title: create data.frame from different number of rows
body: <p>There are objects with basic datas like <code>a&lt;-list(a=1,b="A",c=character())</code></p>
  
  <p>Now I want convert it to a data.frame, but there for I need equal rows. How to fill the empty vectors with NA in easy way to run <code>as.data.frame(a)</code>? the only Idea I have is to ask if one elment of the list has length&lt;1 then set <code>element[1]=NA</code>.</p>