title: UITextfield check if its a number
body: <p>How can I check if a user enters a number in my <code>UITextField</code>. Basically its a mark I'm accepting between 0-100 and it can be a decimal. I heard that I can try to convert it to a <code>NSNumber</code> type and see if it returns null. </p>
  
  <p>I have tried:</p>
  
  <pre><code>if(grade.text doubleValue] == 0){
     //Not a number
  }
  </code></pre>
  
  <p>but then it still can accept "23f".</p>