title: gensim word2vec - array dimensions in updating with online word embedding
body: <p>Word2Vec from gensim 0.13.4.1 to update the word vectors on the fly does not work.</p>
  
  <pre><code>model.build_vocab(sentences, update=False)
  </code></pre>
  
  <p>works fine;  however, </p>
  
  <pre><code>model.build_vocab(sentences, update=True)
  </code></pre>
  
  <p>does not.</p>
  
  <hr>
  
  <p>I am using <a href="http://rutumulkar.com/blog/2015/word2vec" rel="noreferrer">this website</a> to try and emulate what they have done; hence I use the following script at some point:</p>
  
  <pre><code>model = gensim.models.Word2Vec()
  sentences = gensim.models.word2vec.LineSentence("./text8/text8")
  model.build_vocab(sentences, keep_raw_vocab=False, trim_rule=None, progress_per=10000, update=False)
  model.train(sentences)
  </code></pre>
  
  <p>However while this runs with <code>update=False</code>, using <code>update=True</code> gives me the following traceback:</p>
  
  <pre><code>Traceback (most recent call last):
    File "word2vecAttempt.py", line 34, in &lt;module&gt;
      model.build_vocab(sentences, progress_per=10000, update=True)
    File "/home/brownc/anaconda3/lib/python3.5/site-packages/gensim/models/word2vec.py", line 535, in build_vocab
      self.finalize_vocab(update=update)  # build tables &amp; arrays
    File "/home/brownc/anaconda3/lib/python3.5/site-packages/gensim/models/word2vec.py", line 708, in finalize_vocab
      self.update_weights()
    File "/home/brownc/anaconda3/lib/python3.5/site-packages/gensim/models/word2vec.py", line 1070, in update_weights
      self.wv.syn0 = vstack([self.wv.syn0, newsyn0])
    File "/home/brownc/anaconda3/lib/python3.5/site-packages/numpy/core/shape_base.py", line 230, in vstack
      return _nx.concatenate([atleast_2d(_m) for _m in tup], 0)
  ValueError: all the input array dimensions except for the concatenation axis must match exactly
  </code></pre>