title: Using lapply to apply mean function over list of data frame
body: <p>suppose you have a list of n dataframe, for this case the iris base with two variables Petal.Width and Species, I want to use apply or lapply to calculate the average of the Petal.Width column.</p>
  
  <pre><code>df1 = iris[1:10,4:5]
  df2 = iris[11:20,4:5]
  ...
  df15 = iris[141:150,4,5]
  df = list(df1,df2,...,df15)
  </code></pre>
  
  <p>The result that I hope if I only had 2 dataframes would be the following</p>
  
  <pre><code>df = list(df1,df2)
  df = list(df1,df2)
  mean(df[[1]]$Petal.Width);mean(df[[2]]$Petal.Width)
  [1] 0.22
  [1] 0.25
  </code></pre>
  
  <p>Thanks</p>