*Richard M. Heiberger* ,
  You have showed a detailed informaton on order, but it seems that there's
a little difference between my task and your suggestions.
I'd like to give a clear example to show my task:
x <-  c(2, 9, 18, 3, 2)
y <-  c(2,9,8,9,8)
z <-  c(21,5,5,19,7)
a <-  cbind(x, y, z)  #dataset
a
      x y  z
[1,]  2 2 21
[2,]  9 9  5
[3,] 18 8  5
[4,]  3 9 19
[5,]  2 8  7
aa <- a[order(a[,"y"], decreasing=TRUE),]
aaa <- aa[order(aa[,"x"], decreasing=FALSE),]
aaa
    x y  z
[1,]  2 8  7
[2,]  2 2 21
[3,]  3 9 19
[4,]  9 9  5
[5,] 18 8  5
But i hope to get the following results:



      x y  z

[1,]  3 9 19
[2,]  9 9  5
[3,]  2 8  7
[4,] 18 8  5
[5,]  2 2 21
*That's to say to sort the dataset with decreasing y and ascending x
simultaneously*.
BTW,  what's the main difference between order() and sort()? I can't find
some simple introduction on their differnce.
Thanks very much!

On 10/15/06, Richard M. Heiberger <rmh@temple.edu> wrote:
>
> > ?order
> > 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
> > aa <- a[order(a[,"y"], decreasing=TRUE),]
> > aaa <- aa[order(aa[,"x"], decreasing=FALSE),]
> > aaa
>      x   y  z
> [1,]  2 8.0  7
> [2,]  2 2.0 21
> [3,]  3 9.0 19
> [4,]  9 5.6  5
> [5,] 18 5.0  5
> >
>
> a$y doesn't work because $ subscripting requires a data.frame.
> cbind creates an ordinary matrix.  This works with a data.frame.
> > a <- data.frame(x, y, z)
> > aa <- a[order(a$y, decreasing=TRUE),]
> > aaa <- aa[order(aa$x, decreasing=FALSE),]
>
> Please use spaces for legibility on both sides of the assignment
> arrow and after a comma.
>
> If you want all columns ascending (or descending), then you could do it in
> one step
> > aaaa <- a[order(a$x, a$y), ]
>
> See also the example in ?order
> ## Suppose we wanted descending order on y. A simple solution is
> rbind(x,y,z)[, order(x, -y, z)]
>



-- 
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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