How to cin values into a vector

I'm trying to ask the user to enter numbers that will be pushed into a vector, then using a function call to count these numbers.

why is this not working? I'm only able to count the first number.

template <typename T>
void write_vector(const vector<T>& V)
{
   cout << "The numbers in the vector are: " << endl;
  for(int i=0; i < V.size(); i++)
    cout << V[i] << " ";
}

int main()
{
  int input;
  vector<int> V;
  cout << "Enter your numbers to be evaluated: " << endl;
  cin >> input;
  V.push_back(input);
  write_vector(V);
  return 0;
}

Solution 1:

As is, you're only reading in a single integer and pushing it into your vector. Since you probably want to store several integers, you need a loop. E.g., replace

cin >> input;
V.push_back(input);

with

while (cin >> input)
    V.push_back(input);

What this does is continually pull in ints from cin for as long as there is input to grab; the loop continues until cin finds EOF or tries to input a non-integer value. The alternative is to use a sentinel value, though this prevents you from actually inputting that value. Ex:

while ((cin >> input) && input != 9999)
    V.push_back(input);

will read until you try to input 9999 (or any of the other states that render cin invalid), at which point the loop will terminate.

Solution 2:

Other answers would have you disallow a particular number, or tell the user to enter something non-numeric in order to terminate input. Perhaps a better solution is to use std::getline() to read a line of input, then use std::istringstream to read all of the numbers from that line into the vector.

#include <iostream>
#include <sstream>
#include <vector>

int main(int argc, char** argv) {

    std::string line;
    int number;
    std::vector<int> numbers;

    std::cout << "Enter numbers separated by spaces: ";
    std::getline(std::cin, line);
    std::istringstream stream(line);
    while (stream >> number)
        numbers.push_back(number);

    write_vector(numbers);

}

Also, your write_vector() implementation can be replaced with a more idiomatic call to the std::copy() algorithm to copy the elements to an std::ostream_iterator to std::cout:

#include <algorithm>
#include <iterator>

template<class T>
void write_vector(const std::vector<T>& vector) {
    std::cout << "Numbers you entered: ";
    std::copy(vector.begin(), vector.end(),
        std::ostream_iterator<T>(std::cout, " "));
    std::cout << '\n';
}

You can also use std::copy() and a couple of handy iterators to get the values into the vector without an explicit loop:

std::copy(std::istream_iterator<int>(stream),
    std::istream_iterator<int>(),
    std::back_inserter(numbers));

But that’s probably overkill.

Solution 3:

You need a loop for that. So do this:

while (cin >> input) //enter any non-integer to end the loop!
{
   V.push_back(input);
}

Or use this idiomatic version:

#include <iterator> //for std::istream_iterator 

std::istream_iterator<int> begin(std::cin), end;
std::vector<int> v(begin, end);
write_vector(v);

You could also improve your write_vector as:

 #include <algorithm> //for std::copy

template <typename T>
void write_vector(const vector<T>& v)
{
   cout << "The numbers in the vector are: " << endl;
   std::copy(v.begin(), v.end(), std::ostream_iterator<int>(std::cout, " "));
}