title: how to check if the input is a number or not in C?
body: <p>In the main function of C:</p>
  
  <pre><code>void main(int argc, char **argv)
  {
     // do something here
  }
  </code></pre>
  
  <p>In the command line, we will type any number for example <code>1</code> or <code>2</code> as input, but it will be treated as char array for the parameter of argv, but how to make sure the input is a number, in case people typed <code>hello</code> or <code>c</code>?</p>