Работа с <vector>

#include <iostream> 
#include <vector> 
#include <cmath>
using namespace std; 
int main()
{
 
vector<int> vect; 
int x;
cout<<"Input collection size:";
cin>>x; 
for (int i=0; i < x; i++) 
 vect.push_back(1-2*sqrt(i+3)/sqrt(3*i+2)+4/3); 
cout<<"start:"<<endl;
for (int i=0; i < vect.size(); i++) 
 cout << vect.at(i) << ' '; 
cout << endl; 
int count=0; 
for (int i=0; i < vect.size(); i++) 
 if (vect.at(i)<=4)
 count+=i;
cout<<"result of \"round\" number: "<<count<<endl; 
for (int i=0; i < vect.size(); i++) 
 if (vect.at(i)<x)
 vect.at(i)=-1;
cout<<"result with values, changed to -1: "<<endl; 
for (int i=0; i < vect.size(); i++) 
 cout << vect.at(i) << ' '; 
cout << endl;
return 0;
}

#include <iostream>
#include <cmath>
#include <vector>
#include <iomanip>
 
using namespace std;
using vec_t = vector<double>;
double f(int index) noexcept;
 
int main()
{
    const int precision = 5;
    const int P=4;
    size_t N{};
    cout << "Enter amount of elements : "; cin >> N;
    vec_t vec(N);
    cout << "Original sequence : " << endl;
    for(int i=0; i<vec.size(); ++i){
        vec[i] = f(i);
        cout << setw(2) << i << ": " << fixed << setprecision(precision) << vec[i] << endl;
    }
    cout << "Summ of " << P << " elemnts : ";
    double summ{};
    for (int i=0; i<P; ++i)
        summ+=vec[i];
    double e{};
    cout << fixed << setprecision(precision) << summ << endl;
    cout << "Enter a number for condition \"E\" : "; cin >> e;
    for(int i=0; i<vec.size(); ++i)
        if (vec[i] < e)
            vec[i] = -1.;
    
    cout << "Vector after process : " << endl;
    for(int i=0; i<vec.size(); ++i)
        cout << setw(2) << i << ": " << fixed << setprecision(precision) << vec[i] << endl;
    return 0;
}
 
double f(int index) noexcept{
    return (1. - 2*sqrt(index+3))/(sqrt(index*3)+2) + 4.0/3.0;
}

vect.push_back((1 - 2) * sqrt(i + 3) / (sqrt(3 * i) + 2) + (4 / 3));