2

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!!

Will Ness
  • 70,110
  • 9
  • 98
  • 181
Chemical Brewster
  • 43
  • 5
  • Don't write `*(board + i)`. Just write `board[i]`. Don't fight the language. – Tom Karzes Jan 29 '23 at 07:15
  • What values are you entering that cause it to "crash"? You need to provide *complete* information on how to reproduce the problem. – Tom Karzes Jan 29 '23 at 07:17
  • This test is wrong: `(board[i][j] == (-1)) && i >= 0 && i < 8 && j >= 0 && j < 8` You need to perform the bounds checks *before* trying to index into the array, for obvious reasons. As it stands, your program exhibits undefined behavior as a result of this bug. This is quite likely what's causing your "crash". What do you think will happen when `i` is out of bounds? It will index out of bounds, most likely picking up an undefined pointer value which it then tries to dereference. That's a severe bug. Move the `(-1)` check to the *end* of the condition. – Tom Karzes Jan 29 '23 at 07:18
  • @TomKarzes Yup I fixed that and it seems to be fixed now, thanks i will keep this in mind. – Chemical Brewster Jan 29 '23 at 07:21

1 Answers1

2

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;
}
Chemical Brewster
  • 43
  • 5