Locate a number in a range in a matrix

#include <iostream>
#include <vector>
#include <cstdlib> // rand, srand
#include <ctime> // time
using namespace std;

// Find a number in a range of numbers in a matrix.

// These numbers have already been assigned to the matrix.
vector<int> used_numbers;

// The matrix to be filled randomly with numbers 1-225.
int matrix[15][15] = { {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}, {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}, {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}, {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}, {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}, {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}, {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}, {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}, {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}, {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}, {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}, {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}, {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}, {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}, {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0} };

// This function returns true if the number is not
// in the used numbers vector. Otherwise, it returns false.
bool number_is_available(int number, vector<int> numbers)
{
    // Check each number in the vector.
    for (size_t i = 0; i < numbers.size(); ++i)
    {
        // If the specified number is in the vector,
        if (numbers[i] == number)
            return false; // Return false.
    }
    // If the number is not in the used number vector,
    // return true.
    return true;
}

// This function returns the next number to
// assign in the matrix. Each number in the range
// 1 - 225 can only be used once.
int get_next_number_for_matrix()
{
    int random_number = 0; // A random number generated.
    bool available = true; // Has this number been used already?
    while (true) // Forever,
    {
        // Get a number in the range 1 - 225.
        random_number = (rand() % 225) + 1;
        // If the number has not been used in the matrix yet,
        if (number_is_available(random_number, used_numbers))
        {
            // Add it to the used numbers list.
            used_numbers.push_back(random_number);
            return random_number; // Return the number.
        }
    }
}

// This function displays the 2-d matrix.
void display_matrix()
{
    for (int i = 0; i < 15; ++i) // For each row,
    {
        for (int j = 0; j < 15; ++j) // For each item in the row,
        {
            // Display the number in the cell, then tab.
            std::cout << matrix[i][j] << "\t";
        }
        // Go to the next line.
        std::cout << '\n';
    }
}

int main()
{
    bool found = false; // Has the user's number been found?
    srand(time(NULL)); // Prepare the random number generator.
    int users_number = 0; // The number the user wants to find.
    int row_number = 0; // The row location of the user's number.
    int column_number = 0; // The column location of the user's number.

    // Fill the matrix.
    for (int i = 0; i < 15; ++i) // For each row,
    {
        for (int j = 0; j < 15; ++j) // For each item in the row,
        {
            // Get a number that has not been used yet, and
            // assign it to the cell.
            matrix[i][j] = get_next_number_for_matrix();
        }
    }

    std::cout << "The 15-by-15 matrix has numbers 1 to 225.\n"
        "What number would you like to locate? ";
    cin >> users_number; // Get the number from the user to serach for.
    // Validate the unput. While the input is not 1 - 225,
    while ((users_number < 1) || (users_number > 225))
    {
        std::cout << "Please choose a number from 1 to 225. ";
        cin >> users_number; // Get new input.
    }

    // Find the number.
    for (int i = 0; i < 15; ++i) // For each row,
    {
        for (int j = 0; j < 15; ++j) // For each item in the row,
        {
            // If the item in the row matches the user's number,
            if (users_number == matrix[i][j])
            {
                // Indicate the user's number has been found.
                found = true;
                // The row number is the row index + 1.
                row_number = i + 1;
                // The colum number is the column index + 1.
                column_number = j + 1;
                // Stop searching.
                break;
            }
        }
        // If the number has been found, stop searching.
        if (found == true) break;
    }

    // Display the result.
    std::cout << "Number " << users_number << " is located in\n"
        "row " << row_number << ", column " << column_number
        << '\n';

    std::cout << "Here's the randomly-filled matrix:\n\n";
    display_matrix(); // Display the matrix.

    // End the program.
    return 0;
}