title: How do I add a column to each data frame in a list
body: <p>Here's my setup</p>
  
  <pre><code>df1&lt;-data.frame(time=c(1,2,3),y=c(2,3,6))
  df2&lt;-data.frame(time=c(1,2,3),y=c(3,4,7))
  mylist&lt;-list(df1,df2)
  
  mylist
  [[1]]
    time y
  1    1 2
  2    2 3
  3    3 6
  
  [[2]]
    time y
  1    1 3
  2    2 4
  3    3 7
  </code></pre>
  
  <p>I would like to add a column, <code>ratio</code> to each dataframe, where it's the ratio of the value of y relative to the y value at time 1.  It would be equivalent to doing </p>
  
  <pre><code>mylist[[1]]$ratio&lt;-mylist[[1]]$y/mylist[[1]]$y[1]
  mylist[[2]]$ratio&lt;-mylist[[2]]$y/mylist[[2]]$y[1]
  
  mylist
  [[1]]
    time y ratio
  1    1 2   1.0
  2    2 3   1.5
  3    3 6   3.0
  
  [[2]]
    time y    ratio
  1    1 3 1.000000
  2    2 4 1.333333
  3    3 7 2.333333
  </code></pre>
  
  <p>Any suggestions on how to do this?</p>