Perform vector-based operation on data.frame by group

Question

I have seen some fairly similar questions answered here, however, my brain is having some trouble making the leap to apply what I read directly to my data.

I would like

Answer 1

One way is join on 'input' and then assign the value to 'output

library(data.table)
setDT(dfMeasured)[dfRef, output := output - i.output,on = .(input)]
dfMeasured
#    input output group
# 1:     0      0     A
# 2:     1     -2     A
# 3:     2      1     A
# 4:     3      0     A
# 5:     0     -2     B
# 6:     1     -4     B
# 7:     2     -3     B
# 8:     3      7     B
# 9:     0     -3     C
#10:     1     -3     C
#11:     2     -6     C
#12:     3      8     C