How to pass two dimensional array of an unknown size to a function

Question

I want to make class library, a function which its parameter is a matrix of unknown size, and the user will create his own matrix with his own size and pass it to this funct

Answer 1

Multi-dimensional arrays are not very well supported by the built-in components of C and C++. You can pass an N-dimension array only when you know N-1 dimensions at compile time:

calculateDeterminantOfTheMatrix( int matrix[][123])

However, the standard library supplies std::vector container, that works very well for multi-dimension arrays: in your case, passing vector<vector<int> > &matrix would be the proper way of dealing with the task in C++.

int calculateDeterminantOfTheMatrix(vector<vector<int> > &matrix) {
    int res = 0;
    for (int i = 0 ; i != matrix.size() ; i++)
        for(int j = 0 ; j != matrix[i].size() ; j++)
            res += matrix[i][j];
    return res;
}

As an added bonus, you wouldn't need to pass dimensions of the matrix to the function: matrix.size() represents the first dimension, and matrix[0].size() represents the second dimension.

Answer 2

use this method declare an array of pointers ex: int *a[n]; Then allocate memory for them using a for loop ex:

for( int i=0 ; i<n ; i++ )
        a[i] = new int[n];

Now pass the argument like normal array. ex: print_array(a,n); And print_array function looks like

print_array(int **a,int n)//the prototype for the print_array
{
 //access the array using index such as
std:: cout<<a[1][1]<<endl;
}

The above case is for the array of nxn incase mxn is required then allocate the memory like

for( int i=0 ; i<m ; i++ )
        a[i] = new int[n];

then pass the both m,n and to the function and access the array in the for loop.

Answer 3

I would write a simple class wrapper for the matrix with column and row defined.

template <typename T>
class Mat {
  std::size_t _row;
  std::size_t _col;

  T *_mat_elem;
public:
  Mat(std::size_t r, std::size_t c)
   :  _row(r), _col(c), _mat_elem(new T[r*c] {}
  ~Mat() {/* remember to do delete [] here */}

  // element access, for example
  T& at(std::size_t r, std::size_t c)
  {
    return *(_mat_elem+r*_col+c);
  }
};

But actually you are re-inventing the wheels. There are good libraries for matrix handling out there.

Answer 4

There are multiple approaches you could take.

1. C way of doing things -> Pass in a int** but be extremely cautious here. This is not quite a 2D array. You will have to correctly allocate memory to this pointer, or, alternatively, you need to know the size at compile time. (For instance staticly allocating an array of size M * N and then disallowing anything bigger). In order to dynamically allocate the memory, you need to know the number of rows and columns.

2. C++ way -> #include <vector> after which you can simply use vector<vector<int> > &matrix (Careful about the space after the <int> unless you're using c++11 compiler.), which will allocate a vector of int vectors which is basically a 2d array. The memory management will be taken care of for you in this case.

Answer 5

C solution:

In C you can't omit array size (except leftmost) when passing as function parameter.

You can write: int a[]

but can't: int a[][]

just for example: int a[][20]

This constraint is here, because compiler needs to determine proper offsets for accessing array elements. However, you can make it this way:

void print_arbitrary_2D_array(void *arr, int y, int x)
{
    /* cast to 2D array type */
    double (*p_arr)[y][x] = (double (*)[y][x]) arr;

    int i, j;

    for (i = 0; i < y; ++i) {
        for (j = 0; j < x; ++j)
            printf(" %lf", (*p_arr)[i][j]);
        putchar('\n');
    }
}

double arr_1[4][3] = {
    { 3.3, 5.8, 2.3 },
    { 9.1, 3.2, 6.1 },
    { 1.2, 7.9, 9.4 },
    { 0.2, 9.5, 2.4 }
};
double arr_2[2][5] = {
    { 3.6, 1.4, 6.7, 0.1, 4.2 },
    { 8.4, 2.3, 5.9, 1.4, 8.3 }
};

print_arbitrary_2D_array(arr_1, 4, 3);
putchar('\n');
print_arbitrary_2D_array(arr_2, 2, 5);

Answer 6

The Best way to use 2D array in the function that I have found so far is to use Mapping Function. As in the example below , I have use the mapping function to print 2D array

void Print2D(int x[],int ROWS,int COLS)
{
    for(int i=0;i<ROWS;i++)
{
    for(int j=0;j<COLS;j++)
    cout << x[i*COLS+j] << ' ';
    cout << endl;

}
}

Here it is how to use it in main

int main(){

    int x[3][3];
    Print2D(&x[0],3,3);

}

Here &x[0] is the starting address of the First Row of 2D array or more precisely Starting address of 2D array