title: R programming - Normalizing a column of data by another entry in 2 other columns
body: <p>Folks,</p>
  
  <p>From the df as below, I would like to calculate the normalized value @ skew sk0. Sk0 is the central skew for a given group, there is one and only sk0 per group.  The number of cases per group is 1 or more. </p>
  
  <pre><code>group=c("g1", "g1", "g1", "g1", "g2", "g3", "g3", "g3", "g4", "g4")
  skew=c("sk0", "sk1", "sk2", "sk3", "sk0", "sk2", "sk0", "sk1", "sk1", "sk0")
  value=c(0.5, 0.3, 0.8, 1.0, 0.1, 0.4, 0.9, 0.7, 0.6, 0.2)
  df = data.frame(group, skew, value)
  </code></pre>
  
  <p>The desired result would look as below. valueNorm=value/sk0 of the group in question.
  For example. Rows 1-> 4 are of group g1. The central skew sk0 for group g1 is at row 1 and its value is 0.5. therefore, values for rowa 1-> 4 are going to be divided by 0.5         </p>
  
  <pre><code>      group skew  value         GroupSk0  valueNorm
  1     g1    sk0   0.5           0.5       1.00
  2     g1    sk1   0.3           0.5       0.60
  3     g1    sk2   0.8           0.5       1.60
  4     g1    sk3   1.0           0.5       2.00
  5     g2    sk0   0.1           0.1       1.00
  6     g3    sk2   0.4           0.9       0.44
  7     g3    sk0   0.9           0.9       1.00
  8     g3    sk1   0.7           0.9       0.78
  9     g4    sk1   0.6           0.2       3.00
  10    g4    sk0   0.2           0.2       1.00
  </code></pre>
  
  <p>Thx for your help !</p>