Question

I have written a program for the 8-Queens problem.It prints all possible solutions.

queens() finds all the possible solutions.
ok() tells whether the given column and row is safe or not.

The very strange problem is:

‘Count’ won’t increment. I have no idea why.

#include<iostream.h>
#include<stdlib.h>
#include<conio.h>

int arr[8][8]={0};
int count=0;

int ok(int k,int j)
{
 int i,l;
 int tup[8]={0};

 for(i=0;i<k;i++)
 {
  for(l=0;l<8;l++)
  {
   if(arr[i][l]==1)
   tup[i]=l;
  }
 }

 for(i=0;i<k;i++)
 {
  if((abs(tup[i]-j)==abs(i-k))||(tup[i]==j)||(arr[i][j]==1))
  return 0;
 }

 return 1;

}


void queen(int i)
{
 int j,k,temp;

 if(i==8)
 {
  count++;
  cout<<"Solution no. "<<count<<":\n";

  for(i=0;i<8;i++)
  {
   for(j=0;j<8;j++)
   {
    if(arr[i][j]==1)
    cout<<"Q";
    else
    cout<<"=";
    cout<<" ";
   }
   cout<<"\n";
  }
  cout<<"---------------\n";

  getch();

 }

 for(j=0;j<8;j++)
 {
  if(ok(i,j))
  {
   for(k=0;k<8;k++)
   arr[i][k]=0;
   arr[i][j]=1;
   queen(i+1);
  }
 }

 arr[i][j]=0;

}



int main()
{

 clrscr();    

 queen(0);

 return 0;

}

Answer 1

You should return from the queen function after the i==8 part is done. Currently you continue, and at the end you execute arr[i][j]=0; with i==7 and j==8. This is one past the end of the array, and as count resides immediately after arr in memory, it gets reset to zero.

Found using gdb: Break after incrementing count, set (hardware) watchpoint on count, and continue to see where count changes value again.