title: Complete dataframe for few samples, from a vector
body: <pre><code>DF=structure(list(Ech = c(4L, 4L, 4L, 4L, 4L, 13L, 13L, 13L, 13L, 
  13L), pos = c("BD", "BG", "HD", "n_BG", "n_HG", "BD", "HD", "HG", 
  "n_BG", "n_HG"), sm = c(5, 4, 2, 6, 1, 4, 1, 1, 2, 1)), .Names = c("Ech", 
  "pos", "sm"), row.names = c(NA, -10L), class = c("grouped_df", 
  "tbl_df", "tbl", "data.frame"), vars = "Ech", drop = TRUE, indices = list(
      0:4, 5:9), group_sizes = c(5L, 5L), biggest_group_size = 5L, labels = structure(list(
      Ech = c(4L, 13L)), row.names = c(NA, -2L), class = "data.frame", vars = "Ech", drop = TRUE, .Names = "Ech"))
  </code></pre>
  
  <p>The full list of pos value is : </p>
  
  <pre><code>column=c("BD","BG","HD","HG","n_BD","n_BG","n_HD","n_HG") 
  </code></pre>
  
  <p>these value shoulg be present for each <code>Ech</code>. I'm aiming to convert this dataframe:</p>
  
  <p>From</p>
  
  <pre><code>  Ech   pos    sm
     &lt;int&gt; &lt;chr&gt; &lt;dbl&gt;
   1     4    BD     5
   2     4    BG     4
   3     4    HD     2
   4     4  n_BG     6
   5     4  n_HG     1
   6    13    BD     4
   7    13    HD     1
   8    13    HG     1
   9    13  n_BG     2
  10    13  n_HG     1
  </code></pre>
  
  <p>to (complete</p>
  
  <pre><code>  Ech   pos    sm
     &lt;int&gt; &lt;chr&gt; &lt;dbl&gt;
   1     4    BD     5
   2     4    BG     4
   3     4    HD     2
   4     4    HG     0
   5     4  n_BD     0
   6     4  n_BG     6
   7     4  n_HD     0
   8     4  n_HG     1
   9     13   BD     4
   10    13   BG     0
   11    13   HD     1
   12    13   HG     1
   13    13 n_BD     0
   14    13 n_BG     2
   15    13 n_HD     0
   16    13 n_HG     1
  </code></pre>
  
  <p>So I used :</p>
  
  <pre><code>DF = DF %&gt;%
  complete(column=c("BD","BG","HD","HG","n_BD","n_BG","n_HD","n_HG"),nesting(pos,Ech))
  </code></pre>
  
  <blockquote>
    <p>Error :<code>by</code> can't contain join column <code>column</code> which is missing from RHS</p>
  </blockquote>
  
  <p>How to do this ?</p>