> mydata <- data.frame(x1 = c(2,2,6,4),x2 = c(3,4,2,8))
> mydata
  x1 x2
1  2  3
2  2  4
3  6  2
4  4  8
> mydata$sumx <- mydata$x1 + mydata$x2
> mydata
  x1 x2 sumx
1  2  3    5
2  2  4    6
3  6  2    8
4  4  8   12
> mydata$meanx <- (mydata$x1 + mydata$x2)/2
> mydata
  x1 x2 sumx meanx
1  2  3    5   2.5
2  2  4    6   3.0
3  6  2    8   4.0
4  4  8   12   6.0

> attach(mydata) ##解放mydata中的变量x1和x2，以便后续直接调用
The following object is masked _by_ .GlobalEnv:

    meanx

> mydata$sumx2 <- x1 + x2
> mydata
  x1 x2 sumx meanx sumx2
1  2  3    5   2.5     5
2  2  4    6   3.0     6
3  6  2    8   4.0     8
4  4  8   12   6.0    12
> mydata$meanx2 <-  (x1+2)/2
> mydata
  x1 x2 sumx meanx sumx2 meanx2
1  2  3    5   2.5     5      2
2  2  4    6   3.0     6      2
3  6  2    8   4.0     8      4
4  4  8   12   6.0    12      3
> detach(mydata)

> mydata <- transform(mydata,sumx3  = x1 + x2, meanx3 = (x1 + x2)/2) ##为啥可以在transform()中直接调用x1和x2呢？？
> mydata
  x1 x2 sumx meanx sumx2 meanx2 sumx3 meanx3
1  2  3    5   2.5     5      2     5    2.5
2  2  4    6   3.0     6      2     6    3.0
3  6  2    8   4.0     8      4     8    4.0
4  4  8   12   6.0    12      3    12    6.0