title: python pandas get first available datapoint of a year / calculate YTD return
body: <p>I need to calculate the year-to-date relative return of a given dataset. I usually caculate the cumulative relative return with this simple function:</p>
  
  <pre><code>def RelPerf(price):
      RelPerf = (price/price[0])
      return RelPerf  
  </code></pre>
  
  <p>The problem ist that I need to set instead of "price[0]" the price by the start of each year (first available datapoint of the year). Since the dataset does not contain data for each day of the year I can't simply use sth like +365. So the question is how do I get dynamically the location of the first available datapoint into the formula?</p>
  
  <p>This is a short example of the dataframe used:</p>
  
  <pre><code>              CLOSE_SPX    Close_iBoxx  A_Returns  B_Returns  A_Vola    B_Vola
  2014-05-15    1870.85      234.3017    -0.009362   0.003412   0.170535  0.075468   
  2014-05-16    1877.86      234.0216     0.003747  -0.001195   0.170153  0.075378
  2014-05-19    1885.08      233.7717     0.003845  -0.001068   0.170059  0.075384   
  2014-05-20    1872.83      234.2596    -0.006498   0.002087   0.170135  0.075410   
  2014-05-21    1888.03      233.9101     0.008116  -0.001492   0.169560  0.075326   
  2014-05-22    1892.49      233.5429     0.002362  -0.001570   0.169370  0.075341   
  2014-05-23    1900.53      233.8605     0.004248   0.001360   0.168716  0.075333   
  2014-05-27    1911.91      234.0368     0.005988   0.000754   0.168797  0.075294   
  2014-05-28    1909.78      235.4454    -0.001114   0.006019   0.168805  0.075474   
  2014-05-29    1920.03      235.1813     0.005367  -0.001122   0.168866  0.075451   
  2014-05-30    1923.57      235.2161     0.001844   0.000148   0.168844  0.075430   
  2014-06-02    1924.97      233.8868     0.000728  -0.005651   0.168528  0.075641   
  2014-06-03    1924.24      232.9049    -0.000379  -0.004198   0.167852  0.075267
  </code></pre>