What is the 8 queens problem? Write a C program to solve it.

The 8 queens problem is a classic problem using the chess board. This problem is to place 8 queens on the chess board so that they do not attack each other horizontally, vertically or diagonally. It turns out that there are 12 essentially distinct solutions to this problem.


Suppose we have an array t[8] which keeps track of which column is occupied in which row of the chess board. That is, if t[0]==5, then it means that the queen has been placed in the fifth column of the first row. We need to couple the backtracking algorithm with a procedure that checks whether the tuple is completable or not, i.e. to check that the next placed queen 'i' is not menaced by any of the already placed 'j' (j < i):


Two queens are in the same column if t[i]=t[j]
Two queens are in the same major diagonal if (t[i]-t[j])=(i-j)
two queens are in the same minor diagonal if (t[j]-t[i])=(i-j)




Here is some working C code to solve this problem using backtracking


#include
static int t[10]={-1};
void queens(int i);
int empty(int i);

void print_solution();

int main()
{
queens(1);
print_solution();
return(0);
}

void queens(int i)
{
for(t[i]=1;t[i]<=8;t[i]++)
{
if(empty(i))
{
if(i==8)
{
print_solution();
/* If this exit is commented, it will show ALL possible combinations */
exit(0);
}
else
{
// Recurse!
queens(i+1);
}

}// if

}// for
}



int empty(int i)
{
int j;
j=1;

while(t[i]!=t[j] && abs(t[i]-t[j])!=(i-j) &&j<8)j++;

return((i==j)?1:0);
}



void print_solution()
{
int i;
for(i=1;i<=8;i++)printf("\nt[%d] = [%d]",i,t[i]);
}




And here is one of the possible solutions



t[1] = [1] // This means the first square of the first row.
t[2] = [5] // This means the fifth square of the second row.
t[3] = [8] ..
t[4] = [6] ..
t[5] = [3] ..
t[6] = [7] ..
t[7] = [2] ..
t[8] = [4] // This means the fourth square of the last row.