title: Adding a new column to data (from excel file)
body: <p>I am working in RStudio. I have a data from an Excel file named "X12_5_3":</p>
  
  <pre><code>    P,     1,    2,    3
   5350,    17,   44,   50
  25831,   788,  890, 1409
   5594,   968,  218,  344
  10359,   212,  185,  306
  </code></pre>
  
  <p>I want to create a new column called <code>weight</code> and add values <code>0.4</code> for the first two rows, and <code>0.6</code> for the last two rows.</p>
  
  <p>My code for this moment is: </p>
  
  <pre><code>data&lt;- X12_5_3
  </code></pre>