title: simple apply to concat pandas-columns
body: <p>How can I make this program work? The result should be concatenated integers:</p>
  
  <pre><code>import pandas as pd
  d = pd.DataFrame({'A': range(5), 'B': range(5, 10)})
  f = lambda x, y: str(x) + str(y)
  d[['A', 'B']].apply(f, axis = 1)
  </code></pre>
  
  <p>Thx &amp; kind regards </p>