Question

I made this program that should flood fill the matrix but something went wrong. Here’s the code:

queue<Point> Qu;    
int n,m;
cin>>n>>m;
int mat[n][m];
for(int i=0;i<n;++i)    
    for(int j=0;j<m;++j)
        cin>>mat[i][j];        

Point N,W,S,E,bgn;
bgn.x=0;
bgn.y=0;
Qu.push(bgn);
while(!Qu.empty()){
    N.x=Qu.front().x-1; N.y=Qu.front().y;
    S.x=Qu.front().x+1; S.y=Qu.front().y; 
    E.x=Qu.front().x; E.y=Qu.front().y+1; 
    W.x=Qu.front().x; W.y=Qu.front().y-1;

    if(mat[N.x][N.y]==0){mat[N.x][N.y]=2;Qu.push(N);} 
    if(mat[S.x][S.y]==0){mat[S.x][S.y]=2;Qu.push(S);} 
    if(mat[E.x][E.y]==0){mat[E.x][E.y]=2;Qu.push(E);} 
    if(mat[W.x][W.y]==0){mat[W.x][W.y]=2;Qu.push(W);}    
    Qu.pop();
}


for(int i=0;i<n;++i){    
    for(int j=0;j<m;++j)
        cout<<mat[i][j]<<" ";        
    cout<<endl;
} 

Point is a struct I defined earlier in code and it contains only x and y as integers. The program fills the matrix correctly if it’s empty e.g. :
If i enter

3 3
0 0 0
0 0 0
0 0 0

I get the output :

2 2 2
2 2 2
2 2 2

But if i enter:

3 3
0 0 1
0 1 0
0 0 1

I get

2 2 1
2 1 2
2 2 1

instead of

2 2 1
2 1 0
2 2 1

If I check the coordinates after every pop, I can notice that it goes out of boundaries ( e.g. it returns the coordinates 1 -1 and it shouldn’t do it).

Answer 1

You set the coords of N,W,S and E, even if they are invalid; for example if you start at (0,0) and do this:

N.x=Qu.front().x-1; 
N.y=Qu.front().y;
...
if(mat[N.x][N.y]==0) {
    mat[N.x][N.y]=2;
    Qu.push(N);
} 

N will be (-1,0). Instead you only check a direction if you are not at the border of the matrix. For example for N you could do:

if(Qu.front().x > 0) {
    N.x=Qu.front().x-1; 
    N.y=Qu.front().y;
    if(mat[N.x][N.y]==0) {
        mat[N.x][N.y]=2;
        Qu.push(N);
    } 
}