title: Replace value in existing column .csv pandas
body: <p>Let's say I have a csv where a sample row looks like: <code>[' ', 1, 2, 3, 4, 5]</code> where <code></code> indicates an empty cell. I want to iterate through all of the rows in the .csv and replace all of the values in the first column for each row with another value, i.e. <code>[100, 1, 2, 3, 4, 5]</code>. How could this be done? It's also worth noting that the columns don't have labels (they were converted from an .xlsx).</p>
  
  <p>Currently, I'm trying this:</p>
  
  <pre><code>for i, row in test.iterrows():
      value = randomFunc(x, row)
  
      test.loc[test.index[i], 0] = value
  </code></pre>
  
  <p>But this adds a column at the end with the label 0.</p>