Dear friends,
  I want to sort a dataset according to one or two variables in the dataset,
i thought sort could do it , but failed.

e.g.
x <- c(2, 9, 18, 3, 2)
y<-c(2,5.6,5,9,8)
z<-c(21,5,5,19,7)
a<-cbind(x,y,z)
a
      x  y   z
[1,]  2 2.0 21
[2,]  9 5.6  5
[3,] 18 5.0  5
[4,]  3 9.0 19
[5,]  2 8.0  7
I want to sort dataset a according to ascending x and descending y, How to
accomplish it?
BTW,  sort(a$x)  can't works and it shows"null", why is it?
Any suggestions are welcome!


-- 
With Kind Regards,

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Zhi Jie,Zhang ,PHD
Tel:86-21-54237149   epistat@gmail.com
Dept. of Epidemiology,school of public health,Fudan University
Address:No. 138 Yi Xue Yuan Road,Shanghai,China
Postcode:200032
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