C++ Loops: while, for, do-while & Range-based Guide

Module 4: Loops in C++

Loops allow a program to execute the same block of code repeatedly until a specified condition becomes false. They help reduce repetitive code and make programs more efficient.

For example, instead of writing cout << "Hello"; ten times, you can use a loop to print it ten times automatically.

Types of Loops in C++

C++ provides four commonly used loops:

• while
• do...while
• for
• Range-based for

1. while Loop

The while loop executes a block of code as long as the condition is true. The condition is checked before each iteration.

Syntax

cppCopy
1while (condition)
2{
3    // Code to execute
4}

Example

cppCopy
1#include <iostream>
2
3using namespace std;
4
5int main()
6{
7    int i = 1;
8
9    while (i <= 5)
10    {
11        cout << i << " ";
12        i++;
13    }
14
15    return 0;
16}

Output

textCopy
11 2 3 4 5

2. do...while Loop

The do...while loop executes the code block at least once, then checks the condition.

Syntax

cppCopy
1do
2{
3    // Code
4}
5while (condition);

Example

cppCopy
1#include <iostream>
2
3using namespace std;
4
5int main()
6{
7    int i = 1;
8
9    do
10    {
11        cout << i << " ";
12        i++;
13    }
14    while (i <= 5);
15
16    return 0;
17}

Output

textCopy
11 2 3 4 5

Difference Between while and do...while

3. for Loop

The for loop is commonly used when the number of iterations is known.

Syntax

cppCopy
1for (initialization; condition; update)
2{
3    // Code
4}

Example

cppCopy
1#include <iostream>
2
3using namespace std;
4
5int main()
6{
7    for (int i = 1; i <= 5; i++)
8    {
9        cout << i << " ";
10    }
11
12    return 0;
13}

Output

textCopy
11 2 3 4 5

4. Range-based for Loop

The range-based for loop is used to iterate through all elements of an array or container.

Syntax

cppCopy
1for (dataType variable : container)
2{
3    // Code
4}

Example

cppCopy
1#include <iostream>
2
3using namespace std;
4
5int main()
6{
7    int numbers[] = {10, 20, 30, 40, 50};
8
9    for (int number : numbers)
10    {
11        cout << number << " ";
12    }
13
14    return 0;
15}

Output

textCopy
110 20 30 40 50

5. Nested Loops

A nested loop is a loop inside another loop. It is useful for working with tables, matrices, and pattern printing.

Example

cppCopy
1#include <iostream>
2
3using namespace std;
4
5int main()
6{
7    for (int i = 1; i <= 3; i++)
8    {
9        for (int j = 1; j <= 3; j++)
10        {
11            cout << "* ";
12        }
13
14        cout << endl;
15    }
16
17    return 0;
18}

Output

textCopy
1* * *
2* * *
3* * *

Pattern Printing

Pattern 1: Square

cppCopy
1#include <iostream>
2
3using namespace std;
4
5int main()
6{
7    for (int i = 1; i <= 4; i++)
8    {
9        for (int j = 1; j <= 4; j++)
10        {
11            cout << "* ";
12        }
13
14        cout << endl;
15    }
16
17    return 0;
18}

Output

textCopy
1* * * *
2* * * *
3* * * *
4* * * *

Pattern 2: Right Triangle

cppCopy
1#include <iostream>
2
3using namespace std;
4
5int main()
6{
7    for (int i = 1; i <= 5; i++)
8    {
9        for (int j = 1; j <= i; j++)
10        {
11            cout << "* ";
12        }
13
14        cout << endl;
15    }
16
17    return 0;
18}

Output

textCopy
1*
2* *
3* * *
4* * * *
5* * * * *

Pattern 3: Number Triangle

cppCopy
1#include <iostream>
2
3using namespace std;
4
5int main()
6{
7    for (int i = 1; i <= 5; i++)
8    {
9        for (int j = 1; j <= i; j++)
10        {
11            cout << j << " ";
12        }
13
14        cout << endl;
15    }
16
17    return 0;
18}

Output

textCopy
11
21 2
31 2 3
41 2 3 4
51 2 3 4 5

Best Practices

• Choose the appropriate loop for your task.
• Ensure the loop condition eventually becomes false.
• Avoid unnecessary nested loops for better performance.
• Use meaningful loop variable names.
• Keep loop bodies simple and readable.

Common Mistakes

Infinite Loop

cppCopy
1while (true)
2{
3    cout << "Hello";
4}

This loop never ends unless explicitly terminated with break or another exit condition.

Missing Update Statement

cppCopy
1int i = 1;
2
3while (i <= 5)
4{
5    cout << i;
6}

This creates an infinite loop because i is never incremented.

Correct version:

cppCopy
1int i = 1;
2
3while (i <= 5)
4{
5    cout << i;
6    i++;
7}

Practice Exercises

1. Print numbers from 1 to 100 using a while loop.
2. Display the multiplication table of a number using a for loop.
3. Print all even numbers between 1 and 50.
4. Calculate the sum of the first 100 natural numbers.
5. Print a pyramid pattern using nested loops.
6. Traverse an array using a range-based for loop.

Interview Questions

1. What is a loop in C++?
2. What is the difference between while and do...while?
3. When should you use a for loop?
4. What is a range-based for loop?
5. What is a nested loop?
6. What causes an infinite loop?
7. How can you avoid infinite loops?

Summary

In this module, you learned:

• How the while loop works.
• Why the do...while loop always executes at least once.
• How to use the for loop for fixed iterations.
• How the range-based for loop simplifies iterating over arrays and containers.
• How nested loops are used for tables and pattern printing.
• How to create simple star and number patterns.
• Common loop-related mistakes and best practices.

In the next module, you'll learn Functions in C++, including function declaration, definition, parameters, return values, recursion, function overloading, inline functions, default arguments, and lambda expressions.