Subtraction within Groups using R

Question

I have been struggling to find the best way to do this subtraction within groups. I have data frame containing a long list of samples (Sample), each one treate

Answer 1

You can also do that with dplyr:

require(dplyr)

df %.% 
   group_by(Sample) %.% 
   mutate(dValue = value[condition == "A"] - value)

#  Sample condition value dValue
#1   var1         A    12      0
#2   var1         B    14     -2
#3   var1         C    15     -3
#4   var2         A    20      0
#5   var2         B    19      1
#6   var2         C    19      1
#7   var3         A    50      0
#8   var3         B    51     -1
#9   var3         C    48      2