[R] why order doesn't work?

[cowboy]

hi all,
I want to get a cumsum according to the order of some variable.
However, it doesnt' work.
For example,
**********************
test<-data.frame(cbind(x=c(3,5,2,6,7),y=c(8,1,4,9,0)))
test[order(test$x),]$sumy<-cumsum(test[order(test$x),]$y)
**********************
R complians Warning message:
In `[<-.data.frame`(`*tmp*`, order(test$x), , value = list(x = c(2,  :
  provided 3 variables to replace 2 variables.

while the following
***********************
test$sumy<-cumsum(test[order(test$x),]$y)
******************
gives
  x y sumy
1 3 8    4
2 5 1   12
3 2 4   13
4 6 9   22
5 7 0   22

should it gives

  x y sumy
1 3 8    12
2 5 1   13
3 2 4   4
4 6 9   22
5 7 0   22

What am I missing here?
thanks,
Guang