Check out example codes for "declare vector of size n in c++". It will help you in understanding the concepts better.

Code Example 1

#include <vector>

auto n = 20
// create a vector with n=20 integer elements
std::vector<int> arr(n);

Code Example 2

// CPP program to initialize a vector like 
// an array. 
#include <bits/stdc++.h> 
using namespace std; 
  
int main() 
{ 
    vector<int> vect{ 10, 20, 30 }; 
  
    for (int x : vect) 
        cout << x << " "; 
  
    return 0; 
}

Code Example 3

// First include the vector library:
#include <vector>

// The syntax to create a vector looks like this:
std::vector<type> name;

// We can create & initialize "lol" vector with specific values:
std::vector<double> lol = {66.666, -420.69};

// it would look like this: 66.666 | -420.69

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