title: Concatenate DataFrames.DataFrame in Julia
body: <p>I have a problem when I try to concatenate multiple <code>DataFrames</code> (a datastructure from the <code>DataFrames</code> package!) with the same columns but different row numbers. Here's my code:</p>
  
  <pre><code>using(DataFrames)
  DF = DataFrame()
  DF[:x1]   = 1:1000
  DF[:x2]   = rand(1000)
  DF[:time] = append!( [0] , cumsum( diff(DF[:x1]).&lt;0 ) ) + 1
  
  
  
  DF1 = DF[DF[:time] .==1,:]
  DF2 = DF[DF[:time] .==round(maximum(DF[:time])),:]
  DF3 = DF[DF[:time] .==round(maximum(DF[:time])/4),:]
  DF4 = DF[DF[:time] .==round(maximum(DF[:time])/2),:]
  
  DF1[:T] = "initial"
  DF2[:T] = "final"
  DF3[:T] = "1/4"
  DF4[:T] = "1/2"
  
  DF = [DF1;DF2;DF3;DF4]
  </code></pre>
  
  <p>The last line gives me the error </p>
  
  <pre><code>MethodError: Cannot `convert` an object of type DataFrames.DataFrame to an object of type LastMain.LastMain.LastMain.DataFrames.AbstractDataFrame
  This may have arisen from a call to the constructor LastMain.LastMain.LastMain.DataFrames.AbstractDataFrame(...),
  since type constructors fall back to convert methods.
  </code></pre>
  
  <p>I don't understand this error message. Can you help me out? Thanks!</p>