Print 2D Array as a Method in C++

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I want to write a method in C++ which prints a 2D array. This method is void and it has these parameters as inputs: 2D array, number of rows, number of columns. I want to use the 2D matrix as a double pointer to call the function. I want to print the elements in a 2D shape. I get the following error:

error: cannot convert ‘int (*)[2]’ to ‘int**’

EDIT : changing int** list to int (*list)[2] works but is there a way which I don't have to write the column number manually, because I want this method to work for different sizes of array.

#include <iostream>

using namespace std;

void printArray2D(int** list, int row, int col) {
    for (int i = 0; i < row; i++) {
        for (int j = 0; j < col; j++) {
            cout << list[i][j] << " ";
        }
        cout << endl;
    }
}

int main()
{
    int list1[2][2] = { {1,2},{3,4} };

    int row1 = sizeof(list1) / sizeof(list1[0]);
    
    int col1 = sizeof(list1[0]) / sizeof(list1[0][0]);

    printArray2D(list1, row1, col1);

    return 0;
}
multidimensional-array pointers methods arrays c++
1 Answers

Your array is actually flat, meaning that you have to cast its type to int*, not to int**. int** refers to array of pointers to int arrays, but you have no pointers inside array.

In other words you have actually 1-dimensional array of ints (flat). Any C multi-dimensional array like int a[3][5] or int a[3][5][7] are all flat array, 1-dimensional in nature. [3][5] array points to contiguous region in memory containing 3 * 5 = 15 ints, and [3][5][7] array points to 3 * 5 * 7 = 105 ints in contiguous region of memory.

Also after casting to int* information about dimensions is lost, hence you have to do indexing arithmetics manually through using [i * col + j]:

Try it online!

#include <iostream>

using namespace std;

void printArray2D(int* list, int row, int col) {
    for (int i = 0; i < row; i++) {
        for (int j = 0; j < col; j++) {
            cout << list[i * col + j] << " ";
        }
        cout << endl;
    }
}

int main()
{
    int list1[2][2] = { {1,2},{3,4} };

    int row1 = sizeof(list1) / sizeof(list1[0]);
    
    int col1 = sizeof(list1[0]) / sizeof(list1[0][0]);

    printArray2D((int*)list1, row1, col1);

    return 0;
}

Output:

1 2 
3 4

Actually there is a C++ specific solution using templates that can be used to keep information about dimensions and forward this information to function, then you don't have to use manual indexing arithmetics:

Try it online!

#include <iostream>

using namespace std;

template <int Rows, int Cols>
void printArray2D(int const (&list)[Rows][Cols]) {
    for (int i = 0; i < Rows; i++) {
        for (int j = 0; j < Cols; j++) {
            cout << list[i][j] << " ";
        }
        cout << endl;
    }
}

int main()
{
    int list1[2][2] = { {1,2},{3,4} };

    int row1 = sizeof(list1) / sizeof(list1[0]);
    
    int col1 = sizeof(list1[0]) / sizeof(list1[0][0]);

    printArray2D(list1);

    return 0;
}

Output:

1 2 
3 4

Another nice solution in case if you have fixed multi-dimensional array is by using std::array. Then you don't need to forward any dimensions information, just use .size() method of std::array:

Try it online!

#include <iostream>
#include <array>

template <typename ArrT>
void printArray2D(ArrT const & list) {
    for (int i = 0; i < list.size(); i++) {
        for (int j = 0; j < list[i].size(); j++) {
            std::cout << list[i][j] << " ";
        }
        std::cout << std::endl;
    }
}

int main()
{
    std::array<std::array<int, 2>, 2> list1 =
        { std::array{1,2}, std::array{3,4} };
    printArray2D(list1);
    return 0;
}

Output:

1 2 
3 4
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