title: how to make a list of a lists of data frames into a list of data frames
body: <p>I have a list of lists of data frames (or values I guess) that I need to be put into the list-list-dataframe   format to the list-dataframe format.  Please see the simple example below.</p>
  
  <pre><code>library(plyr)
  
  my_function= function(df){
     means = mean(df$mpg)
     sds = sd(df$mpg)  
     # or
     #        means = mtcars *  rnorm(1)
      #       sds   = cars    * rnorm(1)
     output = list(MEAN = means, SD = sds)
     return(output) 
  }
  
  list_of_dfs=plyr::dlply(.data = mtcars, .variables ='cyl')
  output = lapply(list_of_dfs, my_function)
  </code></pre>
  
  <p>Now I would like to aggregate it so all the <code>SD</code> values for CYL=4/6/8 are in one data frame and the <code>MEAN</code> values in a second data frame.</p>
  
  <p>I've tried <code>do.call(rbind, output)</code> but I can't figure it out.</p>
  
  <p>Yes, I am aware I could do some sort of group-by here, <em>however</em>, this is just an example.   You could think of each value in the list as actual data frames rather than calculated output.</p>