title: Evaluation of a data set with conditional selection of columns
body: <p>I want to evaluate a data set with precipitation data. The data is available as a csv file, which I have read in with pandas as dataframe. From this then follows the following table:</p>
  <pre><code>year  month  day      value
  0      1981      1    1   0.522592
  1      1981      1    2   2.692495
  2      1981      1    3   0.556698
  3      1981      1    4   0.000000
  4      1981      1    5   0.000000
  ...     ...    ...  ...        ...
  43824  2100     12   27   0.000000
  43825  2100     12   28   0.185120
  43826  2100     12   29  10.252080
  43827  2100     12   30  13.389290
  43828  2100     12   31   3.523566
  </code></pre>
  <p>Now I want to convert the daily precipitation values into monthly precipitation values and that for each month (for this I would need the sum of each day of a month). For this I probably need a loop or something similar. However, I do not know how to proceed. Maybe via a conditional selection over 'year' and 'month'?!
  I would be very happy about feedback! :)</p>
  <p>That´s what I tried now:</p>
  <pre><code>for i in range(len(dataframe)):
      print(dataframe.loc[i, 'year'], dataframe.loc[i, 'month'])
  </code></pre>