如何在C++中访问向量元素的向量？

Question

#include<bits/stdc++.h>
#include<vector>
using namespace std;
int main(){
    
    int n=5;
    vector<vector<int>> v;
    v[0][0]=1;
    v[1][0]=1;
    v[1][1]=1;
    cout<<2;    //not getting output 2

}

In the above code I am trying to make a dynamic 2D array but the thing is that I am not getting any error but the number 2 is also not getting printed. What may be the reason?

Answer 1

The default constructor of vector creates an empty vector. You are accessing vector elements beyond the vector size which is Undefined Behavior.

You can initialize the vector with initializer lists:

std::vector<std::vector> v{
   {1},
   {1, 1}
};