the program ignores second loop - c++

Question

my program ignores second loop and I can't fill vector v2
vector<int> v1;
vector<int> v2;
int elem1,elem2;
cout<<"Insert v1: ";
while(cin>>elem1){
v1.push_back(elem1);
}
cout<<"Insert v2: ";
while(cin>>elem2){
v2.push_back(elem2);
}

Answer 1

cin keeps going until all output is done, your 2nd loop is never gonna get hit unless you break from your first loop somehow. I would recommend you have some sort of exit condition on the first loop (such as some input marker like 'DONE' or something and once you read that you should break).

Answer 2

I'm guessing that you were expecting the first loop to end when you hit enter or something. but that's not what you coded. You coded 'fill up v1 until there is no input left', so it's not surprising that nothing ever gets put in v2.
Perhaps you could try the following, this reads one line, and put the contents in v1, it then reads a second line and puts the contents in v2. It uses getline to read a single line of text, then put that line into a string stream where you can read from one number at a time.
#include <iostream>
#include <sstream>
#include <string>
#include <vector>
using namespace std;
void fill_vector_from_one_line(vector<int>& v)
{
string str;
if (getline(cin, str))
{
istringstream iss(str);
int elem;
while (iss >> elem)
v.push_back(elem);
}
}
int main()
{
cout << "Insert v1: ";
fill_vector_from_one_line(v1);
cout << "Insert v2: ";
fill_vector_from_one_line(v2);
}
Apologies for any errors, I haven't checked this.

Answer 3

Reset standard input. See below.
#include <iostream>
#include <vector>
int main()
{
std::vector<int> v1;
std::vector<int> v2;
int elem;
std::cout << "Insert v1: ";
while (std::cin >> elem) {
v1.push_back (elem);
}
// Reset std::cin.
std::cin.clear();
std::cin.seekg(0, std::ios::beg);
std::cout << "Insert v2: ";
while (std::cin >> elem) {
v2.push_back (elem);
}
// Just to show that it works.
std::cout << "\nv1 elements:\n";
for (unsigned int ii = 0; ii < v1.size(); ++ii) {
std::cout << "v1[" << ii << "] = " << v1[ii] << "\n";
}
std::cout << "\nv2 elements:\n";
for (unsigned int ii = 0; ii < v2.size(); ++ii) {
std::cout << "v2[" << ii << "] = " << v2[ii] << "\n";
}
return 0;
}
Note that the std::cin.clear(); std:cin.seekg(); calls will have different behaviors depending on the nature of std::cin.
If std::cin is tied to the terminal it will have exactly the behavior that the OP is seeking.
If std::cin is reading from a file, vector v2 will be a carbon copy of vector v1.
If std::cin is reading from a pipe, vector v2 will be empty. You can't seekg() on a pipeline.