title: How to add and extend a new row in Python list?
body: <p>I have to reduce the density of an array, by using a for loop that traverses by steps of 100 and copy the value of my original array into a new array:</p>
  
  <p>soundDataHere is a <code>[7][22000]</code> dim array, and I want cleanSoundData to be a <code>[7][220]</code> dim array</p>
  
  <pre><code>def reduceDensity(soundDataHere):
      for i in range(numberOfFiles):
          for j in range(0, soundDataHere[i].size-1, 100):
              cleanSoundData.extend(soundDataHere[i][j])
  </code></pre>
  
  <p>I keep dont know how to use the append and extend function in a foor loop to recreate a new less dense array.</p>
  
  <p>example: <code>[[1,2,3,4,5],[6,7,8,9,10]]</code> with a <code>step = 2</code> 
  should return <code>[[1,3,5],[6,8,10]]</code> in my new cleanSoundData array</p>
  
  <p>but is only extending it like <code>[1,3,5,6,8,10]</code></p>