Magic Square

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Introduction

Magic square is an ancient mathematical problem that many people try to solve. May be you see it in some magazines or your teacher might have introduced it in a class.

Details

A magic square is an arrangement of numbers from 1 to n² in an [n x n] matrix, with each number occurring exactly once, and such that the sum of the entries of any row, any column, or any main diagonal is the same.

It is not hard to show that this sum must be  n [ ( n² + 1) / 2 ]. If we use this formula for that example output which is below, for [5x5] matrix; 5 [ ( 52 + 1 ) / 2 ] = 65.

Now, I want to show you a way to calculating magic squares in any order by using your talented computer.

Siamese method

This method is useful for calculating magic squares with odd order. It begins by placing a 1 in any location (in the center square of the top row in the above example), then incrementally placing subsequent numbers in the square one unit above and to the right. The counting is wrapped around, so that falling off the top returns on the bottom and falling off the right returns on the left. When a square is encountered which is already filled, the next number is instead placed below the previous one and the method continues as before. The method, also called de la Loubere's method.

For example, if order of square is 5, we have:

void CalculateOddMagicSquare()
{
  n=5;
  int matrix[5][5];

  int nsqr = n * n;
  int i=0, j=n/2;     // start position

  for (int k=1; k<=nsqr; ++k) 
  {
    matrix[i][j] = k;

    i--;
    j++;

    if (k%n == 0) 
    { 
      i += 2; 
      --j; 
    }
    else 
    {
      if (j==n) 
        j -= n;
      else if (i<0) 
        i += n;
    }
  }
}

Here is a very nice flash animation to show you how to fill square's cells. Thanks KIVANÇ HiKMET ANAR, for his flash.

Complete Work

Below is full source code of calculating magic squares in any order.

#include "stdafx.h"
#include <vector>

using namespace std;

void OddMagicSquare(vector<vector<int> > &matrix, int n);
void DoublyEvenMagicSquare(vector<vector<int> > &matrix, int n);
void SinglyEvenMagicSquare(vector<vector<int> > &matrix, int n);
void MagicSquare(vector<vector<int> > &matrix, int n);
void PrintMagicSquare(vector<vector<int> > &matrix, int n);

int main(int argc, char* argv[])
{
  int n;
  printf("Enter order of square: ");
  scanf("%d", &n);

  vector<vector<int> > matrix(n, vector<int> (n, 0));

  if (n<3)
  {
    printf("\nError: n must be greater than 2\n\n");
    return -1;
  }

  MagicSquare(matrix, n);  

  //Print results
  PrintMagicSquare(matrix, n);
    
  return 0;
}

void MagicSquare(vector<vector<int> > &matrix,int n)
{
  if (n%2==1)        //n is Odd
    OddMagicSquare(matrix, n);
  else          //n is even
    if (n%4==0)    //doubly even order
      DoublyEvenMagicSquare(matrix, n);
    else      //singly even order
      SinglyEvenMagicSquare(matrix, n);
}

void OddMagicSquare(vector<vector<int> > &matrix, int n)
{
  int nsqr = n * n;
  int i=0, j=n/2;     // start position

  for (int k=1; k<=nsqr; ++k) 
  {
    matrix[i][j] = k;

    i--;
    j++;

    if (k%n == 0) 
    { 
      i += 2; 
      --j; 
    }
    else 
    {
      if (j==n) 
        j -= n;
      else if (i<0) 
        i += n;
    }
  }
}

void DoublyEvenMagicSquare(vector<vector<int> > &matrix, int n)
{
  vector<vector<int> > I(n, vector<int> (n, 0));
  vector<vector<int> > J(n, vector<int> (n, 0));

  int i, j;

  //prepare I, J
  int index=1;
  for (i=0; i<n; i++)
    for (j=0; j<n; j++)
    {
      I[i][j]=((i+1)%4)/2;
      J[j][i]=((i+1)%4)/2;
      matrix[i][j]=index;
      index++;
    }

  for (i=0; i<n; i++)
    for (j=0; j<n; j++)
    {
      if (I[i][j]==J[i][j])
        matrix[i][j]=n*n+1-matrix[i][j];
    }
}

void SinglyEvenMagicSquare(vector<vector<int> > &matrix, int n)
{
  int p=n/2;

  vector<vector<int> > M(p, vector<int> (p, 0));
  MagicSquare(M, p);
  
  int i, j, k;

  for (i=0; i<p; i++)
    for (j=0; j<p; j++)
    {
      matrix[i][j]=M[i][j];
      matrix[i+p][j]=M[i][j]+3*p*p;
      matrix[i][j+p]=M[i][j]+2*p*p;
      matrix[i+p][j+p]=M[i][j]+p*p;
    }

  if (n==2)
    return;  

  vector<int> I(p, 0);
  vector<int> J;

  for (i=0; i<p; i++)
    I[i]=i+1;

  k=(n-2)/4;
  
  for (i=1; i<=k; i++)
    J.push_back(i);

  for (i=n-k+2; i<=n; i++)
    J.push_back(i);

  int temp;
  for (i=1; i<=p; i++)
    for (j=1; j<=J.size(); j++)
    {
      temp=matrix[i-1][J[j-1]-1];
      matrix[i-1][J[j-1]-1]=matrix[i+p-1][J[j-1]-1];
      matrix[i+p-1][J[j-1]-1]=temp;
    }

  //j=1, i
  //i=k+1, k+1+p
  i=k; 
  j=0;
  temp=matrix[i][j]; matrix[i][j]=matrix[i+p][j]; matrix[i+p][j]=temp;

  j=i;
  temp=matrix[i+p][j]; matrix[i+p][j]=matrix[i][j]; matrix[i][j]=temp;
}


void PrintMagicSquare(vector<vector<int> > &matrix, int n)
{
  for (int i=0; i<n; i++) 
  {
    for (int j=0; j<n; j++)
      printf(" %3d", matrix[i][j]);

    printf("\n");
  }

  printf("\n\n");
}

Conclusion

Enjoy!

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