knights tour code with recursion and backtracking - c

Question

I was recently assigned the knight's tour problem.
Here is my try at it:
#include <stdio.h>
#include <math.h>
#include <stdlib.h>
int count = 1;
int movej[8] = {1, -1,-2, 2, -2, -1, 1, 2};
int movei[8] = {2, 2, 1, 1, -1, -2, -2, -1};
void boardprinter(int **board)
{
for (int i = 0; i < 8; i++)
{
for (int j = 0; j < 8; j++)
{
printf("%d ", board[i][j]);
}
printf("\n");
}
printf("\n");
}
_Bool safe(int **board, int i, int j)
{
if ((board[i][j] == (-1)) && i >= 0 && i < 8 && j >= 0 && j < 8)
{
return 1;
}
return 0;
}
_Bool solve(int **board, int si, int sj)
{
if (count == 64)
{
boardprinter(board);
return 1;
}
int i=0;
while(i<8)
{
if (safe(board, (si + movei[i]), (sj + movej[i])))
{
board[si + movei[i]][sj + movej[i]] = count++;
if (solve(board, (si + movei[i]), (sj + movej[i])))
{
return 1;
}
else
{
board[si + movei[i]][sj + movej[i]] = -1;
}
}
i++;
}
return 0;
}
int main()
{
int **board = (int **)malloc(8 * sizeof(int *));
for (int i = 0; i < 8; i++)
{
*(board + i) = (int *)malloc(8 * sizeof(int));
for (int j = 0; j < 8; j++)
{
board[i][j] = -1;
}
}
// board initiallized
int si, sj;
scanf("%d %d", &si, &sj);
board[si][sj] = 1;
count++;
_Bool c = solve(board, si, sj);
printf("%d",c);
return 0;
}
I applied recursion and backtracking in this but the code crashes after reaching (4,2),now I think this fails because the while loop doesn't seem to behave properly (it gets teminated somehow)But I dont know why..
I have been stuck over this and tried all sorts of things to debug this
Kindly help me out!!

Answer 1

Thanks to Mr Tom Karzes for pointing out the mistakes in my code.
Cause of Error
The Problem with my code was that I was indexing into my array before checking if it was out of bounds,i.e. This happened because I assumed that the condition :
(board[i][j] == (-1)) && i >= 0 && i < 8 && j >= 0 && j < 8 would automatically fail if the array was out of bounds, but it turns out that I forgot that the compiler first checks (board[i][j] == (-1)) before checking the next mentioned
conditions,Which was the cause of the undefined behavior of my program.
Fixed Code
Here is the fixed code:
#include <stdio.h>
#include <math.h>
#include <stdlib.h>
int count = 0;
int movej[8] = {1, -1,-2, 2, -2, -1, 1, 2};
int movei[8] = {2, 2, 1, 1, -1, -2, -2, -1};
void boardprinter(int **board)
{
for (int i = 0; i < 8; i++)
{
for (int j = 0; j < 8; j++)
{
printf("%d ", board[i][j]);
}
printf("\n");
}
printf("\n");
}
_Bool safe(int **board, int i, int j)
{
if (i >= 0 && i < 8 && j >= 0 && j < 8)
{
if((board[i][j] == (-1)))
{
return 1;
}
}
return 0;
}
_Bool solve(int **board, int si, int sj)
{
if (count == 64)
{
boardprinter(board);
return 1;
}
int i=0;
while(i<8)
{
if (safe(board, (si + movei[i]), (sj + movej[i])))
{
board[si + movei[i]][sj + movej[i]] = count++;
if (solve(board, (si + movei[i]), (sj + movej[i])))
{
return 1;
}
else
{
board[si + movei[i]][sj + movej[i]] = -1;
count--;
}
}
i++;
}
return 0;
}
int main()
{
int **board = (int **)malloc(8 * sizeof(int *));
for (int i = 0; i < 8; i++)
{
board[i] = (int *)malloc(8 * sizeof(int));
for (int j = 0; j < 8; j++)
{
board[i][j] = -1;
}
}
// board initiallized
int si, sj;
scanf("%d %d", &si, &sj);
board[si][sj] = 0;
count++;
_Bool c = solve(board, si, sj);
printf("%d",c);
return 0;
}