From: "David A. Black" <dblack@...>
Date: 2009-08-30T20:14:48+09:00
Subject: Re: Sorting an array by multiple elements?

Hi --

On Sun, 30 Aug 2009, Josh Cheek wrote:

> On Sun, Aug 30, 2009 at 1:30 AM, Paul <tester.paul@gmail.com> wrote:
>
>> Hi there, I have an array of arrays that I want to sort by multiple
>> elements.
>>
>> Sample data in the array looks like: [ [ id, date, num, name ], [ id,
>> date, num, name ], ... ]
>>
>> I need to sort by : (1) name, (2) date, and (3) id.
>>
>> I can sort by any one element in the array no problem using something
>> like :
>>> summary_data.sort! { |a,b| a[ 3 ] <=> b[ 3 ] }
>>
>> Unfortunately, I can't do this line more than once because it blows
>> away any previous sorting.
>>
>> I found a few pages on the internet that describe how to "sort an
>> array of Ruby objects by multiple class fields", however, I don't know
>> how to create a "class field".  I'm looking at an array, not a class.
>>
>> Is there a way to do this or do I need to convert my data to something
>> else so I can do what I need?
>>
>> Please help.
>>
>> Thanks.
>>
>>
>
> data = [  [ 1 , Time.now , 4, "barny" ] ,
>          [ 1 , Time.now , 1, "cliff" ] ,
>          [ 2 , Time.now , 3, "alfie" ] ,
>          [ 2 , Time.now , 2, "alfie" ] ]
>
> #you can assign the values of the arrays to specific names by grouping
> data.sort do |(a_id,a_date,a_num,a_name) , (b_id,b_date,b_num,b_name)|
>
>  first  =   a_id <=> b_id     #first condition
>  second =  a_num <=> b_num    #second condition
>
>  first.zero? ? second : first #if they are equal on first
>                               #then return second

You can also do:

   first.nonzero? || second

which I like the look of though I find myself having to kind of step
through it mentally to keep track....

I agree with botp, though, that it's a good case for #sort_by:

   data.sort_by {|id, date, num, name| [id, date, num] }


David

-- 
David A. Black / Ruby Power and Light, LLC / http://www.rubypal.com
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