2D Arrays in C

Let’s say you want to represent a 3 * 3 matrix using an array in C. Here are some fashions that you can apply:

Static 2D Array

• Create Array:

int arr[3][3] = {
    {0, 1, 0},
    {0, 0, 1},
    {0, 1, 0}
  };

• Traverse Array:

for (int i = 0; i < 3; i++) {
  for (int j = 0; j < 3; j++) {
    printf("%d ", arr[i][j]);
  }
  printf("\n");
}

• Free Array:

// No need to worry about, as it's created on the stack.

• Pros:

  • Easy to initialize and use.
  • No dynamic memory allocation required.

• Cons:

  • Limited by stack size (usually signaficantly smaller than heap), not suitable for large arrays.
  • Fixed size, less flexible for varying data.

Jagged 2D Array (Dynamic)

• Create Array:

int cols = 3, rows = 3;
int **arr;

arr = (int **)malloc(rows * sizeof(int*));
for (int i = 0; i < rows; i++){
  arr[i] = (int *)malloc(cols * sizeof(int));
}

• Traverse Array:

for (int i = 0; i < rows; i++) {
  for (int j = 0; j < cols; j++) {
    printf("%d ", arr[i][j]);
  }
  printf("\n");
}

• Free Array:

for (int i = 0; i < rows; i++) {
  free(arr[i]);
}
free(arr);

• Pros:

  • Flexible sizes for each row.
  • Suitable for complex data structures (like where the size of each row is not uniform).

• Cons:

  • Requires careful memory management.
  • Jagged 2D array is not stored contiguously in memory, so it has cache locality issues if you care about performance.

VLA Dynamic 2D Array

• Create Array:

int rows = 3, cols = 3;
int (*arr)[cols] = malloc(rows * sizeof(*arr));

• Traverse Array:

for (int i = 0; i < rows; i++) {
  for (int j = 0; j < cols; j++) {
    printf("%d ", arr[i][j]);
  }
  printf("\n");
}

• Free Array:

free(arr);

• Pros:
  • Allows dynamic allocation with variable sizes at runtime.
  • Easier to handle than a jagged array.
• Cons:
  • Not supported in all C standards. (VLA is a feature after C99).

1D Array Simulation

• Create Array:

int rows = 3, cols = 3;
int *arr = malloc(rows * cols * sizeof(int));

• Traverse Array:

for (int i = 0; i < rows; i++) {
  for (int j = 0; j < cols; j++) {
    printf("%d ", arr[i * cols + j]);
  }
  printf("\n");
}

• Free Array:

free(arr);

• Pros:
  • Simulates 2D arrays with simple memory management.
  • Flexible for dynamic size adjustments.
• Cons:
  • Accessing elements can be less intuitive.

Reference:

• CppReference: C99 variable length arrays