title: Appending arrays to dataframe (python)
body: <p>So I ran a time series model on a small sales data set, and forecasted sales for next 12 periods. With the following code:</p>
  
  <pre><code> mod1=ARIMA(df1, order=(2,1,1)).fit(disp=0,transparams=True)
      y_future=mod1.forecast(steps=12)[0]
  </code></pre>
  
  <p>where df1 contains the sales values with months being the index. Now I'm storing the predicted values in the following manner:</p>
  
  <pre><code>pred.append(y_future)
  </code></pre>
  
  <p>Now, I need to append the forecasted values to the original dataset df1, preferably with the same index. I'm trying to use the following code:</p>
  
  <pre><code>df1.append(pred, ignore_index=False)
  </code></pre>
  
  <p>But I'm getting the following error:</p>
  
  <pre><code>TypeError: cannot concatenate a non-NDFrame object
  </code></pre>
  
  <p>I've tried converting pred variable to list and then appending, but to no avail.
  Any help will be appreciated. Thanks.</p>