[R] Subtraction with aggregate

[Gang Chen]

With the following data in data.frame:

subject   QM    emotion     yi
  s1   75.1017   neutral  -75.928276
  s2  -47.3512   neutral -178.295990
  s3  -68.9016   neutral -134.753906
  s1   17.2099  negative -104.168312
  s2  -53.1114  negative -182.373474
  s3  -33.0322  negative -137.420410

I can obtain the average between the two emotions with

mydata <- read.table('clipboard', header=TRUE)
aggregate(mydata[,c('yi', 'QM')], by=list(subject=mydata$subject), mean)

My question is, what is a nice way to get the difference between the
two emotions?

Thanks,
Gang