[R] I don't understand the 'order' function

[Duncan Murdoch]

On 16/04/2013 1:51 PM, Julio Sergio wrote:
> I thought I've understood the 'order' function, using simple examples like:
>
>     order(c(5,4,-2))
>     [1] 3 2 1
>
> However, I arrived to the following example:
>
>     order(c(2465, 2255, 2085, 1545, 1335, 1210, 920, 210, 210, 505, 1045))
>     [1]  8  9 10  7 11  6  5  4  3  2  1
>
> and I was completely perplexed!
> Shouldn't the output vector be  11 10 9 8 7 6 4 1 2 3 5 ?
> Do I have a damaged version of R?

You are probably confusing order() and rank().  What we want is that

x[order(x)]

is in increasing order.   This is the inverse permutation of what 
rank(x) gives, so (if there are no ties) rank(x)[order(x)]  and
order(x)[rank(x)] should both give 1:length(x).

Duncan


>
> I became still more astonished when I used the sort function and got the
> right answer:
>
>     sort(c(2465, 2255, 2085, 1545, 1335, 1210,  920,  210,  210,  505, 1045))
>     [1]  210  210  505  920 1045 1210 1335 1545 2085 2255 2465
> since 'sort' documentation claims to be using 'order' to establish the right
> order.
>
> Please help me to understand all this!
>
>    Thanks,
>
>    -Sergio.
>
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