From: Josh Cheek <josh.cheek@...>
Date: 2013-05-19T21:16:24+09:00
Subject: Re: Need help to understand the Enumerable methods

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On Sun, May 19, 2013 at 4:03 AM, Love U Ruby <lists@ruby-forum.com> wrote:

> I am very eager to know about the Enum#sort{|a,b|..}
>
> http://www.ruby-doc.org/core-2.0/Enumerable.html#method-i-sort
>
> Now my question is about the internal mechanism.
>
> ***Returns an array containing the items in enum sorted, either
> according to their own <=> method, or by using the results of the
> supplied block. The block should return -1, 0, or +1 depending on the
> comparison between a and b.**
>
> I took the below as an example:
>
> dogs = [
>   { name: "Rover", gender: "Male" },
>   { name: "Max", gender: "Male" },
>   { name: "Fluffy", gender: "Female" },
>   { name: "Cocoa", gender: "Female" }
> ]
>
> # gender asc, name asc
> p(dogs.sort do |a, b|
>   [a[:gender], a[:name]] <=> [b[:gender], b[:name]]
> end)
> #=> [{:name=>"Cocoa", :gender=>"Female"}, {:name=>"Fluffy",
> :gender=>"Female"}, {:name=>"Max", :gender=>"Male"}, {:name=>"Rover",
> :gender=>"Male"}]
>
> My question is using the method `<=>` returns value `-1,0,+1`,how the
> method sorting the source array ? What the things are actually going on
> with the result of `<=>`?
>
> --
> Posted via http://www.ruby-forum.com/.
>
>
The spaceship operator (<=>) returns a negative number if the value on the
left is less than the value on the right, 0 if they are equal, and a
positive number if the value on the left is more than the value on the
right.

'a' <=> 'b'  # => -1
'b' <=> 'b'  # => 0
'c' <=> 'b'  # => 1


So the sort method just checks for these to see which element should go
first. Something like this:

def sort(unsorted_ary, &comparison)
  comparison ||= lambda { |a, b| a <=> b }

  unsorted_ary.each_with_object [] do |to_add, sorted|
    index = sorted.index { |already_added| 0 <
comparison.call(already_added, to_add) }
    sorted.insert (index || -1), to_add
  end
end


sort([*1..10].shuffle) { |a, b| a <=> b }  # => [1, 2, 3, 4, 5, 6, 7, 8, 9,
10]
sort([*1..10].shuffle) { |a, b| b <=> a }  # => [10, 9, 8, 7, 6, 5, 4, 3,
2, 1]
sort([*1..10].shuffle)                     # => [1, 2, 3, 4, 5, 6, 7, 8, 9,
10]
sort([*1..10].shuffle) do |a, b|
  a.even? && b.even? ? b <=> a :
  a.odd?  && b.odd?  ? a <=> b :
  a.even?            ? -1      :
                       1
end                                        # => [10, 8, 6, 4, 2, 1, 3, 5,
7, 9]

