title: How to change observation labels in R?
body: <p>Assume we have the following data frame df in R:  </p>
  
  <pre><code>          Var1        Var2   Var3        Var4          Var5    Var6
  1       0.00699        0.09  0.00000    -0.04546      0.18479   0.00609
  2       0.01198        0.65  0.00000     0.60825      0.18331   0.00240
  3       0.02098        0.19  0.00000    -0.21057      0.23096  -0.04050
  4      -0.01434        0.19  0.00000     0.57732      0.28627   0.01788
  5       0.00895        0.19  0.00001    -2.43361      0.05339   0.02511
  </code></pre>
  
  <p>There is no column peculiar to observation labels (years) in my data frame df, and for some reasons I do not want to create a column for this. The leftmost column in R (that is not part of df) has the values 1,2,3,4,5. I wanna change these labels to 2001,2002,...,2005. This way, without creating a column for observation points, I will be able to represent them. I really need this, searched in SOF, but not found an answer. Simply, I wanna change the above to:</p>
  
  <pre><code>              Var1        Var2   Var3        Var4          Var5    Var6
  2001       0.00699        0.09  0.00000    -0.04546      0.18479   0.00609
  ...........................................................................
  2005       0.00895        0.19  0.00001    -2.43361      0.05339   0.02511
  </code></pre>
  
  <p>Any idea?</p>