From: Jan Svitok <jan.svitok@...>
Date: 2007-01-10T00:26:27+09:00
Subject: Re: sortung a structured array on more than one criteria

On 1/9/07, dblack@wobblini.net <dblack@wobblini.net> wrote:
> Hi --
> On Wed, 10 Jan 2007, Josselin wrote:
>
> > On 2007-01-09 15:53:13 +0100, "Jan Svitok" <jan.svitok@gmail.com> said:
> >
> >> On 1/9/07, Josselin <josselin@wanadoo.fr> wrote:
> >>> On 2007-01-09 15:29:43 +0100, Josselin <josselin@wanadoo.fr> said:
> >>>
> >>> > I can sort an array like that :
> >>> >
> >>> > results = rs.sort_by{|p| p.km}
> >>> >
> >>> > but I tried to add a scrond sorting criteria like that :
> >>> >
> >>> > results = rs.sort_by{|p| p.km, p.updated_at }
> >>> >
> >>> > and I got obviously an error ...   parse error, unexpected '}',
> >>> > expecting tCOLON2 or '[' or '.'
> >>> >
> >>> > canot find the correct writing..
> >>> >
> >>> > appreciate any help...
> >>> >
> >>> > thanks
> >>> >
> >>> > joss
> >>>
> >>> thanks to both of you... and HNY%2007    (Dry version of Happy New Year
> >>> 2007)   ;-))
> >>>
> >>> I forgot to ask about a DESC sort....    p.km is Ascending and
> >>> p.updated_at should be DESC..
> >>>
> >>> I am not sure that is possible (at least not documented.....)
> >>>
> >>> joss
> >>
> >> results = rs.sort_by{|p| p.km, -p.updated_at } # if updated_at is Numeric
> >>
> >> or
> >>
> >> results = rs.sort { |a,b| [a.km, b.updated_at] <=> [b.km, a.updated_at] }
> >
> > thanks ,  'updated_at'  is  a string  ( "2007-01-04 13:48:30")
> > I try to understand the writing...  why the first parameter will be Ascending
> > and the second Descending  ...
> >
> > how would you write it, if both parameters  shoudl be Descending ?
>
> results = rs.sort_by {...}.reverse

Ok, two things:

1. how does my code works:

The block in sort {|x,y| } implements x<=>y (<=>, comparison operator,
sometimes called starship operator, returns -1, 0 or 1 for x <y, x==y
and x> y)

[a,b] <=> [c,d] (array comparision) is implemented as first doing
a<=>c and if they are equal doing b<=>d. if I exchange b and d, I
effectively reverse the result of the second comparision.

If I wanted the reverse sort, I could exchange both pairs, i.e.
instead of doing x<=>y I'd be doing y <=> x.

2. sort_by is usually faster than sort because it does n block
evaluations, while sort does O(n*n) evaluations. Therefore sort_by
{}.reverse is usually better than sort{|a,b| b<=> a} (except some edge
cases where computaton is fast, and the block's result is large)