title: Merge unequal dataframes and replace missing rows with 0
body: <p>I have two data.frames, one with only characters and the other one with characters and values.</p>
  
  <pre><code>df1 = data.frame(x=c('a', 'b', 'c', 'd', 'e'))
  df2 = data.frame(x=c('a', 'b', 'c'),y = c(0,1,0))
  merge(df1, df2)
    x y
  1 a 0
  2 b 1
  3 c 0 
  </code></pre>
  
  <p>I want to merge df1 and df2. The characters a, b and c merged good and also have 0, 1, 0 but d and e has nothing. I want d and e also in the merge table, with the 0 0 condition. Thus for every missing row at the df2 data.frame, the 0 must be placed in the df1 table, like:</p>
  
  <pre><code>  x y
  1 a 0
  2 b 1
  3 c 0
  4 d 0
  5 e 0
  </code></pre>