title: how to find which row values are not changing a lot within a threshold
body: <p>I have a data which looks like below </p>
  
  <pre><code>df&lt;-structure(list(C1 = c(0.003926348, 0.001642442, 6.72e-05, 0.000314789, 
  0.00031372, 0.000196342, 0.01318432, 8.86e-05, 0.005671017, 0.003616196, 
  0.026635645, 0.001136402, 0.000161111, 0.005777738, 0.000145104, 
  0.000996546, 4.27e-05, 0.000114159, 0.001152384, 0.002860251, 
  0.000284873), C2 = c(0.003901373, 0.001526195, 6.3e-05, 0.000387266, 
  0.000312458, 0.000256647, 0.012489205, 0.00013071, 0.005196136, 
  0.003059834, 0.024624562, 0.001025486, 0.000144964, 0.005659078, 
  0.000105755, 0.000844871, 5.88e-05, 0.000118831, 0.000999354, 
  0.002153167, 0.000214486), T1 = c(0.003646894, 0.001484503, 4.93e-05, 
  0.00036715, 0.000333378, 0.000244199, 0.010286787, 6.48e-05, 
  0.006180042, 0.00387491, 0.025428464, 0.001075376, 0.000122088, 
  0.005448152, 0.000103301, 0.000974826, 4.32e-05, 0.000109876, 
  0.001030364, 0.002777244, 0.000221169), T2 = c(0.00050388, 0.001135969, 
  0.000113829, 2.14e-06, 0.00010293, 0.000315704, 0.01160593, 8.46e-05, 
  0.004495437, 0.003062559, 0.018662663, 0.002096675, 0.000214814, 
  0.002177849, 8.61e-05, 0.001057254, 3.27e-05, 0.000115822, 0.008133257, 
  0.021657018, 0.000261339), G1 = c(0.001496712, 0.001640965, 0.000129124, 
  3.02e-06, 0.000122839, 0.000305686, 0.01378774, 0.000199637, 
  0.00534668, 0.00300097, 0.023290941, 0.002595433, 0.000262479, 
  0.002926346, 0.000125655, 0.001302624, 4.89e-05, 0.000122862, 
  0.009851791, 0.017621282, 0.000197561), G2 = c(0.00114337, 0.001285636, 
  0.000122848, 2.46e-06, 9.1e-05, 0.000288897, 0.012288087, 0.000122286, 
  0.002575368, 0.002158011, 0.022008677, 0.002017026, 0.000241754, 
  0.003340175, 0.00013424, 0.001517655, 4.78e-05, 0.000110353, 
  0.008293286, 0.018999466, 0.000191129)), .Names = c("C1", "C2", 
  "T1", "T2", "G1", "G2"), row.names = c("A", "B", "C", "D", "E", 
  "F", "G", "H", "I", "J", "K", "L", "M", "N", "O", "P", "PP", 
  "TT", "EE", "FF", "AS"), class = "data.frame")
  </code></pre>
  
  <p>what I want to know is to find those rows that are higher or lower than average of the first two columns (row-wise)
  for example</p>
  
  <pre><code>   C1     C2    C3   C4
      A 1.2   1.3   1.6   1.9  
      B 1.2   1.0   0.1   0.2
  
  Average 1.2 +1.3 =1.25
  Average 1.6 +1.9 =1.75 
  </code></pre>
  
  <p>It means the A is true because the average of the first two row values are 0.5 different than the average of the second two values on the same row</p>
  
  <p>The same for the B</p>
  
  <pre><code>Average 1.2 +1.0 =1.1
  Average 0.1   0.2 = 0.15
  </code></pre>
  
  <p>It means that the B is also true</p>
  
  <p>I just want to compare each two columns based on the first two columns (in above data we have 3 columns) so we will have for the first two columns (1 and 2) versus column (3 and 4) and first two column (1 and 2) versus the last two pairs (5 and 6) </p>