title: Subtract paired column
body: <p>I have a data frame with 192 columns. I want to make a 96 column data frame by subtracting matching column based in pairing.  The pairing info is found in the data frame <code>Pairing</code> in the column <code>Match</code> . The column <code>Pos</code>  matches as substring the column names of my data frame that I want to subtract. </p>
  
  <p>How do I use the pairing info in <code>Pairing</code> to identify which column to subtract. </p>
  
  <pre><code>  &gt; Pairing
  Match              Pos
  Control_70         001_A01
  Control_56         001_A02
     Case_70         001_A03
     Case_56         001_A04
  Control_21         001_A05
     Case_21         001_A06
  
  
  &gt; head(matures.cpm.spike.batch[,1:6])
                001_A01_S1 001_A02_S2 001_A03_S3 001_A04_S4 001_A05_S5 001_A06_S6
  hsa-let-7a-5p  16.566813  11.415796  12.400252  22.701457   8.864882  20.442599
  hsa-let-7b-5p  15.574190  11.107133  12.196465  17.954547   8.527478  25.788286
  hsa-let-7c-5p   5.976763   4.372978   5.984685   9.821348   6.341252   7.480211
  hsa-let-7d-3p  16.508818  10.697730  11.001534  18.375286   7.583910  24.974774
  hsa-let-7d-5p  13.273824   5.134547   9.456675  11.567230   7.096485  13.294108
  hsa-let-7f-5p  13.900711   9.804384  11.481614  20.002110   7.878241  17.295909
  </code></pre>