title: How to modify cells in a pandas DataFrame?
body: <p>I need to change individual elements in a DataFrame. I tried doing something like this, but it doesn't work:</p>
  
  <pre><code>for index, row in df.iterrows():
      if df.at[row, index] == 'something':
          df.at[row, index] = df.at[row, index] + 'add a string'
      else:
          df.at[row, index] = df.at[row, index] + 'add a value'
  </code></pre>
  
  <p>How can I do that?</p>