c++ sudoku program using Back tracking, Segmentation fault (core dumped) ,all suggestions are welcomed

Question

#include <iostream>
using namespace std;
//defining 9X9 grid.

int a[9][9] ={{0,0,3,0,9,2,6,0,0},
              {1,0,0,3,0,0,8,0,0},          
              {0,0,5,0,1,0,0,4,0},
              {0,3,0,0,0,0,2,5,8},  
              {2,4,0,0,5,0,0,0,0},
              {0,0,0,6,2,0,0,0,3},
              {0,1,4,0,0,9,0,3,0},
              {6,0,0,7,0,0,1,0,0},
              {3,0,0,0,0,4,0,0,2} };

// class sudoku.

class sudoku{
   public:
   int row,col,i,j,num;

//to check presence of element in particular row.

bool rowCheck(int a[9][9],int &row,int num)
{
   for(j=0;j<9;j++)
   {
     if(a[row][j]==num)
     return true;
   }
   return false;
}

//to check presence of element in particular column.

bool colCheck(int a[9][9], int &col, int num)
{
   for(j=0;j<9;j++)
   {
     if(a[j][col]==num)
     return true;
   }
   return false;
}

//to check presence of element in particular 3X3 grid.

bool boxCheck(int a[9][9],int &row ,int &col ,int num)
{ 
  int x,y;
  if(row<3)
  x=0;
  else if(row>=3 && row<6)
  x=3;
  else
  x=6;

  if(col<3)
  int y=0;
  else if(col>=3 && col<6)
  y=3;
  else
  y=6;

  for(i=x;i<x+3;i++)
  {
    for(j=y;j<y+3;j++)
    {
        if(a[i][j]==num)
        return true;
    }
  }
  return false;
}

//to check index which is unassigned.

bool unAssigned(int a[9][9],int &row,int &col)
{
  for(row=0;row<9;row++)
  {
    for(col=0;col<9;col++)
    {
        if(a[row][col]==0){
        return true;}
    }
   }
   return false;
}

//to return true if position is suitable to insert .

bool isSafe(int a[9][9],int &row,int &col,int num)
{
  if(!rowCheck(a,row,num) && !colCheck(a,col,num) && 
    !boxCheck(a,row,col,num))
     return true;
  else
  return false;
}

//function to solve sudoku.

bool sudokuSolver(int a[9][9])
{
   if(!unAssigned(a,row,col))   
   return true;

   for(i=1;i<=9;i++)
   {
     if(isSafe(a,row,col,i))
     {
        a[row][col]=i;
        cout<<a[row][col];

        if(sudokuSolver(a))
        return true;

        a[row][col]=0;
     }
 }
 return false;
}
void display(int a[9][9])
{
  for(i=0;i<9;i++)
  {
        for(j=0;j<9;j++)
        {
            cout<<a[i][j]<<" ";
        }
        cout<<endl;
  }
}
//class ends

};

//main method

int main()
{
  sudoku s;                
  s.sudokuSolver(a);
  s.display(a);
  return 0;
}

Answer 1

After calling: unAssigned(a,row,col) the value of row is 9 and the value of colis 9 when unAssigned() returns false. This is a consequence of using references to row and col.

bool unAssigned(int a[9][9],int &row,int &col)
{
  for(row=0;row<9;row++)
  {
    for(col=0;col<9;col++)
    {
        if(a[row][col]==0){
        return true;}
    }
   }
   // here: row is 9 and col is 9
   return false;
}

This means that you can return from sudokuSolver() with row and col out of bounds. This will trigger a segmentation fault in the following line:

    if(sudokuSolver(a))
        return true;
    // here row or col are equal to 9 which is out of bounds
    a[row][col]=0; // seg-fault here

Answer 2

You never initialize row and col which leads to undefined behaviour once you use their values.

Apart from that, I would suggest you to avoid hard coded array bounds and raw loops. If you use containers and iterators instead you can avoid out of bounds errors completely (not the problem here, but a line for(i=1;i<=9;i++) looks very suspicious and makes me think twice to realize that it is ok). Moreover, dont pass by reference if the parameter is actually not modified by the method. E.g. bool colCheck(int a[9][9], int &col, int num) does not modify col, thus it is rather confusing why it takes col as reference. Also it is confusing that both row and col are members of the class but at the same time you pass them between the methods. I would suggest to rename the members to max_row and max_col, respectively.