title: Subtract a constant from a column in a pandas dataframe
body: <p>I have a dataframe as follows:</p>
  
  <pre><code>year,value
  1970,2.0729729191557147
  1971,1.0184197388632872
  1972,2.574009084167593
  1973,1.4986879160266255
  1974,3.0246498975934464
  1975,1.7876222478238608
  1976,2.5631745148930913
  1977,2.444014336917563
  1978,2.619502688172043
  1979,2.268273809523809
  1980,2.6086169818316645
  1981,0.8452720174091145
  1982,1.3158922171018947
  1983,-0.12695212493599603
  1984,1.4374230626622169
  1985,2.389290834613415
  1986,2.3489311315924217
  1987,2.6002265745007676
  1988,1.2623717711036955
  1989,1.1793426779313878
  </code></pre>
  
  <p>I would like to subtract a constant from each of the values in the second column. This is the code I have tried:</p>
  
  <pre><code>df = pd.read_csv(f1, sep=",", header=0)
  df2 = df["value"].subtract(1)
  </code></pre>
  
  <p>However when I do this, df2 becomes this:</p>
  
  <pre><code>70    1.072973
  71    0.018420
  72    1.574009
  73    0.498688
  74    2.024650
  75    0.787622
  76    1.563175
  77    1.444014
  78    1.619503
  79    1.268274
  80    1.608617
  81   -0.154728
  82    0.315892
  83   -1.126952
  84    0.437423
  85    1.389291
  86    1.348931
  87    1.600227
  88    0.262372
  89    0.179343
  </code></pre>
  
  <p>The year becomes only the last two digits. How can I retain all of the digits of the year?</p>