title: Simple way to Dask concatenate (horizontal, axis=1, columns)
body: <p><strong>Action</strong>
  Reading two csv (data.csv and label.csv) to a single dataframe.</p>
  
  <pre><code>df = dd.read_csv(data_files, delimiter=' ', header=None, names=['x', 'y', 'z', 'intensity', 'r', 'g', 'b'])
  df_label = dd.read_csv(label_files, delimiter=' ', header=None, names=['label'])
  </code></pre>
  
  <p><strong>Problem</strong>
  Concatenation of columns requires known divisions. However setting an index will sort the data, which I explicitly do not want, because order of both files is their match.</p>
  
  <pre><code>df = dd.concat([df, df_label], axis=1)
  ---------------------------------------------------------------------------
  ValueError                                Traceback (most recent call last)
  &lt;ipython-input-11-e6c2e1bdde55&gt; in &lt;module&gt;()
  ----&gt; 1 df = dd.concat([df, df_label], axis=1)
  
  /uhome/hemmest/.local/lib/python3.5/site-packages/dask/dataframe/multi.py in concat(dfs, axis, join, interleave_partitions)
      573             return concat_unindexed_dataframes(dfs)
      574         else:
  --&gt; 575             raise ValueError('Unable to concatenate DataFrame with unknown '
      576                              'division specifying axis=1')
      577     else:
  
  ValueError: Unable to concatenate DataFrame with unknown division specifying axis=1
  </code></pre>
  
  <p><strong>Tried</strong>
  Adding an <code>'id'</code> column</p>
  
  <pre><code>df['id'] = pd.Series(range(len(df)))
  </code></pre>
  
  <p>However, the length of Dataframe results in a Series larger than memory.</p>
  
  <p><strong>Question</strong>
  Apparently Dask knows both Dataframe have the same length:</p>
  
  <pre><code>In [15]:
  df.index.compute()
  Out[15]:
  Int64Index([      0,       1,       2,       3,       4,       5,       6,
                    7,       8,       9,
              ...
              1120910, 1120911, 1120912, 1120913, 1120914, 1120915, 1120916,
              1120917, 1120918, 1120919],
             dtype='int64', length=280994776)
  In [16]:
  df_label.index.compute()
  Out[16]:
  Int64Index([1, 5, 5, 2, 2, 2, 2, 2, 2, 2,
              ...
              3, 3, 3, 3, 3, 3, 3, 3, 3, 3],
             dtype='int64', length=280994776)
  </code></pre>
  
  <p>How to exploit this knowledge to simply concatenate?</p>