title: Add a new column on multiple dataframes using mutate
body: <p>I am a new user in R.</p>
  
  <p>I have a bunch of dataframe like this:</p>
  
  <pre><code> Date         ID    Value conversion
  1 2018-07-16  123450  617     10
  2 2018-07-23  123450  476     20
  3 2018-07-30  123450  44.6     30
  4 2018-08-06  123450  248     10  
  5 2018-08-13  123450  177     40 
  </code></pre>
  
  <p>All the dataframes have the name pattern like this:</p>
  
  <pre><code>df_1
  df_2
  df_3
  </code></pre>
  
  <p>I will need to add a column to calculate the "weight of the conversion", which means that this column is dividing the conversion in each row by the aggregated total sum of the conversions column (which is 100 in the example above). My intention is to calculate the weighted Standard deviation of each dataframe later.
  So, ideally, the result should looks like this, and hopefully the outcome can be exported as dataframe in the environment as well (can also replace the original one):</p>
  
  <pre><code> Date         ID    Value conversion  weight
  1 2018-07-16 123450  617     10        0.1
  2 2018-07-23 123450  476     20        0.2
  3 2018-07-30 123450  44.6    30       0.3
  4 2018-08-06 123450  248     10        0.1
  5 2018-08-13 123450  177     30        0.3
  </code></pre>
  
  <p>I've tried to reproduce the results from <a href="https://stackoverflow.com/questions/57109390/apply-changes-group-by-on-multiple-dataframes-using-for-loop">Apply changes (group by) on multiple dataframes using for loop</a></p>
  
  <p>but I received the error: "no applicable method for 'mutate_' applied to an object of class list". But wasnt sure how I can do this using dplyr or for loop or dataframe list..</p>
  
  <p>Thank you!</p>
  
  <p><strong>EDIT</strong></p>
  
  <p>When I have my result (a list of dataframes), I did this:
  <code>list2env(list ,envir=.GlobalEnv)</code>
  however, I have the error:</p>
  
  <pre><code>  names(x) must be a character vector of the same length as x 
  </code></pre>
  
  <p>Does anyone know how can I resolve this?
  Thanks a lot!</p>