c++arraysfaultmagic-square

Magic Square in C++


Yet another magic square problem. I'm creating an odd magic square program in C++, and for some reason the program keeps giving a segmentation fault error and quitting. Here is the code:

#include <iostream>

using std::cin;
using std::cout;

#include <cstring>

using std::memset;

int *generateOddSquare(int n) {
    if (n % 2 != 0 && n >= 3) {
        int row = 0, col = n / 2, square = n * n;
        int **matrix = new int *[n], *dest = new int[square];

        memset(matrix, 0, sizeof(matrix[0][0]) * square);

        for (int i = 1; i <= square; i++) {
            matrix[row][col] = i;

            if (i % n == 0)
                row++;
            else {
                if (row == 0)
                    row = n - 1;
                else
                    row--;

                if (col == (n - 1))
                    col = 0;
                else
                    col++;
            }
        }

        for (int i = 0; i < n; i++) {
            for (int j = 0; j < n; j++) {
                dest[(i * n) + j] = matrix[i][j];
            }
        }

        return dest;
    } else
        return NULL;
}

int main() {
    int *arr = generateOddSquare(3);

    for (int i = 0; i < 9; i++) {
        cout << arr[i] << "\n";
    }
}

What is wrong with it? Is the way I'm declaring my pointers correct?


Solution

  • You create an array of pointers:

    int **matrix = new int *[n]
    

    but don't initialise those to point to anything; hence the segementation fault when you try to dereference them. If you really must juggle pointers, then allocate an array for each to point at:

    for (int i = 0; i < n; ++i) {
        matrix[i] = new int[n];
    }
    

    and don't forget to delete all of these allocations, if you care about memory leaks.

    Unless this is an exercise in masochism, use the standard library to make life easier:

    std::vector<std::vector<int>> matrix(n, std::vector<int>(n));
    

    and return std::vector<int> rather than int* to save the caller the hassle of juggling and deleting the pointer.