title: insert values into column in R data.frame
body: <p>I am really new to R and came accros the following problem.
  My dataframe looks like that.</p>
  
  <pre><code>    &gt;   Entity  Substance conc..mM. variable value
      1     STD5    Sulfate     5.000     Area 70761
      2     STD5    Formate     5.000     Area    NA
      3     STD5    Acetate     5.000     Area 70777
      4     STD5    Lactate     5.000     Area 74329
      5     STD5 Propionate     5.000     Area 84198
      6     STD5   Butyrate     5.000     Area 91598
      7     STD5  Valeriate        NA     Area 40609
      8     STD5   Caproate        NA     Area 40763
      9   STD2_5    Sulfate     2.500     Area 34350
      10  STD2_5    Formate     2.500     Area    NA
      11  STD2_5    Acetate     2.500     Area 34863
      12  STD2_5    Lactate     2.500     Area 36262
      13  STD2_5 Propionate     2.500     Area 41849
      14  STD2_5   Butyrate     2.500     Area 44574
      15  STD2_5  Valeriate     2.500     Area 39664
      16  STD2_5   Caproate     2.500     Area 39867
  </code></pre>
  
  <p>Now I wanted to add a column where each row of a givent entity contains the value of caproate from the respective entity. 
  Is there a smart way to achieve this?</p>
  
  <p>Expected output</p>
  
  <pre><code>    &gt;   Entity  Substance conc..mM. variable value area caproate
      1     STD5    Sulfate     5.000     Area 70761   40763
      2     STD5    Formate     5.000     Area    NA   40763 
      3     STD5    Acetate     5.000     Area 70777   40763
      4     STD5    Lactate     5.000     Area 74329   40763
      5     STD5 Propionate     5.000     Area 84198   40763
      6     STD5   Butyrate     5.000     Area 91598   40763
      7     STD5  Valeriate        NA     Area 40609   40763
      8     STD5   Caproate        NA     Area 40763   40763
      9   STD2_5    Sulfate     2.500     Area 34350   39867
      10  STD2_5    Formate     2.500     Area    NA   39867
      11  STD2_5    Acetate     2.500     Area 34863   39867
      12  STD2_5    Lactate     2.500     Area 36262   39867
      13  STD2_5 Propionate     2.500     Area 41849   39867
      14  STD2_5   Butyrate     2.500     Area 44574   39867
      15  STD2_5  Valeriate     2.500     Area 39664   39867
      16  STD2_5   Caproate     2.500     Area 39867   39867
  </code></pre>