Why my code is giving runtime error ? (leetcode problem number 200)

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My code for problem Number of Islands in Leetcode is giving Runtime error.

class Solution {
public:
    vector<pair<int,int>> movements={{1,0},{-1,0},{0,1},{0,-1}};
    bool isValid(int i,int j,int n,int m){
        if((i>=n && i<n) && (j>=m && j<m)){
            return 1;
        }
        return 0;
    }
    int numIslands(vector<vector<char>>& grid) {
        int count=0;
        queue<pair<int,int>> q;
        int row=grid.size();
        int col=grid[0].size();
        vector<vector<bool>> visited;
        for(int i=0;i<row;i++){
            for(int j=0;j<col;j++){
                visited[i][j]=0;
            }
        }
        for(int i=0;i<row;i++){
            for(int j=0;j<col;j++){
                if(grid[i][j]=='1'){
                    q.push(make_pair(i,j));
                }
            }
        }
        while(!q.empty()){
            auto it=q.front();
            q.pop();
            int x=it.first;
            int y=it.second;
            visited[x][y]=1;
            count++;
            for(auto movement:movements){
                int childx=movement.first+x;
                int childy=movement.second+y;
                if(isValid(childx,childy,row,col) && !visited[childx][childy]){
                    if(grid[childx][childy]=='1'){
                    visited[childx][childy]=1;
                    q.push(make_pair(childx,childy));
                    }        
                }
            }
        }
        return count;
    }
};

What I have tried:

I tried looking in Discuss section of Leetcode other people BFS code is just as same as mine.
The error Leetcode debugger throwing is:

Line 1034: Char 9: runtime error: reference binding to null pointer of type 'std::vector<bool, std::allocator<bool>>' (stl_vector.h)
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_vector.h:1043:9

Posted 22-Jul-22 14:31pm

Animesh _1SG20IS008

Updated 23-Jul-22 5:59am

v2

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Comments

Peter_in_2780 22-Jul-22 22:07pm

Time to learn how to use the debugger. We can't help if we don't even know what error it's throwing up.

Animesh _1SG20IS008 23-Jul-22 0:09am

Line 1034: Char 9: runtime error: reference binding to null pointer of type 'std::vector<bool, std::allocator<bool="">>' (stl_vector.h)
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_vector.h:1043:9

Patrice T 22-Jul-22 23:06pm

And you plan to tell us which error message and position ?
Or we have to guess ?

Animesh _1SG20IS008 23-Jul-22 0:09am

Line 1034: Char 9: runtime error: reference binding to null pointer of type 'std::vector<bool, std::allocator<bool="">>' (stl_vector.h)
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_vector.h:1043:9

Patrice T 23-Jul-22 0:11am

Use Improve question to update your question.
So that everyone can pay attention to this information.

Patrice T 23-Jul-22 0:19am

which line is 1034 ?

Animesh _1SG20IS008 23-Jul-22 1:51am

Don't know Sir I'm also confused around this one.

4 solutions

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Rick York · Answer 1 · 2022-07-22T17:48:00

Solution 1

I think this is incorrect :

if((i>=n && i<n) && (j>=m && j<m))

so IsValid will always return false.

I would guess that it should be :

if( ( i >= 0 && i < n ) && ( j >= 0 && j < m ) )

Posted 22-Jul-22 17:48pm

Rick York

Comments

Animesh _1SG20IS008 23-Jul-22 0:16am

Thanks Rick for the help but still its giving the same error.

Rick York · Answer 2 · 2022-07-22T20:48:00

Solution 2

I see the real problem. It's this section :

C++

vector<vector<bool>> visited;
for(int i=0;i<row;i++){
    for(int j=0;j<col;j++){
        visited[i][j]=0;
    }
}

You are accessing an uninitialized vector. Here's one way you could initialize it :

C++

using vbool = std::vector< bool >;
using vvbool = std::vector< vbool >;

vvbool visited;
for( int i = 0; i < row; ++i )
{
    vbool rowvec( col, 0 );
    visited.push_back( rowvec );
}

Posted 22-Jul-22 20:48pm

Rick York

Comments

Animesh _1SG20IS008 23-Jul-22 7:59am

Thanks it worked.

merano99 23-Jul-22 12:13pm

Why is the creation of rowvec in the loop?

Rick York 23-Jul-22 12:37pm

Because visited is a two dimensional vector - a vector of vectors. That means each row is a vector also so vectors must be pushed into the vector.

merano99 23-Jul-22 14:36pm

The two dimensions are clear, but the vector's values are probably copied every time, aren't they?
Just tested with VS. The end result is the same if you don't keep updating the vector rowvec in the loop. In your version, however, many new classes are created and destroyed.

Richard MacCutchan · Answer 3 · 2022-07-22T21:35:00

Solution 3

C++

if((i>=n && i<n) && (j>=m && j<m)){

The value of i cannot be greater than or equal to n and less than n at the same time. The same goes for j and m.

Posted 22-Jul-22 21:35pm

Richard MacCutchan

merano99 · Answer 4 · 2022-07-23T05:59:00

Since the main program is missing, the program is difficult to test. If the solution is empty, the program will certainly raise exceptions here.

C++

int row = grid.size();
int col = grid[0].size();

This could easily be fixed by increasing the container grid to a minimum size. The same applies to the container visited.

C++

int row = grid.size();
if (row == 0)
  grid.resize(1);
int col = grid[0].size();

It is noticeable that the containers grid and visited always have to be the same size and maybe also belong in the solution.
There you could keep them consistent and always have access and size appropriate. The fields do not have to be transferred or created anew each time.
So I propose a member function InitGrid().

C++

class Solution {
private:
   vector<vector<char>> grid;
   vector<vector<bool>> visited;
public:
   vector<pair<int, int>> const movements = { { 1,0 },{ -1,0 },{ 0,1 },{ 0,-1 } };
   bool InitGrid(int row, int col);
   bool isValid(int i, int j, int n, int m);
   int numIslands();
};

C++

bool Solution::InitGrid(int row, int col)
{
  ...
  return true;
}

For the isValid function, Rick's suggestion makes sense. Otherwise the program would be useless, but it wouldn't crash either.
Since the function declares a return value of type bool, it should also return true or false.

Why my code is giving runtime error ? (leetcode problem number 200)

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