Flatten a 2D Array into a 1D Array in C

To flatten a 2D array into a 1D array in C, we need to traverse the 2D array row-wise or column-wise and store its elements in a 1D array. This process converts a matrix-like structure into a linear sequence of values. We achieve this using nested loops that iterate over rows and columns while maintaining an index for the 1D array.

Examples to Flatten a 2D Array

1. Flattening a 2D Array Row-wise

In this example, we will take a 2D array and flatten it row-wise into a 1D array. The nested loop will iterate over rows first, then columns, copying elements sequentially into the 1D array.

main.c

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#include <stdio.h>

#define ROWS 2
#define COLS 3

int main() {
    int matrix[ROWS][COLS] = {{1, 2, 3}, {4, 5, 6}};
    int flatArray[ROWS * COLS];
    int index = 0;

    // Flattening the 2D array row-wise
    for (int i = 0; i < ROWS; i++) {
        for (int j = 0; j < COLS; j++) {
            flatArray[index++] = matrix[i][j];
        }
    }

    // Printing the flattened 1D array
    for (int i = 0; i < ROWS * COLS; i++) {
        printf("%d ", flatArray[i]);
    }

    return 0;
}

Explanation:

  1. We define a 2D array matrix of size 2x3 with values {{1, 2, 3}, {4, 5, 6}}.
  2. We declare a 1D array flatArray of size ROWS * COLS to store the flattened values.
  3. We use a nested loop to iterate over rows and columns of the 2D array.
  4. Inside the inner loop, we copy elements from matrix[i][j] to flatArray[index], incrementing index each time.
  5. Finally, we print the contents of the 1D array.

Output:

1 2 3 4 5 6

2. Flattening a 2D Array Column-wise

In this example, we will flatten a 2D array column-wise instead of row-wise. This means we traverse elements column by column instead of row by row.

main.c

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#include <stdio.h>

#define ROWS 2
#define COLS 3

int main() {
    int matrix[ROWS][COLS] = {{1, 2, 3}, {4, 5, 6}};
    int flatArray[ROWS * COLS];
    int index = 0;

    // Flattening the 2D array column-wise
    for (int j = 0; j < COLS; j++) {
        for (int i = 0; i < ROWS; i++) {
            flatArray[index++] = matrix[i][j];
        }
    }

    // Printing the flattened 1D array
    for (int i = 0; i < ROWS * COLS; i++) {
        printf("%d ", flatArray[i]);
    }

    return 0;
}

Explanation:

  1. We define the 2D array matrix of size 2x3 with the same values.
  2. We declare a 1D array flatArray of size ROWS * COLS.
  3. Instead of iterating row-wise, we swap the loop order and iterate column-wise first.
  4. We copy elements from matrix[i][j] into flatArray[index] in column-major order.
  5. Finally, we print the 1D array.

Output:

1 4 2 5 3 6

Conclusion

In this tutorial, we learned how to flatten a 2D array into a 1D array using two different approaches:

  1. Row-wise Flattening: Extracts elements from left to right, top to bottom.
  2. Column-wise Flattening: Extracts elements from top to bottom, left to right.

Both approaches use nested loops to iterate through the 2D array and store values in a 1D array sequentially.