title: concatenate two columns in pandas dataframe
body: <p>I have year and month column in my pandas data frame and i want to concatenate them by as a string so that i can convert them into datetime.</p>
  
  <p>df.info() gives</p>
  
  <pre><code>Data columns (total 3 columns):
  years         372 non-null object
  months        372 non-null object
  windSpeedM    371 non-null float64
  </code></pre>
  
  <p>and the data looks like</p>
  
  <pre><code>years  month windSpeedM
  1985    1   0.4
  </code></pre>
  
  <p>i am trying this</p>
  
  <pre><code>ff2['date'] = ff2['years']+'-'+ff2['months']+'-'+'01'
  </code></pre>
  
  <p>i am getting an error</p>
  
  <pre><code>TypeError: ufunc 'add' did not contain a loop with signature matching types dtype('S21') dtype('S21') dtype('S21')
  </code></pre>