title: C++ cin positive integers only
body: <p>This is my first time on Stackoverflow.
  I was making a program to find out MPG for a car. I was wondering how can I make the cin statement only accept positive integers only? and also, if you do enter a invalid input, can you reset it? I am not sure if that makes sense. I didn't have to do this for class. I was just curious on how to do it. Here is the code.</p>
  
  <pre><code>#include &lt;iostream&gt;
  using namespace std;
  
  int main()
  {
  double tank, miles, mpg;
  
  cout &lt;&lt; "Hello. This is a program that calculates the MPG ( Miles Per Gallon) for      your\n" ;
  cout &lt;&lt; "vehicle\n" &lt;&lt; endl;
  cout &lt;&lt; "Please enter how many gallons your vehicle can hold\n" &lt;&lt; endl;
  cin &gt;&gt; tank;
  cout &lt;&lt; endl;
  cout &lt;&lt; "Please enter how many miles that have been driven on a full tank\n" &lt;&lt;endl;
  cin &gt;&gt; miles;
  cout &lt;&lt; endl;
  
  mpg = (miles)/(tank);
  cout &lt;&lt; "Your vehicle recieves " &lt;&lt; mpg &lt;&lt; " miles per gallon\n" &lt;&lt; endl;
  system ("pause");
  return 0;
  }
  </code></pre>