Question

Need help.. Please tell me why ” A.clear() ” does not clear the first column too?
………………………………………………..

#include <iostream>
#include <vector>
using namespace std;

int N, M;
vector< vector<int> > A;
int main()
{
    cin >> N >> M;
    A.resize(N + 2);
    for (int i = 0; i <= N; ++i)
    {
        A[i].resize(M + 2);
    }
    A.clear();
    for (int i = 0; i <= N; ++i)
    {
        for (int j = 0; j <= M; ++j)
        {
            cout << A[i][j] << ' ';
        }
        cout << '\n';
    }
    return 0;
}

Answer 1

clear removes all elements of the vector as opposed to setting all of them to 0 as you seem to be expecting. After calling clear the size of your vector is 0. Thus when you try to read A[i][j] you are accessing an index out of bounds and anything may happen(your code causes undefined behavior).