C++ Nested Loops: Patterns, Examples, and Tracing

1. Learning Objectives

By the end of this lecture, students will be able to:

• Understand nested loops.
• Explain the relationship between outer and inner loops.
• Trace nested loops step by step.
• Create star and number patterns.
• Use break to terminate a loop.
• Use continue to skip an iteration.

2. What Is a Nested Loop?

A nested loop is a loop placed inside another loop.

Think of a classroom with 3 rows of students, with 4 students in each row. The outer loop represents the rows, while the inner loop represents the students in each row.

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The outer loop runs once for each row. For every outer-loop iteration, the inner loop completes all its iterations.

3. Example 1: Print a Rectangle of Stars

#include <iostream>
using namespace std;

int main()
{
    for (int i = 1; i <= 3; i++)
    {
        for (int j = 1; j <= 4; j++)
        {
            cout << "* ";
        }

        cout << endl;
    }

    return 0;
}

Output:

* * * *
* * * *
* * * *

Understand the Code

• The outer loop (i) runs 3 times, producing 3 rows.
• The inner loop (j) runs 4 times for every row, printing 4 stars.
• cout << endl; moves to the next line after each row.

Total stars: \(3 \times 4 = 12\)

Trace the Execution

4. Example 2: Right-Angle Triangle of Stars

#include <iostream>
using namespace std;

int main()
{
    for (int i = 1; i <= 5; i++)
    {
        for (int j = 1; j <= i; j++)
        {
            cout << "* ";
        }

        cout << endl;
    }

    return 0;
}

Output:

*
* *
* * *
* * * *
* * * * *

Understand the Code

The key statement is:

for (int j = 1; j <= i; j++)

Unlike the previous example, the inner loop depends on the outer loop's counter i.

• When i = 1, print 1 star.
• When i = 2, print 2 stars.
• When i = 3, print 3 stars.
• Continue until 5 rows are printed.

Modify the Code

1. Print a triangle with 7 rows.
2. Replace * with #.
3. Print numbers instead of stars, as shown below.

1
1 2
1 2 3
1 2 3 4
1 2 3 4 5

5. Example 3: Number Pattern

#include <iostream>
using namespace std;

int main()
{
    for (int i = 1; i <= 4; i++)
    {
        for (int j = 1; j <= i; j++)
        {
            cout << i << " ";
        }

        cout << endl;
    }

    return 0;
}

Output:

1
2 2
3 3 3
4 4 4 4

Understand the Code

Notice the difference between i and j:

• i determines the row number and the value printed.
• j determines how many times that value is printed.

Modify the Code

Produce this pattern:

1 1 1 1
2 2 2 2
3 3 3 3
4 4 4 4

Hint: Keep the inner loop's condition independent of i, so every row contains four numbers.

6. Example 4: Multiplication Tables Using Nested Loops

#include <iostream>
using namespace std;

int main()
{
    for (int i = 1; i <= 3; i++)
    {
        cout << "Table of " << i << endl;

        for (int j = 1; j <= 5; j++)
        {
            cout << i << " x " << j
                 << " = " << i * j << endl;
        }

        cout << endl;
    }

    return 0;
}

Sample output:

Table of 1
1 x 1 = 1
1 x 2 = 2
1 x 3 = 3
1 x 4 = 4
1 x 5 = 5

Table of 2
2 x 1 = 2
2 x 2 = 4
...

Understand the Code

The outer loop selects the table. The inner loop calculates five multiplication results for each table.

Modify the Code

1. Display tables from 1 to 10.
2. Display each table up to 10.
3. Ask the user how many tables to display.

7. The break Statement

The break statement immediately terminates the nearest enclosing loop. Execution continues with the statement after that loop.

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Real-life example: A student is searching a list for a particular student ID. Once the ID is found, there is no need to continue searching.

Example: Stop at Number 5

#include <iostream>
using namespace std;

int main()
{
    for (int i = 1; i <= 10; i++)
    {
        if (i == 5)
        {
            break;
        }

        cout << i << " ";
    }

    return 0;
}

Output:

1 2 3 4

Understand the Code

• The loop starts at 1.
• When i becomes 5, the condition i == 5 is true.
• break terminates the loop immediately.
• The number 5 is not printed because break executes before cout.

Modify the Code

1. Stop the loop when i == 7.
2. Print numbers from 1 to 100, but stop at 15.
3. Ask the user to enter numbers and stop when they enter 0.

8. The continue Statement

The continue statement skips the remaining statements in the current iteration and moves to the next iteration. In a for loop, the update expression still executes.

image

Break and continue – Learn C++

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Example: Skip Number 5

#include <iostream>
using namespace std;

int main()
{
    for (int i = 1; i <= 10; i++)
    {
        if (i == 5)
        {
            continue;
        }

        cout << i << " ";
    }

    return 0;
}

Output:

1 2 3 4 6 7 8 9 10

Understand the Code

When i becomes 5, continue skips the cout statement for that iteration. The loop then moves to the next value, 6.

Modify the Code

1. Skip number 3.
2. Skip all even numbers.
3. Print numbers from 1 to 20, skipping multiples of 5.

9. Difference Between break and continue

Easy way to remember:

• break = Stop.
• continue = Skip and continue.

10. Important Concept: break in Nested Loops

A break inside an inner loop terminates only the inner loop. The outer loop continues normally.

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#include <iostream>
using namespace std;

int main()
{
    for (int i = 1; i <= 3; i++)
    {
        for (int j = 1; j <= 4; j++)
        {
            if (j == 3)
            {
                break;
            }

            cout << j << " ";
        }

        cout << endl;
    }

    return 0;
}

Output:

1 2
1 2
1 2

The inner loop stops whenever j reaches 3. The outer loop still creates all three rows.

11. Coding Exercises

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Basic

1. Rectangle of stars

Use nested loops to print 4 rows, with 6 stars in each row.

Basic

2. Number triangle

Print a triangle where row 1 contains 1, row 2 contains 1 2, and so on up to row 6.

Intermediate

3. Multiplication tables

Print the multiplication tables from 2 to 5, each up to 10.

Intermediate

4. Skip multiples

Print numbers from 1 to 50 but use continue to skip multiples of 3.

Intermediate

5. Stop at a value

Print numbers from 1 to 100 but use break to stop when the counter reaches 25.

Challenge

6. User-defined pattern

Ask the user for the number of rows and print a right-angle triangle of stars.

12. Quick Revision Quiz

1. What does a nested loop mean?

A loop inside another loop

A loop without a condition

A loop that executes only once

2. What does `break` do?

Skips one output

Stops the nearest enclosing loop

Restarts the program

3. What does `continue` do in a `for` loop?

Ends the entire program

Skips the current iteration and proceeds to the next

Repeats the same value forever

4. If the outer loop runs 3 times and the inner loop prints 4 stars each time, how many stars are printed?

7

12

16

Check Answers

13. Lesson Summary

• Nested loops are useful for patterns, tables, and grids.
• The outer loop commonly controls rows, while the inner loop controls columns.
• break stops the nearest enclosing loop.
• continue skips the current iteration.
• Practise tracing the values of both loop counters before writing more complex patterns.

Recommended next lesson: Loop applications and problem-solving — factorials, number sums, digit counting, and menu-driven programs.