Loop constructs allow sections of code to repeat a specified number of times. There are three main types of loops in C++: 1. For loops use a counter variable that is initialized, tested against a condition on each iteration, and incremented/decremented until false. 2. While loops continuously execute a block of code as long as a condition is true. 3. Do-while loops are similar but execute the block of code once before checking the condition, ensuring it runs at least once.

1. Loop constructs
Department of Software Engineering,
Capital University of Science and Technology, Islamabad

2. 2
Loops
• Loops cause a section of your program to be
repeated a certain number of times
• loops are used to repeat a block of code.
• Repeats until the condition remains true
• Terminates when the condition becomes
false
• For example, let's say we want to show a
message 100 times. Then instead of writing the
print statement 100 times, we can use a loop.

3. 3
Loops in C++
1. for loop
2. while loop
3. do while loop

4. Loops
Counter-controlled Loops
• Depends on the value of a variable known as counter
variable. The value of the variable is incremented or
decremented in each iteration.
Example: for loop
Sentinel-Controlled Loops / Conditional loop
• A loop that terminates when something happens inside
the loop body indicating that loop should be exited. Also
known as conditional loops
• OR
• A condition controlled loop is programming structure
that. causes a statement or set of statements to repeat as
long as a. condition evaluates to True.
Example: while and do while loops

5. for loop - Syntax
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6. 6
Example: for loop - Syntax
for (int j=0; j<10; j++)
cout << j * j <<endl;
Initialization
expression
Test Condition
Increment expression
cout << j*2 <<endl;
cout << j*j*j <<endl;

7. for loop – Flow Chart
7

8. Example 1: for loop
Printing Numbers From 1 to 5
#include <iostream>
using namespace std;
int main()
{
for (int i = 1; i <= 5; ++i)
{
cout << i << " ";
}
return 0;
}
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9. Example 2: for loop
// C++ Program to display a
text 5 times
#include <iostream>
using namespace std;
int main()
{
for (int i = 1; i <= 5; ++i)
{
cout << "Hello World! " <<
endl;
}
return 0;
}
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10. Example 3: for loop
Find the sum of first n Natural Numbers
#include <iostream>
using namespace std;
int main()
{
int num, sum;
sum = 0;
cout << "Enter a positive integer: ";
cin >> num;
for (int i = 1; i <= num; ++i)
{
sum += i;
}
cout << "Sum = " << sum << endl;
return 0;
}
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11. 11
Example 4: for loop
#include <iostream>
using namespace std;
int main()
{
int j;
for (j=0; j<10; j++)
cout << j * j <<endl;
return 0;
}

12. 12
Example 5: for loop
- Get a number from user and calculate its factorial:
For any positive number n, it's factorial is given by:
• factorial = 1*2*3...*n
• Factorial of negative number cannot be found and
factorial of 0 is 1.
In this program below, user is asked to enter a
positive integer. Then the factorial of that number is
computed and displayed in the screen.

13. Cont…
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14. 14
for loop - Variable Visibility
int main()
{
int j;
for(j=0; j<10; j++) {
int k=0;
k = j*j;
cout<<“nValue of k: “<<k;
}
// k = 23; Cannot do this!
}

15. for loop - Variable Visibility
int main()
{
for(int j=0; j<5; j++)
cout<<“nValue of j: “<<j;
cout<<“nValue of j: “<<j; // ERROR
}
Loop body
15

16. for loop – optional expressions
int j=0;
for(; j<10; j++)
cout<<“nHello world“;
int j=0;
for(; j<10;)
{
cout<<“nHello world“;
j++;
}
for(; ;)
cout<<“nHello world“;
Infinite loop
(it never terminates) 16

17. Infinite for loop
17
If the condition in a for loop is always true, it runs forever (until memory is full).
For example,
In the above program, the condition is always true which will then run the code
for infinite times.

18. While Loop- Syntax
18
• A while loop evaluates the condition
• If the condition evaluates to true, the code inside the while loop is executed.
• The condition is evaluated again.
• This process continues until the condition is false.
• When the condition evaluates to false, the loop terminates.

19. Flowchart of while Loop
19

20. Example 1: Display Numbers from 1
to 5
// C++ Program to print numbers from 1 to 5
#include <iostream>
using namespace std;
int main() {
int i = 1;
// while loop from 1 to 5
while (i <= 5) {
cout << i << " ";
++i;
}
return 0;
}
20

21. Example 2: Sum of Positive Numbers Only
#include <iostream>
using namespace std;
int main() {
int number;
int sum = 0;
// take input from the user
cout << "Enter a number: ";
cin >> number;
while (number >= 0) {
// add all positive numbers
sum += number;
// take input again if the number is positive
cout << "Enter a number: ";
cin >> number;
}
// display the sum
cout << "nThe sum is " << sum << endl;
return 0;
} 21

22. Example 3: Write a program to print the table of 10
using a while loop
22
#include<iostream>
using namespace std;
int main()
{
int t=1;
cout<<"Table of 10"<<endl;
while(t!=11)
{
cout<<"10 X "<<t<<"="<<10*t<<endl;
t=t+1;
}
return(0);
}

23. do...while Loop - Syntax
23
• The body of the loop is executed at first. Then the condition is evaluated.
• If the condition evaluates to true, the body of the loop inside the do statement is
executed again.
• The condition is evaluated once again.
• If the condition evaluates to true, the body of the loop inside the do statement is
executed again.
• This process continues until the condition evaluates to false. Then the loop
stops.

24. Flowchart of do...while Loop
24

25. Example 4: Display Numbers from 1 to 5
// C++ Program to print numbers from 1 to 5
#include <iostream>
using namespace std;
int main() {
int i = 1;
// do...while loop from 1 to 5
do {
cout << i << " ";
++i;
}
while (i <= 5);
return 0;
}
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26. Example 5: Sum of Positive Numbers Only
#include <iostream>
using namespace std;
int main() {
int number = 0;
int sum = 0;
do {
sum += number;
// take input from the user
cout << "Enter a number: ";
cin >> number;
}
while (number >= 0);
// display the sum
cout << "nThe sum is " << sum << endl;
return 0;
}
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27. Infinite while Loop/ Do…While Loop
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