title: How do I overwrite the value of a specific index/column in a DataFrame?
body: <p>I have a dataframe <code>Exposure</code> with zeros constructed as follows:</p>
  
  <pre><code>Exposure = pd.DataFrame(0, index=dates, columns=tickers)
  </code></pre>
  
  <p>and a DataFrame <code>df</code> with data.</p>
  
  <p>I want to fill some of the data from <code>df</code> to <code>Exposure</code>:</p>
  
  <pre><code>    for index, column in df.iterrows():
          # value of df(index,column) to be filled at Exposure(index,column)
  </code></pre>
  
  <p>How do I overwrite the value of at <code>(index,column)</code> of <code>Exposure</code> with the value of <code>df(index,column)</code>?</p>