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

Try this

> transform(df, dValue=ave(value, Sample, FUN=function(x) c(0, x[1]-x[-1])))
  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

I'm assuming df looks like this

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