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On Sun, May 19, 2013 at 4:03 AM, Love U Ruby <span dir=3D"ltr">&lt;<a href=
=3D"mailto:lists@ruby-forum.com" target=3D"_blank">lists@ruby-forum.com</a>=
&gt;</span> wrote:<br><div class=3D"gmail_quote"><blockquote class=3D"gmail=
_quote" style=3D"margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:=
1ex">
I am very eager to know about the Enum#sort{|a,b|..}<br>
<br>
<a href=3D"http://www.ruby-doc.org/core-2.0/Enumerable.html#method-i-sort" =
target=3D"_blank">http://www.ruby-doc.org/core-2.0/Enumerable.html#method-i=
-sort</a><br>
<br>
Now my question is about the internal mechanism.<br>
<br>
***Returns an array containing the items in enum sorted, either<br>
according to their own &lt;=3D&gt; method, or by using the results of the<b=
r>
supplied block. The block should return -1, 0, or +1 depending on the<br>
comparison between a and b.**<br>
<br>
I took the below as an example:<br>
<br>
dogs =3D [<br>
=A0 { name: &quot;Rover&quot;, gender: &quot;Male&quot; },<br>
=A0 { name: &quot;Max&quot;, gender: &quot;Male&quot; },<br>
=A0 { name: &quot;Fluffy&quot;, gender: &quot;Female&quot; },<br>
=A0 { name: &quot;Cocoa&quot;, gender: &quot;Female&quot; }<br>
]<br>
<br>
# gender asc, name asc<br>
p(dogs.sort do |a, b|<br>
=A0 [a[:gender], a[:name]] &lt;=3D&gt; [b[:gender], b[:name]]<br>
end)<br>
#=3D&gt; [{:name=3D&gt;&quot;Cocoa&quot;, :gender=3D&gt;&quot;Female&quot;}=
, {:name=3D&gt;&quot;Fluffy&quot;,<br>
:gender=3D&gt;&quot;Female&quot;}, {:name=3D&gt;&quot;Max&quot;, :gender=3D=
&gt;&quot;Male&quot;}, {:name=3D&gt;&quot;Rover&quot;,<br>
:gender=3D&gt;&quot;Male&quot;}]<br>
<br>
My question is using the method `&lt;=3D&gt;` returns value `-1,0,+1`,how t=
he<br>
method sorting the source array ? What the things are actually going on<br>
with the result of `&lt;=3D&gt;`?<br>
<span class=3D"HOEnZb"><font color=3D"#888888"><br>
--<br>
Posted via <a href=3D"http://www.ruby-forum.com/" target=3D"_blank">http://=
www.ruby-forum.com/</a>.<br>
<br>
</font></span></blockquote></div><br><div>The spaceship operator (&lt;=3D&g=
t;) returns a negative number if the value on the left is less than the val=
ue on the right, 0 if they are equal, and a positive number if the value on=
 the left is more than the value on the right.</div>
<div><br></div><div><div>&#39;a&#39; &lt;=3D&gt; &#39;b&#39; =A0# =3D&gt; -=
1</div><div>&#39;b&#39; &lt;=3D&gt; &#39;b&#39; =A0# =3D&gt; 0</div><div>&#=
39;c&#39; &lt;=3D&gt; &#39;b&#39; =A0# =3D&gt; 1</div></div><div><br></div>=
<div><br></div>
<div>So the sort method just checks for these to see which element should g=
o first. Something like this:</div><div><br></div><div><div>def sort(unsort=
ed_ary, &amp;comparison)</div><div>=A0 comparison ||=3D lambda { |a, b| a &=
lt;=3D&gt; b }</div>
<div>=A0=A0</div><div>=A0 unsorted_ary.each_with_object [] do |to_add, sort=
ed|</div><div>=A0 =A0 index =3D sorted.index { |already_added| 0 &lt; compa=
rison.call(already_added, to_add) }</div><div>=A0 =A0 sorted.insert (index =
|| -1), to_add</div>
<div>=A0 end</div><div>end</div></div><div><br></div><div><br></div><div><d=
iv>sort([*1..10].shuffle) { |a, b| a &lt;=3D&gt; b } =A0# =3D&gt; [1, 2, 3,=
 4, 5, 6, 7, 8, 9, 10]</div><div>sort([*1..10].shuffle) { |a, b| b &lt;=3D&=
gt; a } =A0# =3D&gt; [10, 9, 8, 7, 6, 5, 4, 3, 2, 1]</div>
<div>sort([*1..10].shuffle) =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 # =3D&g=
t; [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]</div><div>sort([*1..10].shuffle) do |a, =
b|=A0</div><div>=A0 a.even? &amp;&amp; b.even? ? b &lt;=3D&gt; a :</div><di=
v>=A0 a.odd? =A0&amp;&amp; b.odd? =A0? a &lt;=3D&gt; b :</div>
<div>=A0 a.even? =A0 =A0 =A0 =A0 =A0 =A0? -1 =A0 =A0 =A0:</div><div>=A0 =A0=
 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A01</div><div>end =A0 =A0 =A0 =A0 =A0=
 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0# =3D&gt; [10, =
8, 6, 4, 2, 1, 3, 5, 7, 9]</div></div><div><br></div>

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