Loop running infinitely C++ for Knight Tour Problem

Question

I'm coding the Knight Tour problem in C++. But the is running indefinitely. I checked my logic and is similar to the logic mentioned here

#include<bits/stdc++.h>
using namespace std;

bool isvalid(vector<vector<int>>board,int i,int j)
{
    return i >= 0 && j >= 0 && i < 8 && j < 8 ;
};

bool checkcompletion(vector<vector<int>>board)
{
    for(int i=0;i<8;i++)
    {
        for(int j=0;j<8;j++)
            if(board[i][j]==0)
                return false;
    }
    return true;
}
int tour(vector<vector<int>>board,int i,int j,int steps)
{
    //CHECK IF IT IS A VALID STEP IF NOT VALID THEN RETURN 0
    if(!isvalid(board,i,j))
    {
        return 0;
    }

    //CHECK IF THAT PLACE IS ALREADY VISITED(MOVED OUT OF ISVALID AS THAT FUNCTION ME 
    //GET A OUT OF BOUND REQUEST WHICH WILL CAUSE SEGMENTATION FAULT)
    // cout<<i<<" "<<j<<endl;
    if(board[i][j]!=0) return 0;

    //CHECK IF ALL ARE MARKED FOR TERMINATION
    if(steps==64)return 1;
    //if(checkcompletion(board)) return 1;


    //BACKTRACKING IDEA VARIABLE
    board[i][j]=++steps;

    for(int i=0;i<8;i++)
    {
        for(int j=0;j<8;j++)
        {
            if(board[i][j]>9)
                cout<<" "<<board[i][j]<<" ";
            else
                cout<<" "<<board[i][j]<<"  ";
        } 
        cout<<"\n";
    }
    cout<<"================================\n";
    tour(board,i+2,j+1,steps);
    tour(board,i+1,j+2,steps);
     tour(board,i-1,j+2,steps);
     tour(board,i-2,j+1,steps);
    tour(board,i-2,j-1,steps);
    tour(board,i-1,j-2,steps);
   tour(board,i+1,j-2,steps);
    tour(board,i+2,j-1,steps);
    
    

    board[i][j]=0;
}

int main()
{
    //for marking the visited part as well as noting the number os steps
    vector<vector<int>>board (8,vector<int>(8,0));
    int steps = 0;

    //MADE steps =1 for 0,0
    //board[0][0]=++steps;
    //initil position 0,0
    tour(board,0,0,steps);

}

Can someone tell me what's wrong with this code? When I checked the intermediate status of the array it was just working fine. Is it because the algorithm is complex and hence it is running for a longer time and I'm mistaking it as an infinite loop? I checked this answer which describes how complex this problem is.

Now is a very good time to learn something that competition/online judge sites won't teach you: ***Debugging!*** Using a *debugger* you can step through your code statement by statement while monitoring variables and their values and see how they change. This is the normal way to solve problems like this. — Some programmer dude, Jul 08 '20 at 08:23
Another few tips to help you create working programs: [Find a simpler problem](https://ericlippert.com/2014/03/21/find-a-simpler-problem/). With that I mean that you don't write large pieces of code (and definitely not whole programs) in one go, only to test it when it's hard to test and debug. Instead just write the smallest piece of code possible (beginning with an empty `main` function), build (with extra warnings enabled) and test. Only when that piece of code you continue with the next small and simple piece. Do this until the program is finished. — Some programmer dude, Jul 08 '20 at 08:27
It would certainly run faster if you passed your board by reference. — john, Jul 08 '20 at 08:39
[Why should I not `#include `?](https://stackoverflow.com/questions/31816095/why-should-i-not-include-bits-stdc-h), [Why is `using namespace std;` considered bad practice?](https://stackoverflow.com/questions/1452721/why-is-using-namespace-std-considered-bad-practice) — Evg, Jul 08 '20 at 09:08
@Someprogrammerdude Thanks for the advice. I tried to debug the code and the code works fine for the last few iterations until I forcefully stop it. But the code doesn't seem to terminate even though the logic is correct. — IshduttT, Jul 09 '20 at 01:44

score 1 · Accepted Answer · answered Jul 08 '20 at 13:35

1

what you are trying to do is recursive function but you do not have exit condition so the function calls itself in a loop. You should put a return in someplace in your function according to the target of your function. for example:

if (tour(board, i + 2, j + 1, steps) == 0)
{
    return steps;
}

(this is an example I don't know what is your condition)

if you want to learn more about recursive functions here are websites:

https://beginnersbook.com/2017/08/cpp-recursion/#:~:text=The%20process%20in%20which%20a,f(n)%20%3D%201.

https://www.geeksforgeeks.org/recursion/

In addition, I recommend you use your debugger to understand why and where your code get stuck.

Hope that was helpful :)

answered Jul 08 '20 at 13:35

רן מורסקי

86
2

1

Hey, @רן מורסקי Thanks! my code actually terminated, but I didn't get it. Why are we returning steps, whenever we call the recursive function? We are returning the steps when the condition is satisfied anyway. Isn't it sufficient? – IshduttT Jul 09 '20 at 01:58
1

Sure! if you are returning only in the first condition the program will not stop except if the first condition is the last step, so you need to put a return in every condition (hope it was clear, if not you can try to add one more comment to this or search on youtube for recursive function and examples of chess or something like this) Good Luck!!! – רן מורסקי Jul 09 '20 at 20:46

Loop running infinitely C++ for Knight Tour Problem

1 Answers1