Diagram locations of digits in a 2-d array

#include <iostream>
#include <string>
using namespace std;

// Print nine separate diagrams of
// locations of each digit 1 - 9 in
// a matrix.
// Numbers in left and top headings are
// multiplied, then modulo 9 to fill the cells.
// Ex: (5 x 4) mod 9 = 2
// 0 1 2 3 4 5 6 7 8 9
// 1 1 2 3 4 5 6 7 8 9
// 2 2 4 6 8 1 3 5 7 9
// 3 3 6 9 3 6 9 3 6 9
// 4 4 8 3 7 2 6 1 5 9
// 5 5 1 6 2 7 3 8 4 9
// 6 6 3 9 6 3 9 6 3 9
// 7 7 5 3 1 8 6 4 2 9
// 8 8 7 6 5 4 3 2 1 9
// 9 9 9 9 9 9 9 9 9 9

const int ROWS = 10; // Number of rows.
const int COLUMNS = 10; // Number of columns.
// Printed when the number is there.
const char dot = '@';
// Printed when the number is not there.
const char space = '.';
// Indent from the edge of the console window.
const string indent_3 = "   ";

// The 2-d array.
int matrix[ROWS][COLUMNS] = { {0, 1, 2, 3, 4, 5, 6, 7, 8, 9},
    {1, 1, 2, 3, 4, 5, 6, 7, 8, 9}, {2, 2, 4, 6, 8, 1, 3, 5, 7, 9},
    {3, 3, 6, 9, 3, 6, 9, 3, 6, 9}, {4, 4, 8, 3, 7, 2, 6, 1, 5, 9},
    {5, 5, 1, 6, 2, 7, 3, 8, 4, 9}, {6, 6, 3, 9, 6, 3, 9, 6, 3, 9},
    {7, 7, 5, 3, 1, 8, 6, 4, 2, 9}, {8, 8, 7, 6, 5, 4, 3, 2, 1, 9},
    {9, 9, 9, 9, 9, 9, 9, 9, 9, 9} };

// Show the locations of a specific digit.
void print_digit_location(int arr[ROWS][COLUMNS], int number)
{
    // For each row starting with index 1,
    // (ignore the top row)
    for (int i = 1; i < ROWS; ++i)
    {
        // Start the diagram three spaces from
        // the edge of the window.
        cout << indent_3;

        // For each column starting with index 1,
        for (int j = 0;  j < COLUMNS; ++j)
        {
            if (j != 0) // Ignore the left column.
            {
                // If the specified number is
                // at this location,
                if (arr[i][j] == number)
                {
                    // Print a dot character.
                    cout << dot;
                }
                else // If it's not at this
                    // location,
                {
                    // Print a space character.
                    cout << space;
                }
            }
        }
        cout << endl; // Go to the next line.
    }
}

int main()
{
    cout << "These are the patterns created by digits in "
        "the array.\n";

    for (int i = 1; i <= 9; ++i) // For each digit,
    {
        // Print a heading
        cout << "\nDigit " << i << "\n\n";
        // Print diagram of all locations.
        print_digit_location(matrix, i);
        cout << "\n\n"; // Add blank lines below
    }

    // End the program.
    return 0;
}