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

Assuming your dataset is called dat, here's a couple of data.table solutions:

Method 1:

require(data.table) ## >= 1.9.2
setDT(dat)[, dValue := value[condition == "A"] - value, by=Sample]

Method 2:

require(data.table) ## >= 1.9.2
setkey(setDT(dat), Sample)
dat[dat[condition == "A"], dValue := i.value-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