title: I want to split a list of strings to a two dimensional array
body: <p>I have three sentences in a list:</p>
  
  <pre><code>sentences = []
  sentence.append("This is my first sentence.")
  sentence.append("This is my second sentence.")
  sentence.append("This is my third sentence.")
  </code></pre>
  
  <p>I need to split them, so the result should be:
      [['This', 'is', 'my', 'first', 'sentence.'], 
       ['This', 'is', 'my', 'second', 'sentence.'],
       ['This', 'is', 'my', 'third', 'sentence.']]</p>
  
  <p>I tried assign a new list as followed:</p>
  
  <pre><code>sentencesplit = []
  for i in range(0, 3):
     sentencesplit.extend(sentence[i].split())
  </code></pre>
  
  <p>The result is a one dimensional list containing all the splitted strings in the sentences. I have even tried to declare</p>
  
  <pre><code>sentencesplit[[]]
  for i in range(0, 3):
     sentencesplit[i].extend(sentence[i].split())
  </code></pre>
  
  <p>but this just result in the error message List index out of range.</p>
  
  <p>Any idea how to solve this?</p>