title: Subset columns based on row value
body: <p>This may be a simple question, but I haven't been able to find any answer. Consider you have a dataframe with n columns with molecular features. In the last row of each column, a coefficient of variance is expressed. </p>
  
  <p>Example data set: </p>
  
  <pre><code>a &lt;- data.frame(matrix(runif(30),ncol=3))
  b &lt;- c(50.23,45.23,21)
  a&lt;-rbind(a,b)
  
          X1          X2          X3
  1   0.1097075  0.78584027  0.20925033
  2   0.6081752  0.39669748  0.65559913
  3   0.9912855  0.68462073  0.54741795
  4   0.8543848  0.53776889  0.43789447
  5   0.2579654  0.92188090  0.61292895
  6   0.6203840  0.73152279  0.82866311
  7   0.6643195  0.84953926  0.62192976
  8   0.5760624  0.30949900  0.11032929
  9   0.8888167  0.04530598  0.08089825
  10  0.8926815  0.61736284  0.19834310
  11 50.2300000 45.23000000 21.00000000
  </code></pre>
  
  <p>How do I subset so I only get the columns with CV>50 in the last row? So my new data.frame would be:</p>
  
  <pre><code>X1   
  1   0.1097075
  2   0.6081752
  3   0.9912855
  4   0.8543848
  5   0.2579654
  6   0.6203840 
  7   0.6643195
  8   0.5760624
  9   0.8888167
  10  0.8926815
  11 50.230000
  </code></pre>