runtime error: addition of unsigned offset/UndefinedBehaviorSanitizer: undefined-behavior

Question

I am not able to find where is my vector going out of bound. This is solution of leetcode question 695.

Error:

Line 1034: Char 34: runtime error: addition of unsigned offset to 0x610000000040 overflowed to 0x610000000028 (stl_vector.h) SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_vector.h:1043:34

Code:

int maxAreaOfIsland(vector<vector<int>>& grid) {
    int n = grid.size();
    int m = grid[0].size(); 
    
    vector<vector<int>> vis(n,vector<int>(m,0));
    int ans =0 ;
    for(int i=0;i<n;i++) {
        for(int j=0;j<m;j++) {
            int count  = 0;
            if(grid[i][j]==1 && !vis[i][j]) {
                vis[i][j]=1;
                queue<pair<int,int>> q;
                q.push({i,j});
                count++;
                while(!q.empty()) {
                    int  a = q.front().first;
                    int  b = q.front().second;
                    q.pop();
                    
                    int r[] = {-1, 1, 0, 0};
                    int c[] = {0, 0, 1, -1};
                    
                    for(int z=0; z<4; z++) {
                       int x = a + r[z];
                       int y = b + c[z]; 
                       if(x<n && y<m && grid[x][y]==1 && !vis[x][y]) {
                           vis[x][y]=1;
                           q.push({x,y});
                           count++;
                       }
                    }
                    
                }
                ans = max(ans,count);
            }
        }
    }
    return ans;
}

Answer 1

I expect

if(x<n && y<m && grid[x][y]==1 && !vis[x][y]) {

should be

if (x>=0 && y>=0 && x<n && y<m && grid[x][y]==1 && !vis[x][y]) {

Notice in the error addition of unsigned offset to 0x610000000040 overflowed to 0x610000000028. In other words the unsigned offset is negative (when regarded as a signed quantity).

Always be careful mixing signed and unsigned arithmetic.