From: Robert Klemme <shortcutter@...>
Date: 2012-01-19T21:56:22+09:00
Subject: Re: Elegant expression of sorting

On Thu, Jan 19, 2012 at 11:49 AM, Garthy D
<garthy_lmkltybr@entropicsoftware.com> wrote:
> On 19/01/12 19:52, Robert Klemme wrote:

>> foo.sort! {|a,b|
>>  # Ascending, primary.
>>  (a.key0<=>  b.key0).nonzer0? ||
>>  # Descending, secondary.
>>  (b.key1<=>  a.key1).nonzero? ||
>>  # A tricky and expensive comparison, tertiary.
>>  a.magic(b)
>> }
>
>
> Very, very neat; and nice find on "nonzero?". :) My first thought was that
> "nonzero?" surely returns a boolean and this would break it- but it looks
> like "nonzero?" with numeric return was designed for this *exact* problem.
> :)

Right. :-)

> I was wondering if there was a nice way to drop the "rv" from my solution-
> this looks like the best way.
>
>
>> foo.sort_by {|a|
>>   [
>>   # Ascending, primary.
>>   a.key0,
>>   # Descending, secondary, only if numeric.
>>   -a.key1,
>>    # A tricky and expensive comparison, tertiary.
>>   a.create_magic_key
>>   ]
>> }
>
> I'd stumbled on sort_by initially, I suspect I'll use it when I can
> precompute the comparisons easily (this is sometimes the case, sometimes
> not).

Note, with #sort_by you do not precompute comparisons but you compute
a comparison key!  In the example it is an Array because Array#<=>
uses fields from the beginning to the end, i.e. sorts according to
fields with decreasing precedence (first pos 0, if that comparison
returns 0 field pos 1 etc.).

> I'm not at all familiar with the "enum.sort_by&YourClass::SORT_FOO" syntax
> at this point, so I might need to read up on that. I can at least partly
> guess from the context though.

It just passes the lambda as a block as if it the block was passed
directly.  Note, that technically behind the scenes the syntax foo(&x)
invokes x.to_proc which is also provided by Symbol:

irb(main):001:0> a = 5.times.to_a
=> [0, 1, 2, 3, 4]
irb(main):002:0> a.map {|x| x.to_s}
=> ["0", "1", "2", "3", "4"]
irb(main):003:0> a.map(&:to_s)
=> ["0", "1", "2", "3", "4"]

> Thankyou for your very detailed input on this one, excellent stuff! :)

You're welcome!

Kind regards

robert

-- 
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