title: How could I change string in a 2d list?
body: <p>In Python, how could I change a string in a 2d list to 'X'?</p>
  
  <p>This is the sample code.</p>
  
  <p>I want to change <code>'9'</code> to <code>'X'</code></p>
  
  <p>example: <code>cavityMap(['1112', '1912', '1892', '1234'])</code></p>
  
  <p>result: <code>1112, 1X12, 18X2, 1234</code></p>
  
  <p>this is my incomplete code:</p>
  
  <pre><code>def cavityMap(grid):
      t = '9'
  
      for i, j in enumerate(grid):
          if i == 0:
              continue
          if i == len(grid)-1:
              break
          j = list(j)
          for a, b in enumerate(j):
              if b == t:
                  pass
  
      return grid
  </code></pre>