[R] question about order

[Achim Zeileis]

On Mon, 28 Jan 2008, Waverley wrote:

> I have a data vector as following:
> > z
>  [1] 183.1370 201.9610 113.7250 140.7840 156.2750  42.1569  42.1569  42.1569
>  [9] 240.1960 308.4310  42.1569  42.1569  42.1569  42.1569  42.1569  42.1569
> [17]  42.1569  42.1569  42.1569  42.1569 279.8040  42.1569  42.1569
>
> when I sort, it gave me the right order
>
> > sort(z)
>  [1]  42.1569  42.1569  42.1569  42.1569  42.1569  42.1569  42.1569  42.1569
>  [9]  42.1569  42.1569  42.1569  42.1569  42.1569  42.1569  42.1569 113.7250
> [17] 140.7840 156.2750 183.1370 201.9610 240.1960 279.8040 308.4310
>
> BUT when I use the order, the returned index is strange and not right.
>  You can check the first 4 values.
> > order (z)
>  [1]  6  7  8 11 12 13 14 15 16 17 18 19 20 22 23  3  4  5  1  2  9 21 10

Seems ok to me. Maybe you are looking for rank()?

Order gives the ordering permutation, rank the inverse, i.e.:

R> z2 <- sort(z)
R> identical(z2, z[order(z)])
[1] TRUE
R> identical(z, z2[rank(z)])
[1] TRUE

hth,
Z

> I am not sure why R does not order it correctly when handling a vector
> with repetitive values.
>
> I use just the first 4 values of z, then it ordered correctly.
> > order (z[1:4])
> [1] 3 4 1 2
>
> Can someone help?  What is the problem here? Is this a R bug?  How to
> order when handling a vector with repetitive values?
>
> --
> Waverley @ Palo Alto
>
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