program crashes on certain inputs - c++

Question

I have tried writing this code to output an odd-order magic square based on user input of an odd number. When I enter 1 or 3, it works fine. Whenever I enter anything above that such as 5, 7, 9, 11, etc the program crashes the moment I press enter. I've reviewed my code and I can't pinpoint where the problem is. I get no error messages.
Small note: if you know what a magic square is, my algorithm here (given to us by the professor in English to translate to C++) does not output the correct values since they don't all add up to the same number.
#include <iostream>
#include <iomanip>
using namespace std;
int main()
{
int n; //n = order
cout << "Enter an odd integer for the order of the Magic Square: ";
cin >> n;
cout << endl;
if(n%2 == 0) //only allows program to accept odd numbers
{
cout << "The number you have entered is not odd" << endl;
return 0;
}
int x, y; //x and y access the columns and rows of the following matrix
int magicsquare[n][n]; //creates a n by n matrix to set up magic square
int counter, square = n*n; //square is upper boundary
for(x=0; x<n; x++) //initialize all spaces in matrix with zeros
{
for(y=0; y<n; y++)
magicsquare[x][y] = 0;
}
/*Beginning of the magic square algorithm*/
x = 0, y = n/2; //initialize algorithm at the middle column of the top row
for (counter = 1; counter <= square; counter++) //magic square will contain the integers from 1 to n squared
{
magicsquare[x][y] = counter; //places current counter number at current position in the matrix or square
x--; //moves position diagonally up
y++; //and to the right
/*If a move takes you above the top row in the jth column, move to the bottom of the jth column*/
if(x<0)
x = n - 1;
/*If a move takes you outside to the right of the square in the ith row, move to the left side of the ith row*/
else if(y==n)
y = 0;
/*If a move takes you to an already filled square or if you move out of the square at the upper right
hand corner, move immediately below position of previous number*/
else if((magicsquare[x][y] != 0) || (x<0 && y==n))
{
y--; //move one space to the left back into the square
x = x+2; //move two spots down into the square and below previous number
}
}
for(x=0; x<n; x++)
{
for(y=0; y<n; y++)
cout << setw(5) << magicsquare[x][y];
cout << endl;
}
return 0;
}

Answer 1

I can't follow the logic in my head to know if this can ever actually happen, but in this code:
if(x<0)
x = n - 1;
/*If a move takes you outside to the right of the square in the ith row, move to the left side of the ith row*/
else if(y==n)
y = 0;
If both conditions would have been true, you won't fix up y and the next iteration you'll run off the end of the matrix.
Also note that int magicsquare[n][n]; is a compiler extension and not supported by the C++ standard, since n is not a compile time constant. You almost certainly want to use vector instead.

Answer 2

The following is illegal:
int magicsquare[n][n];
Did you ignore errors, or are you using a compiler that doesn't give errors at all? I suggest you to use an IDE that hints you when a mistake is made, so you can easily see your mistake. Please do not use notepad to write C++, that is horrible.
Fixed version:
int** magicsquare = new int*[n]; //creates a n by n matrix to set up magic square
for(int i = 0; i < n+1; ++i)
magicsquare[i] = new int[n];
Now, together with Mark B's hint, you will get this running up in no time.
Do not forget to cleanup magicsquare by the way using delete.

Answer 3

So I don't really know anything about magic squares. But I think that this is the behavior that you are trying to achieve:
for (int counter = 1, x = 0, y = (n / 2); counter <= n * n; ++counter){
magicsquare[x][y] = counter; //places current counter number at current position in the matrix or square
if (counter % n == 0){ //moves down into the square and below previous number
x = (x + 1) % n;
}
else //moves position diagonally up and to the right
{
x = (x + n - 1) % n;
y = (y + 1) % n;
}
}
Two additional points:
Until we can use the Array Extensions Technical Specification I think you should avoid declaring C99's runtime-sized arrays in your code. Even though gcc will allow it. You might look into doing something like: vector<vector<int>> magicsquare(n, vector<int>(n));
This doesn't match the behavior illustrated by Wikipedia's article but you can get there by tweaking the start values and order of indexing.