title: Sparse Efficiency Warning while changing the column
body: <pre><code>def tdm_modify(feature_names,tdm):
      non_useful_words=['kill','stampede','trigger','cause','death','hospital'\
          ,'minister','said','told','say','injury','victim','report']
      indexes=[feature_names.index(word) for word in non_useful_words]
      for index in indexes:
          tdm[:,index]=0   
      return tdm
  </code></pre>
  
  <p>I want to manually set zero weights for some terms in tdm matrix. Using the above code I get the warning. I don't seem to understand why? Is there a better way to do this?</p>
  
  <pre><code>C:\Anaconda\lib\site-packages\scipy\sparse\compressed.py:730: SparseEfficiencyWarning: Changing the sparsity structure of a csr_matrix is expensive. lil_matrix is more efficient.
    SparseEfficiencyWarning)
  </code></pre>