title: Using "combn" to create list of dataframes for all combinations of a variable
body: <p>Using my dataframe below and working in R, I would like to generate a list of new dataframes in which each dataframe is based on a unique combination of 4 different values of my Taxon vector. In the dataframe below I have 10 different Taxon (many have multiple replicate entries) and I would like to select 4 Taxon at a time, for all combinations of 4. I believe that there should be 210 different combinations of 4 Taxon from these 10 Taxon (when sampled without replacement and order is not important). So ultimately I would like a list containing 210 dataframes, each containing a different combination of 4 Taxon (with all replicate rows for each Taxon)!!! </p>
  
  <p>Though selection is based on the Taxon vector, I would like the new dataframes to include the other columns of information. For instance, if "Aphididae.sp.3" is selected, i would like the new dataframe to also have -25.92(C), 1.69(N), sap(func.group), herbivore(trophic.group) listed in columns alongside. </p>
  
  <p>So far I used the "combn" function to generate all combinations of 4 taxon using the "unique" command, but I can't get all the other columns of information included in this, and it does not give all the replicate entries for each taxon! I'd be grateful for any help!</p>
  
  <p>my code:</p>
  
  <pre><code>combos&lt;-combn(unique(df$Taxon),4)
  </code></pre>
  
  <p>df:</p>
  
  <pre><code>                  Taxon      C    N     func.group trophic.grp
  1           Aphididae.sp.3 -25.92 1.69        sap   herbivore
  2           Aphididae.sp.3 -25.91 1.78        sap   herbivore
  3           Aphididae.sp.3 -26.05 1.74        sap   herbivore
  4  Cicadellidae.mixed.juvs -28.94 1.19        sap   herbivore
  5  Cicadellidae.mixed.juvs -29.25 2.24        sap   herbivore
  6  Cicadellidae.mixed.juvs -28.17 1.88        sap   herbivore
  7  Cicadellidae.mixed.juvs -28.29 1.94        sap   herbivore
  8        Cicadellidae.sp.1 -27.69 2.25        sap   herbivore
  9        Cicadellidae.sp.1 -27.67 2.41        sap   herbivore
  10       Cicadellidae.sp.1 -26.65 3.26        sap   herbivore
  11       Cicadellidae.sp.1 -28.30 3.20        sap   herbivore
  12       Cicadellidae.sp.1 -28.08 1.88        sap   herbivore
  13       Cicadellidae.sp.2 -26.59 2.89        sap   herbivore
  14       Cicadellidae.sp.3 -26.82 5.16        sap   herbivore
  15       Cicadellidae.sp.4 -26.54 3.46        sap   herbivore
  16       Cicadellidae.sp.4 -26.55 4.05        sap   herbivore
  17       Cicadellidae.sp.4 -27.20 3.14        sap   herbivore
  18       Cicadellidae.sp.4 -26.48 3.80        sap   herbivore
  19       Cicadellidae.sp.5 -27.54 4.17        sap   herbivore
  20       Cicadellidae.sp.5 -27.18 3.43        sap   herbivore
  21       Cicadellidae.sp.5 -27.46 4.03        sap   herbivore
  22       Cicadellidae.sp.6 -26.71 1.09        sap   herbivore
  23       Cicadellidae.sp.6 -26.33 1.56        sap   herbivore
  24       Cicadellidae.sp.6 -25.59 0.59        sap   herbivore
  25       Cicadellidae.sp.6 -25.07 0.84        sap   herbivore
  26       Cicadellidae.sp.6 -26.56 0.97        sap   herbivore
  27       Cicadellidae.sp.7 -25.84 1.08        sap   herbivore
  28       Cicadellidae.sp.7 -24.96 1.36        sap   herbivore
  29       Cicadellidae.sp.7 -26.15 1.90        sap   herbivore
  30       Cicadellidae.sp.7 -26.58 2.63        sap   herbivore
  31       Cicadellidae.sp.8 -28.02 2.28        sap   herbivore
  32       Cicadellidae.sp.8 -27.90 2.01        sap   herbivore
  33       Cicadellidae.sp.8 -27.70 1.92        sap   herbivore
  34       Cicadellidae.sp.8 -26.85 1.04        sap   herbivore
  </code></pre>