Multidimensional Array

General Multidimensional Array

• Multidimensional arrays - arrays of arrays

  int a[10];  // 1d local array
  int a1[2][3];  // 2d local array
  int a2[2][3][4];  // 3d local array
  int **a3;  // pointer of pointer to int, used for 2D dynamic array
  int ***a4;  // pointer of pointer of pointer to int, used for 3D dynamic array
  

int matrix[10][10]; // a local 10 x 10 2D array

Two dimensional array

• 2d local array

  Example: a local array a1 with 2 rows, 3 columns

  • int a1[2][3];

  • a1 has the pointer part of the array with the int ** type

  • a1 is an array of 2 element, and each element is an array of 3 int

  • memory allocated on stack memory region

  • initialization syntax

    // initialization of 2d array
    // a1[0] is {1, 2, 3}, a1[1] is {4, 5, 6}
    int a1[2][3] = {{1, 2, 3}, {4, 5, 6}};
    int a2[2][3] = {1, 2, 3, 4, 5, 6};
    int a3[][3] = {1, 2, 3, 4, 5, 6};  // first dimension can be inferred
    

• 2d dynamic array

  Example: a dynamic array dynArr1 with 2 rows, 3 columns

  // declaration
  int **dynArr1;
  
  // allocate memory (on Heap)
  dynArr1 = new int *[2];
  for (int i = 0; i < 2 ; ++i)
    dynArr1[i] = new int[3];
  //now you can access element using array syntax like dynArr1[0][0]
  
  // put values into the array
  for (int i = 0; i < 2 ; ++i)
    for (int j = 0; j < 3; ++j)
      dynArr1[i][j] = i * 3 + j + 1;
  // now dynArr1 is {{1, 2, 3}, {4, 5, 6}}
  
  // release memory
  for (int i = 0; i < 2 ; ++i)
    delete [] dynArr1[i];
  delete [] dynArr1;
  

• 2d array as a function parameter

  1. pass a 2 x 3 local array

     • Declared like int a1[2][3];
     • Not useful in real projects because of the limitation on dimension

     // ---- wrong ----
     void matrixOp(int matrix[rows][cols], int rows, int cols);
     void matrixOp(int matrix[][], int rows, int cols);
     void matrixOp(int matrix[2][], int rows, int cols);
     
     // ---- correct ----
     // only the first dimension can be empty, use int literals
     // or const variables for other dimensions
     void matrixOp(int matrix[2][3], int rows, int cols);
     void matrixOp(int matrix[][3], int rows, int cols);
     
     // using const variables
     const int ROWS = 2;
     const int COLS = 3;
     void matrixOp(int matrix[ROWS][COLS], int rows, int cols);
     void matrixOp(int matrix[][COLS], int rows, int cols);
     
     // to call
     int a1[2][3] = {{1, 2, 3}, {4, 5, 6}};
     matrixOp(a1, 2, 3)
     

  2. passing a dynamic array, a much nicer solution + declared like int **dyArr1

     // correct version 1
     void matrixOp(int ** matrix, int rows, int cols) {
       for (int i = 0; i < rows ; ++i)
         for (int j = 0; j < cols; ++j)
           cout << matrix[i][j] << " ";  // same syntax as local array
     }
     // correct version 2
     void matrixOp(int *matrix[], int rows, int cols);
     
     // to call the function
     int **dyArr1;
     dyArr1 = new int *[2];
     for (int i = 0; i < 2; ++i) {
       dyArr1[i] = new int[3];
       // initialize values
     }
     matrixOp(dynArr1, 2, 3);
     // for loop delete []
     for (int i = 0; i < 2; ++i)
       delete [] dyArr1[i];
     delete [] dyArr1;
     

• Returning a Multidimensional array from a function

  Just make a dynamic array and return the pointer:

  int **createMatrix(int rows, int cols, int value);
  
  int **createMatrix(int rows, int cols, int value) {
    int **result;
    result = new int *[rows];
    for (int i = 0; i < rows; ++i) {
      result[i] = new int[cols];
      for (int j = 0; j < cols; ++j)
        result[i][j] = value;
    }
    return result;
  }
  
  int main() {
    int **matrix = createMatrix(3, 5, 1);
  
    // work on the matrix
  
    // release memory
    for (int i = 0; i < rows; ++i)
      delete [] matrix[i];
    delete [] matrix;
  }
  

Pitfalls

// bad local array declaration
int arr[rows][cols];  // rows and cols are not compilation-time constants
int arr[][];
int arr[2][];
int arr[][3];

// bad dynamic array related
int **arr = new int[2][3];  // cannot use more than one pair of [] with new
delete arr;  // missing []
delete [2][3] arr;
delete [][] arr;
delete arr[2][3];

// bad function declarations that take local array parameters
void func1(int arr[][], int rows, int cols);
void func1(int arr[2][], int rows, int cols);
void func1(int arr[rows][cols], int rows, int cols);


// bad function declarations that take dynamic array parameters
void func1(int *arr[], int rows, int cols);