Generating an NxN magic square using a dynamically allocated 2D array in C++ with user-input dimension

Question

I'm trying to generate and solve an NxN odd-numbered magic square through dynamic memory allocation but whenever I run the code, it displays nothing on the terminal and the programme ends. I reckon it has something to do with the dynamic allocation of the 2D array, as when I make a normal NxN array with a constant size, the programme runs fine. I'd appreciate any help regarding this!

#include<bits/stdc++.h>
#include<cmath>

using namespace std;

void calcuateMagicSquare(int N)

{

    int **Array;
    Array=new int*[N];
        for(int i=0; i<N; i++)
        {
            Array[i]=new int[N];
        }
        memset(Array, 0, sizeof(Array));
    int nSquare=N*N;
    int i=N/2;
    int j=N-1;
        for(int k=1; k<=nSquare;)
        {
            if(i==-1 && j==N)
            {
                j=N-2;
                i=0;
            }
            else
            {
                if(j==N)
                {
                    j=0;
                }
                if(i<0)
                {
                    i=N-1;
                }
            }
            if(Array[i][j])
            {
                j=j-2;
                i++;
                continue;
            }
            else
            {
                Array[i][j]=k++;
            }
            j++;
            i--;
        }
    int SolutionMagicSquare=N*(N*N+1)/2;
        cout << "Solution of the magic Square: " << SolutionMagicSquare << endl;
        cout << "MAGIC SQUARE: \n" << endl;
        for(int i=0; i<N; i++)
        {
            for(int j=0; j<N; j++)
            {
                cout << setw(4) << Array[i][j] << " ";
                cout << endl;
            }
        }

}

int main()

{

    int N;
        cout << "Please enter the dimension of the magic square:" << endl;
        cin >> N;
        calcuateMagicSquare(N);

}

First of all, [*never* include ``](https://stackoverflow.com/questions/31816095/why-should-i-not-include-bits-stdc-h). Then don't do manual memory handling and use pointers. Use `std::vector` instead. And your use (or rather misuse) of pointers is what leads to some of your problems: The size of a pointer is the size of the pointer itself. The compiler don't know what it points to, or its size. And calling `memset` after the assignments will destroy the data you just assigned, so it's wrong as well (and leave you with one or more null pointers in your "array"). — Some programmer dude, Mar 05 '22 at 18:39

score 0 · Accepted Answer · answered Mar 05 '22 at 20:00

This isn't too bad.

    int ** Array=new int*[N];
    for(int i=0; i<N; i++)
    {
        Array[i]=new int[N];
    }
    memset(Array, 0, sizeof(Array));

The memset is causing your trouble, and it's wrong for two reasons. First, let's say you really wanted to do that. You don't, but let's say you did. How big is sizeof(Array). Array is an int **. On a 64-bit machine, that's 8 bytes. So conveniently, you only destroyed the first pointer.

What you really need to do is this:

    int ** Array=new int*[N];
    // Don't do this memset, but showing you what it should look like)
    memset(Array, 0, sizeof(int *) * N);
    for(int i=0; i<N; i++)
    {
        Array[i]=new int[N];
        // But this one is safe
        memset(Array[i], 0, sizeof(int) * N);
    }

Your version was erasing the first pointer -- Array[0]. I put that first memset in there so you could see what it would look like if you needed it. It's the second one you need. You're clearing out the size of an int times the number of ints that you have.

Generating an NxN magic square using a dynamically allocated 2D array in C++ with user-input dimension

1 Answers1