From: Rob Biedenharn <Rob@...>
Date: 2007-10-18T01:04:30+09:00
Subject: Re: sort_by

On Oct 17, 2007, at 11:12 AM, Paulo Carvalho wrote:
> Farrel Lifson wrote:
>> On 17/10/2007, Paulo Carvalho <pjcarvalho@gmail.com> wrote:
>>> I have a list of rows which I want to order by a column.
>>> I make it with
>>> @values_ordered = @values.sort_by(&:id_column)
>>>
>>> What if I want to inverse the order (like order by x desc in SQL)
>>>
>>> How could I do that using sort_by? is that possible?
>>
>> values_orderd = @values.sort_by{|value| -value.id_column}
>
> Hello
>
> Thanks for your quick answer.
>
> However i am having a problem.
>
> I used the syntax that you give to me like this :
>
> To order (ASC way):
> values_ordered = @demandes.sort_by{|value| value.eluxid}
> (This one works fine)
>
> To order (DESC way)
> values_ordered = @demandes.sort_by{|value| -value.eluxid}
> This one send to me a syntax error (I think he dont like the '-'):
> undefined method `-@' for ["3CC11A"]:Array
>
> (note: "3CC11A" is the column value of my first row)
>
> Any idea of where is the problem?
>
> Thanks again
>
> --  
> Posted via http://www.ruby-forum.com/.

If you don't have a way of giving your sortable value an inverse, you  
may have to use .sort_by{|v|v.eluxid}.reverse or just use .sort {| 
a,b| b.eluxid <=> a.eluxid } (note the swapping of the left- and  
right-hand sides).

When @demandes.size is "small enough" it's not necessarily faster to  
use sort_by than just sort (particularly if computing a DESC version  
of the sort key is "expensive").

-Rob

Rob Biedenharn		http://agileconsultingllc.com
Rob@AgileConsultingLLC.com