title: Use of undeclared identifier 'cardNum'
body: <p>I'm using a function of the library CS50 on cs50's environment, and this is how you store a value on a variable gathering it with an input:</p>
  
  <pre><code>long i = get_long("Enter a long: ");
  </code></pre>
  
  <p>Here comes my code:</p>
  
  <pre><code>#include &lt;stdio.h&gt;
  #include &lt;cs50.h&gt;
  #include &lt;math.h&gt;
  
  int main(void)
  {
  
  do
  {
      long cardNum = get_long("Card number:\n");
  }
  while (0 &lt;= cardNum &lt;= 9999999999999999);
  
  
  if (340000000000000 &lt;= cardNum &lt;= 379999999999999)
  {
  </code></pre>
  
  <p>...and so on.</p>
  
  <p>The issue WAS</p>
  
  <blockquote>
    <p>use of undeclared identifier 'cardNum'</p>
  </blockquote>
  
  <p>I've added:</p>
  
  <pre><code>long cardNum;
  </code></pre>
  
  <p>BEFORE the do-while loop; PROBLEM SOLVED.</p>