Backtracking in C

Question

I've heard about backtracking and I've searched a little bit .I thought I got the idea and wrote this piece of code to solve sudoku , but it seems to give wrong solutions (for example repeated numbers in a row) , what exactly i am doing wrong here ?

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>


#define N 9

bool not_in_row(int temp , int i , int grid[N][N]){

int f ;
for(f = 0 ; f < N ; f++)
    if(grid[i][f] == temp)
        return false ;
return true ;
 }

bool not_in_column(int temp , int j , int grid[N][N]){

int f ;
for(f = 0 ; f < N ; f++)
    if(grid[f][j] == temp)
        return false ;
return true ;
}


bool not_in_sq(int i , int j , int grid[N][N] , int temp){

int k , t ; 
int s = i - (i % 3) + 3, v = j - (j % 3) + 3;
for(k =  i - (i % 3) ; i < s ; i++)
    for(t = j - (j % 3) ; j < v ; j++)
        if(grid[k][t] == temp)
          return false ;
return true ;
       }


bool sudoku_Solver(int grid[N][N] , int position){



if(position == 81)
    return true ;
int i = position / 9 , j = position % 9 ;

if(grid[i][j] != 0)
    return sudoku_Solver(grid , position + 1) ;


else{

int temp ;

for(temp = 1 ; temp <= N ; temp++){
    if(not_in_row(temp , i , grid) && not_in_column(temp , j , grid) && not_in_sq(i , j , grid , temp))
    {
        grid[i][j] = temp ;
        if(sudoku_Solver(grid , position + 1))
                return true ;
            }
                }

            }
grid[i][j] = 0 ;    
return false ;
}


int main(int argc, char *argv[]) {

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

sudoku_Solver(grid , 0) ;

for(i = 0 ; i < 9 ; i++){
    for(j = 0 ; j < 9 ; j++){
        printf("%d " , grid[i][j]) ;
                }
    printf("\n") ;
        } 
return 0;
       }

Can you be a bit more precise. What is "wrong" about the solutions? Can you show the output? Can you explain what you expected? — David Schwartz, Apr 17 '14 at 01:55
it just gives wrong solutions , for example repeated numbers in a row . — user3543510, Apr 17 '14 at 01:57
Solving Suduko is more complex than this. For example two 3/3 boxes may have two possibilities in two rows. Therefore the third box will have that number. The location of that number may require similar logic. this is not catered for in this algorithm — Ed Heal, Apr 17 '14 at 02:00
@EdHeal It is. The algorithm does accept the first solution it finds, but other than that, if there's a solution, it will find it. — David Schwartz, Apr 17 '14 at 02:16
@DavidSchwartz - You are right! Brute force approach. That leads to an interesting question - How many Sudukos are there with only one solution? — Ed Heal, Apr 17 '14 at 02:24
@EdHeal Purists will tell you that if it doesn't have only one solution, it's not a Sudoku. (But I think this is based on a combination of an etymological fallacy and a myth about the origin of the name.) — David Schwartz, Apr 17 '14 at 03:44

score 1 · Accepted Answer · answered Apr 17 '14 at 01:59

Your loops in not_in_sq are wrong:

for(k =  i - (i % 3) ; i < s ; i++)
    for(t = j - (j % 3) ; j < v ; j++)

Look closely at i < s, i++, j < v, and j++. They are obviously incorrect. Here they are fixed:

for(k =  i - (i % 3) ; k < s ; k++)
    for(t = j - (j % 3) ; t < v ; t++)

Backtracking in C

1 Answers1