Unusual issue with vector use

Question

I was trying to write a program and I encountered some problem with vector. I have a logic where the values of the vector are to be compared at every iteration of the loop. So I initialize the vector and I push one element to each of the vector. Now since I have one element in both of these vectors, I should be able to compare and iterate as the following code should do, but it isn't. There might be a bug, but its difficult to find.

int M = 3; // I am passing values of M and N to this function
int N = 3; 
std::vector<int> X;
std::vector<int> Y;
X.push_back(1);
Y.push_back(1);
int x = 0;
for(int i = 1; i <= N; i++){
    for(int j = 1; j <= M; j++){
        if((X[x] <= i) && (Y[x] <= j)){
            if (x > 0){
                X.push_back(i);
                Y.push_back(j);
                x = x + 1;
            }
            else{
                X[0] = 1;
                Y[0] = 1;
                x = x + 1;
            }
        }
    }
}
for(int i = 0; i < X.size(); i++){
    cout << X[i] << "; " << Y[i] << endl;
}

I get the following output with the above program.

`1; 1`

But I need to produce this.

`1; 1`
`1; 2`
`2; 2`

Answer 1

You have undefined behavior.

When x is zero you don't push a new entry into the vectors, you do however increase x and the next iteration you access X[1] and Y[1] which is out of bounds.