Use of nested for loops involves creating complex, variable-bound patterns, efficiently processing multi-dimensional data structures like matrices, and understanding techniques for controlling the flow across multiple loop levels.

Uses of Nested for Loops in C 🔢

While a basic nested loop creates a simple grid, its  applications are key to solving more complex problems in algorithms and data manipulation.

1. Generating Complex Patterns

The key to creating non-rectangular patterns (like triangles or pyramids) is to make the inner loop's condition dependent on the outer loop's variable.

Example: Printing a Number Pyramid

The outer loop controls the rows. The inner loop, which prints the numbers, runs i times, where i is the current row number.

C

#include <stdio.h>

int main() {

    int rows = 5;

    // Outer loop iterates through each row

    for (int i = 1; i <= rows; i++) {

        // Inner loop prints numbers from 1 up to the current row number 'i'

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

            printf("%d ", j);

        }

        // Move to the next line after the row is printed

        printf("\n");

    }

    return 0;

}

Output:

1

1 2

1 2 3

1 2 3 4

1 2 3 4 5

2. Processing 2D Arrays (Matrices)

Nested for loops are the standard and most efficient way to process every element in a 2D array. The outer loop typically iterates over the rows, and the inner loop iterates over the columns within each row.

 Example: Matrix Transposition

Transposing a matrix means swapping its rows and columns. The element at matrix[i][j] moves to transpose[j][i].

C

#include <stdio.h>

#define ROWS 2

#define COLS 3

int main() {

    int matrix[ROWS][COLS] = { {1, 2, 3}, {4, 5, 6} };

    int transpose[COLS][ROWS];

    // Transpose the matrix

    for (int i = 0; i < ROWS; i++) {

        for (int j = 0; j < COLS; j++) {

            transpose[j][i] = matrix[i][j];

        }

    }

    // Print the transposed matrix

    printf("Transposed Matrix:\n");

    for (int i = 0; i < COLS; i++) {

        for (int j = 0; j < ROWS; j++) {

            printf("%d ", transpose[i][j]);

        }

        printf("\n");

    }

    return 0;

}

Output:

Transposed Matrix:

1 4

2 5

3 6

3. Breaking Out of Nested Loops

A common challenge is that a break statement only exits the innermost loop. The cleanest way to break out of multiple loops at once is often with a goto statement.

Example: Searching in a 2D Array

This code searches a matrix for the first negative number and stops all searching as soon as it's found.

C

#include <stdio.h>

int main() {

    int matrix[3][3] = { {5, 8, 2}, {9, -1, 4}, {3, 7, 6} };

    for (int i = 0; i < 3; i++) {

        for (int j = 0; j < 3; j++) {

            if (matrix[i][j] < 0) {

                printf("Negative number found at (%d, %d). Stopping.\n", i, j);

                goto found; // Jump out of both loops

            }

        }

    }

found:

    printf("Search complete.\n");

    return 0;

}

4. Flattening Nested Loops (Optimization)

For fixed-size multi-dimensional arrays, you can sometimes improve performance by converting nested loops into a single loop. This can help the compiler's optimizer by simplifying the control flow.

• The Technique: A single loop iterates from 0 to ROWS * COLS - 1. The row and column indices are then calculated from the single loop variable using division and modulo.

• row = i / NUM_COLS;

• col = i % NUM_COLS;

Example: Single-Loop Array Initialization

C

#include <stdio.h>

#define ROWS 2

#define COLS 5

int main() {

    int matrix[ROWS][COLS];

    // Flattened loop to initialize the matrix

    for (int k = 0; k < ROWS * COLS; k++) {

        int row = k / COLS;

        int col = k % COLS;

        matrix[row][col] = k;

    }

    // Print the matrix to verify

    for (int i = 0; i < ROWS; i++) {

        for (int j = 0; j < COLS; j++) {

            printf("%2d ", matrix[i][j]);

        }

        printf("\n");

    }

    return 0;

}