title: Why isdigit() is not working properly in C
body: <p>Why is <code>isdigit()</code> printing <code>0</code> (false) for both the integer and the character?</p>
  <p>Here is the code:</p>
  <pre><code>#include &lt;stdio.h&gt;
  #include &lt;ctype.h&gt;
  
  int main( void )
  {
      printf( &quot;with input of integer, output is %d\n&quot;, isdigit(1) );
      printf( &quot;with input of char, output is %d\n&quot;, isdigit('k') );
  }
  </code></pre>
  <p>Here is the output:</p>
  <pre><code>with input of integer, output is 0
  with input of char, output is 0
  </code></pre>