Nested Loops

A nested loop means putting one loop inside another. In C, you can use for, while, or do-while loops inside each other to perform repeated tasks within a repeated task.

Syntax:

for (int i = 1; i <= outer_limit; i++) {
    for (int j = 1; j <= inner_limit; j++) {
        // inner loop code
    }
    // outer loop code
}

Example: Print Rectangle of Stars

#include <stdio.h>

int main() {
    int rows = 3, cols = 5;
    for (int i = 1; i <= rows; i++) {
        for (int j = 1; j <= cols; j++) {
            printf("*");
        }
        printf("\n");
    }
    return 0;
}

Output:

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

##Example: Multiplication Table

Let's create a multiplication table using nested loops:

#include <stdio.h>

int main() {
    printf("Multiplication Table (1-5):\n");
    
    for (int i = 1; i <= 5; i++) {        // Outer loop
        for (int j = 1; j <= 5; j++) {    // Inner loop
            printf("%d x %d = %d\t", i, j, i * j);
        }
        printf("\n");  // New line after each row
    }
    
    return 0;
}

Output:

1 x 1 = 1    1 x 2 = 2    1 x 3 = 3    1 x 4 = 4    1 x 5 = 5
2 x 1 = 2    2 x 2 = 4    2 x 3 = 6    2 x 4 = 8    2 x 5 = 10
3 x 1 = 3    3 x 2 = 6    3 x 3 = 9    3 x 4 = 12   3 x 5 = 15
4 x 1 = 4    4 x 2 = 8    4 x 3 = 12   4 x 4 = 16   4 x 5 = 20
5 x 1 = 5    5 x 2 = 10   5 x 3 = 15   5 x 4 = 20   5 x 5 = 25

How It Works

1. Outer loop starts with i = 1
2. Inner loop runs completely from j = 1 to j = 5
3. Outer loop moves to i = 2
4. Inner loop runs completely again from j = 1 to j = 5
5. This continues until the outer loop finishes

Pattern Printing Example

Nested loops are perfect for creating patterns:

#include <stdio.h>

int main() {
    int rows = 5;
    
    // Print a right triangle of stars
    for (int i = 1; i <= rows; i++) {        // Outer loop for rows
        for (int j = 1; j <= i; j++) {       // Inner loop for columns
            printf("* ");
        }
        printf("\n");
    }
    
    return 0;
}

Output:

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

Different Loop Combinations

You can mix different types of loops:

#include <stdio.h>

int main() {
    int i = 1;
    
    while (i <= 3) {                    // Outer while loop
        printf("Outer loop: %d\n", i);
        
        for (int j = 1; j <= 2; j++) {  // Inner for loop
            printf("  Inner loop: %d\n", j);
        }
        
        i++;
    }
    
    return 0;
}

Key Points to Remember

Aspect	Description
Execution Order	Inner loop completes fully before outer loop advances
Total Iterations	Outer iterations × Inner iterations
Performance	Can be expensive with large datasets
Use Cases	2D arrays, patterns, tables, matrix operations

Common Pitfalls

• Infinite loops: Make sure both loops have proper termination conditions
• Variable scope: Inner loop variables are only accessible within the inner loop
• Performance: Nested loops can be slow with large numbers of iterations

Think of nested loops like reading a book with chapters and pages - you read all pages in chapter 1, then all pages in chapter 2, and so on. The outer loop is the chapters, and the inner loop is the pages!