C Nested Loops

Nested loops in C are loops inside other loops. They are useful for performing tasks that require multiple iterations for each iteration of the outer loop, such as working with multi-dimensional arrays or creating patterns.

Syntax of Nested Loops

A nested loop has the following structure:

for (initialization; condition; increment) {
    // Outer loop code

    for (initialization; condition; increment) {
        // Inner loop code
    }

    // Outer loop continues
}

You can also have other types of loops (like while or do...while) nested inside for loops and vice versa.

Example 1: Nested for Loops

Here’s an example of using two for loops to print a 2D grid:

#include <stdio.h>

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

    return 0;
}

Output:

i = 1, j = 1
i = 1, j = 2
i = 1, j = 3
i = 2, j = 1
i = 2, j = 2
i = 2, j = 3
i = 3, j = 1
i = 3, j = 2
i = 3, j = 3

In this example:

• The outer loop (i) runs 3 times.
• For each iteration of the outer loop, the inner loop (j) runs 3 times.
• This results in 9 total iterations (3 outer iterations × 3 inner iterations).

Example 2: Printing a Multiplication Table (2D Array)

A common use of nested loops is to print multiplication tables.

#include <stdio.h>

int main() {
    for (int i = 1; i <= 5; i++) {  // Outer loop for rows
        for (int j = 1; j <= 5; j++) {  // Inner loop for columns
            printf("%d\t", i * j);  // Print multiplication of i and j
        }
        printf("\n");  // Move to the next line after each row
    }

    return 0;
}

Output:

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

Here:

• The outer loop controls the rows (from 1 to 5).
• The inner loop controls the columns (from 1 to 5).
• For each combination of i and j, the multiplication result is printed.

Example 3: Nested while Loops

You can nest while loops inside each other as well. Here’s an example that prints a pattern using nested while loops.

#include <stdio.h>

int main() {
    int i = 1;

    while (i <= 3) {  // Outer loop
        int j = 1;
        
        while (j <= 3) {  // Inner loop
            printf("* ");
            j++;
        }

        printf("\n");
        i++;
    }

    return 0;
}

Output:

* * * 
* * * 
* * * 

Here:

• The outer while loop runs 3 times.
• The inner while loop also runs 3 times for each iteration of the outer loop, printing * each time.

Example 4: Nested do...while Loops

You can also nest do...while loops. Here’s an example of printing a pattern with nested do...while loops:

#include <stdio.h>

int main() {
    int i = 1;

    do {  // Outer loop
        int j = 1;
        
        do {  // Inner loop
            printf("* ");
            j++;
        } while (j <= 3);

        printf("\n");
        i++;
    } while (i <= 3);

    return 0;
}

Output:

* * * 
* * * 
* * * 

This example is similar to the previous one, but it uses do...while loops instead of while loops. The outer loop runs 3 times, and the inner loop also runs 3 times for each iteration of the outer loop.

Example 5: Using Nested Loops for 2D Arrays

Nested loops are often used to process elements of 2D arrays (arrays of arrays). Here’s an example where we declare a 2D array and print its elements:

#include <stdio.h>

int main() {
    int arr[3][3] = {
        {1, 2, 3},
        {4, 5, 6},
        {7, 8, 9}
    };

    for (int i = 0; i < 3; i++) {  // Outer loop for rows
        for (int j = 0; j < 3; j++) {  // Inner loop for columns
            printf("%d ", arr[i][j]);  // Accessing elements of the array
        }
        printf("\n");
    }

    return 0;
}

Output:

1 2 3 
4 5 6 
7 8 9 

In this example:

• The outer loop iterates over the rows of the 2D array.
• The inner loop iterates over the columns of each row.
• We access each element of the array using arr[i][j].

Example 6: Using break and continue in Nested Loops

You can use break and continue within nested loops to control their flow. Here’s an example that prints a multiplication table but skips the iteration when i * j equals 12:

#include <stdio.h>

int main() {
    for (int i = 1; i <= 5; i++) {
        for (int j = 1; j <= 5; j++) {
            if (i * j == 12) {
                continue;  // Skip the iteration when the product is 12
            }
            printf("%d\t", i * j);
        }
        printf("\n");
    }

    return 0;
}

Output:

1	2	3	4	5	
2	4	6	8	10	
3	6	9	11	15	
4	8	12	16	20	
5	10	15	20	25	

In this example:

• The continue statement skips the printing of 12 when i * j equals 12.

Summary

• Nested loops are loops placed inside another loop, and they are commonly used when dealing with multi-dimensional arrays or when you need to perform repetitive tasks that involve multiple variables.
• You can nest for, while, and do...while loops within each other.
• The flow of nested loops can be controlled using break and continue.
• Nested loops are a powerful tool but should be used carefully to avoid unnecessarily complex or inefficient code, especially for large datasets.