title: String to Int without atoi/isdigit
body: <p>How can I convert a string to an array without using <code>atoi</code>, <code>atol</code>, <code>isdigit</code>, anything like that?</p>
  <p>Assume I have a <code>const char *str</code> and a <code>int *converted_val</code> as parameters.</p>
  <p>Here's what I have:</p>
  <pre><code>const char *c;
      for(c = str; (*c != '\0') &amp;&amp; isdigit(*c); ++c){
                      *converted_value = *converted_value*10 + *c - '0';
      }
      return true;
  </code></pre>
  <p>but once again, I can't do it without isdigit. And I'm not sure how to handle strings that are large (for instance: &quot;10000000000000000&quot;)</p>