title: Julia DataFrames.jl - filter data with NA's (NAException)
body: <p>I am not sure how to handle <code>NA</code> within Julia DataFrames. </p>
  
  <p>For example with the following DataFrame:</p>
  
  <pre><code>&gt; import DataFrames
  &gt; a = DataFrames.@data([1, 2, 3, 4, 5]);
  &gt; b = DataFrames.@data([3, 4, 5, 6, NA]);
  &gt; ndf = DataFrames.DataFrame(a=a, b=b)
  </code></pre>
  
  <p>I can successfully execute the following operation on column <code>:a</code></p>
  
  <pre><code>&gt; ndf[ndf[:a] .== 4, :]
  </code></pre>
  
  <p>but if I try the same operation on <code>:b</code> I get an error <code>NAException("cannot index an array with a DataArray containing NA values")</code>. </p>
  
  <pre><code>&gt; ndf[ndf[:b] .== 4, :]
  
  NAException("cannot index an array with a DataArray containing NA values")
  while loading In[108], in expression starting on line 1
  
  in to_index at /Users/abisen/.julia/v0.3/DataArrays/src/indexing.jl:85
  in getindex at /Users/abisen/.julia/v0.3/DataArrays/src/indexing.jl:210
  in getindex at /Users/abisen/.julia/v0.3/DataFrames/src/dataframe/dataframe.jl:268
  </code></pre>
  
  <p>Which is because of the presence of NA value. </p>
  
  <p>My question is how should DataFrames with <code>NA</code> should typically be handled? I can understand that <code>&gt;</code> or <code>&lt;</code> operation against <code>NA</code> would be <code>undefined</code> but <code>==</code> should work (no?).</p>