R help - Mar 2013 - How to obtain the original indices of elements after sorting

Dear All,

Suppose I have a vector X = (x_1,  x_2, ....,  x_n), X_sort = sort(X)
= (x_(1), x_(2), ... , x(n) ),
and I would like to know the original position of these ordered x_(i)
in X, how can I do it?

case 1: all values are unique
x <- c( 3, 5, 4, 6)
x.sort <- sort(x) #
# I would like to obtain a vector (1, 3, 2, 4) which indicates that 3
in x is still the 1st element in x.sort, 5 is at the 3rd position in
x.sort. etc.

case 2: some x_i's have the same value
x <- c(3, 3, 5, 5, 4, 4, 6, 6)
I would like to obtain a vector as (1, 2, 5, 6, 3, 4, 7, 8)

I do not want to use which and loop over the vector to do it since for
a long vector it is not fast enough.
Thank you for your suggestion.

Best wishes,
Jie

?order

     cheers,

         Rolf Turner

On 03/11/2013 02:24 PM, Jie wrote:
> Dear All,
>
> Suppose I have a vector X = (x_1,  x_2, ....,  x_n), X_sort = sort(X)
> = (x_(1), x_(2), ... , x(n) ),
> and I would like to know the original position of these ordered x_(i)
> in X, how can I do it?
>
> case 1: all values are unique
> x <- c( 3, 5, 4, 6)
> x.sort <- sort(x) #
> # I would like to obtain a vector (1, 3, 2, 4) which indicates that 3
> in x is still the 1st element in x.sort, 5 is at the 3rd position in
> x.sort. etc.
>
> case 2: some x_i's have the same value
> x <- c(3, 3, 5, 5, 4, 4, 6, 6)
> I would like to obtain a vector as (1, 2, 5, 6, 3, 4, 7, 8)
>
> I do not want to use which and loop over the vector to do it since for
> a long vector it is not fast enough.
> Thank you for your suggestion.

Hi,

The answer to your question is written on the help page.

?sort

HTH,
Pascal


On 11/03/13 10:24, Jie wrote:
> Dear All,
>
> Suppose I have a vector X = (x_1,  x_2, ....,  x_n), X_sort = sort(X)
> = (x_(1), x_(2), ... , x(n) ),
> and I would like to know the original position of these ordered x_(i)
> in X, how can I do it?
>
> case 1: all values are unique
> x <- c( 3, 5, 4, 6)
> x.sort <- sort(x) #
> # I would like to obtain a vector (1, 3, 2, 4) which indicates that 3
> in x is still the 1st element in x.sort, 5 is at the 3rd position in
> x.sort. etc.
>
> case 2: some x_i's have the same value
> x <- c(3, 3, 5, 5, 4, 4, 6, 6)
> I would like to obtain a vector as (1, 2, 5, 6, 3, 4, 7, 8)
>
> I do not want to use which and loop over the vector to do it since for
> a long vector it is not fast enough.
> Thank you for your suggestion.
>
> Best wishes,
> Jie
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

Hi
> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of Jie
> Sent: Monday, March 11, 2013 2:24 AM
> To: r-help
> Subject: [R] How to obtain the original indices of elements after
> sorting
> 
> Dear All,
> 
> Suppose I have a vector X = (x_1,  x_2, ....,  x_n), X_sort = sort(X) >
(x_(1), x_(2), ... , x(n) ), and I would like to know the original
> position of these ordered x_(i) in X, how can I do it?
> 
> case 1: all values are unique
> x <- c( 3, 5, 4, 6)
> x.sort <- sort(x) #
That is why order is included.
> x <- c( 3, 5, 4, 6)
> order(x)
[1] 1 3 2 4
>
I believe that it is mentioned in sort help page.

Regards
Petr
> # I would like to obtain a vector (1, 3, 2, 4) which indicates that 3
> in x is still the 1st element in x.sort, 5 is at the 3rd position in
> x.sort. etc.
> 
> case 2: some x_i's have the same value
> x <- c(3, 3, 5, 5, 4, 4, 6, 6)
> I would like to obtain a vector as (1, 2, 5, 6, 3, 4, 7, 8)
> 
> I do not want to use which and loop over the vector to do it since for
> a long vector it is not fast enough.
> Thank you for your suggestion.
> 
> Best wishes,
> Jie
> 
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-
> guide.html
> and provide commented, minimal, self-contained, reproducible code.