title: Update the nth element of the nth element in nested lists
body: <p>Lists of lists are kicking my butt. I've done a fair amount of research trying to find how to update the nth element of the nth list in a list of lists python data structure. I see a lot of advice on updating the nth element or combining lists into an existing list, but nothing about updating a particular element within a nested list.</p>
  
  <p>My actual code reads through a file searching for tid:somestring then adds tid:somestring to a set. I now want to loop through the set and make that value the 0th y for each occurrence. My next step in the code will add another search item to the 1st y for the corresponding tid value that I previously placed in the 0th y position (and so on, for the remaining search values). I plan to increment with a counter to move the index value for x...Essentially, I need update a two dimensional array. There are 9 values that will be on the y axis. The x axis is dependent on the number of unique instances of tid:somestring, which is 10,000 or more for each file.</p>
  
  <p>How do I update/insert or otherwise change the nth item of the nth list in my list of lists?</p>
  
  <pre><code>samlfile="2013-08-18 22:47:55,248 tid:b7c31fbac DEBUG" #I have a file with 10000+ "tid"
  tidset = set()
  tidcount = len(tidset)
  tidassert = [[0 for x in range(tidcount)] for y in range(9)]
  xcount = 0
  
  
  for line in samlfile:
  
      if "tid:" in line:
          str=line
          tid = re.search(r'(tid:.*?)(?= )', str)
          if tid.group() not in tidset:
              tidset.add(tid.group())
              tid = tid.group()
              for tid in tidset:
                  tidassert[xcount][0] = tid
              xcount = xcount + 1
  
          print tidassert
  </code></pre>