How to fill a two dimensional array?

Question

There is a two dimensional array with 9 rows and 9 columns (81 elements). The task is to fill this array in a spiral way: from center to the left, then down, then to the right, then up. Just like shown here. Notice: the numbers on the picture are indexes of elements!

How to do that? I only know how to fill an array in this way (first fill all columns of the first row, then all columns of the second row and so on):

#include <iostream>
using namespace std;

int main()
{
const int rows = 9;
const int cols = 9;
int ARR[rows][cols];

for (int i = 0; i < rows; i++)
{
    for (int j = 0; j < cols; j++)
    {
        ARR[i][j] = rand() % 10;
    }
}

for (int i = 0; i < rows; i++)
{
    for (int j = 0; j < cols; j++)
    {
        cout << ARR[i][j] << "  ";
    }
    cout << endl;
}
}

So how can I do this task: to fill an array in a spiral way?

Answer 1

There is no simple way. Here is an algorithm that changes the direction as soon as possible, starting left, and stops when going out of the array:

#include <iostream>
#include <iomanip>

using namespace std;

int main()
{
    // use constexpr to have true constants
    constexpr const int rows = 9;
    constexpr const int cols = 9;

    int ARR[rows][cols];

    // Initialize the array to 0 values to detect unmodified slots
    for (int (&row)[cols]: ARR) {
       for (auto &val: row) {
           val = 0;
       }
    }

    // Use symbols for directions
    enum Dir {
        left = 0,
        down,
        up,
        right,
    } dir = left;

    // Define the starting point and starting value
    int x = rows / 2;
    int y = cols / 2;
    int val = 1;
    
    // A flag to know when to stop
    bool stop = false;

    // Let's go...
    for (;;) {
        ARR[x][y] = val++;
        switch (dir) {
        case left:
            y -= 1;
            if (y < 0) stop = true;
            else if (ARR[x+1][y] == 0) dir = down;
            break;
        case down:
            x += 1;
            if (x > rows) stop = true;
            else if (ARR[x][y+1] == 0) dir = right;
            break;
        case right:
            y += 1;
            if (y >= rows) stop = true;
            else if (ARR[x-1][y] == 0) dir = up;
            break;
        case up:
            x -= 1;
            if (x < 0) stop = true;
            else if (ARR[x][y-1] == 0) dir = left;
        }
        if (stop) break;
    }

    // Display the values
    for (int (&row)[cols]: ARR) {
       for (auto val: row) {
           cout << setw(3) << val;
       }
       cout << '\n';
    }

    return 0;
}