize

42. while and for (cont) Check condition #include <stdio.h>
int main() {
float nextNum, sum = 0.0;
int count, totalNumbers;
scanf("%d", &totalNumbers);
count = 0;
while (count < totalNumbers)
scanf("%f", &nextNum);
sum += nextNum;
count++; }
printf("Sum was %f\n",sum);
printf("Mean was %f\n",
sum/count);
return 0;
#include <stdio.h>
int main() {
float nextNum, sum = 0.0;
int count, totalNumbers;
scanf("%d", &totalNumbers);
for ( count=0;
count < totalNumbers;
count++ )
scanf("%f", &nextNum);
sum += nextNum; }
printf("Sum was %f\n",sum);
printf("Mean was %f\n",
sum/count);
return 0;
Check condition

43. while and for (cont) Update #include <stdio.h> int main() {
float nextNum, sum = 0.0;
int count, totalNumbers;
scanf("%d", &totalNumbers);
count = 0;
while (count < totalNumbers)
scanf("%f", &nextNum);
sum += nextNum;
count++; }
printf("Sum was %f\n",sum);
printf("Mean was %f\n",
sum/count);
return 0;
#include <stdio.h>
int main() {
float nextNum, sum = 0.0;
int count, totalNumbers;
scanf("%d", &totalNumbers);
for ( count=0;
count < totalNumbers;
count++ )
scanf("%f", &nextNum);
sum += nextNum; }
printf("Sum was %f\n",sum);
printf("Mean was %f\n",
sum/count);
return 0;
Update