title: C programming about "double" multiplication
body: <p>I'm currently learning the C language and I'm having trouble in the <code>double</code> multiplication topic.</p>
  
  <p>I need to to print the original value and then the <code>2*value</code> of the <code>double</code>.</p>
  
  <pre><code>double num = 34.39;
  printf("Original value = %d,   2x original value = %d", num, num*2);
  </code></pre>
  
  <p>How do I make it so that the 2x value will be really 2x the original value?</p>