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Passing 2D Arrays

Multi-dimensional arrays, and especially 2-dimensional arrays (i.e., matrices), come in handy very often when approaching several computational problems.
Unfortunately, the C programming language does not provide a built-in support to multi-dimensional arrays, in fact there are several ways to simulate them.
The way to pass such arrays to a function depends on how multiple dimensions are simulated. To this end, we here propose 3 different methods as follows.

1) Use an array of arrays. This can only be used if your array bounds are fully determined at compile time.


#define N_ROWS 4
#define N_COLS 5

void initMatrix(int matrix[N_ROWS][N_COLS]) {
      int i, j;

      for (i=0; i<N_ROWS; i++) {
          for (j=0; j<N_COLS; j++) {
              matrix[i][j] = i*j;
          }
      }
}

int main() {

    int matrix[N_ROWS][N_COLS];
    initMatrix(matrix);

}

2) Use a dynamically-allocated array of pointers to dynamically-allocated arrays. This is used mostly when the array bounds are not known until runtime.


void initMatrix(int** matrix, int n_rows, int n_cols) {
      int i, j;

      for (i=0; i<n_rows; i++) {
          for (j=0; j<n_cols; j++) {
              matrix[i][j] = i*j;
          }
      }
}

int main() {
    
    int n_rows, n_cols, i;
    int **matrix;

    /* obtain values for n_rows & n_cols */

    /* dynamically allocate the matrix using malloc */
    matrix = malloc(n_rows * sizeof *matrix);
    
    for (i=0; i<n_rows; i++) {
        matrix[i] = malloc(n_cols * sizeof *matrix[i]);
    }

    /* use the array */
    initMatrix(matrix, n_rows, n_cols);

    /* deallocate the matrix */
    for (i=0; i<n_rows; i++) {
        free(matrix[i]);
    }
    free(matrix);

}

3) Use a 1-dimensional array and fixup the indices. This can be used with both statically-allocated (i.e., fixed-size) and dynamically-allocated arrays.


void initMatrix(int* matrix, int n_rows, int n_cols) {
    int i, j;

    for (i=0; i<n_rows; i++) {
        for (j=0; j<n_cols; j++) {
            matrix[i*n_rows+j] = i*j;
        }
    }
}

int main() {
    
    int n_rows, n_cols;
    int *matrix;

    /* obtain values for n_rows & n_cols */

    /* allocate the matrix */
    matrix = malloc(n_rows * n_cols * sizeof *matrix);

    /* use the array */
    initMatrix(matrix, n_rows, n_cols);

    /* deallocate the array */
    free(matrix);

}

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