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comp.lang.ruby

defining sort <=> for a class

Adam Akhtar

8/24/2008 12:04:00 PM

If i have a class

class Example

attr_accessor :value

def initialize
@value = 0
end


end

and I have an array of examples and want to sort them based on value how
do i define a <=> for my class. I know its possible as it was a side
comment on an earlier post. Seeing as this is a totally different
subject to that post i decided to ask in a new post - it will be easier
for others who may have the same questoin to search for.

i tried this

class Example

attr_accessor :value

def initialize
@value = 0
end

def <=>(other_example)
@value <=> other_example
end
end

but i keep getting an error of undefined method for nill class when i
try to call it like this with an array of example objects

ojects.sort do |one, another|
one.<=>(another)
end

i also tried
ojects.sort do |one, another|
one <=>(another)
end

(i.e no period between one and <=>)

where am i going wrong???
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6 Answers

Thomas Wieczorek

8/24/2008 12:28:00 PM

0

Heya!

On Sun, Aug 24, 2008 at 2:03 PM, Adam Akhtar <adamtemporary@gmail.com> wrote:
> i tried this
>
> class Example
>
> attr_accessor :value
>
> def initialize
> @value = 0
> end
>

Here's the error:

> def <=>(other_example)
> @value <=> other_example
> end

You want to sort by the value, so you have to compare the values:

def <=>(other_example)
@value <=> other_example.value
end

A simple example:
a = Example.new
b = Example.new
a.value = 10
arr = [a,b]
# => [#<Example:0x2b5d6a8 @value=10>, #<Example:0x2b5b7cc @value=0>]
arr.sort!
# => [#<Example:0x2b5b7cc @value=0>, #<Example:0x2b5d6a8 @value=10>],
means arr = [b, a]

Adam Akhtar

8/24/2008 1:54:00 PM

0

ahh so the <=> thing is called implicitly, i dont have to call it
automatically in my code. I just do sort!.

thanks very much, ill go away and fix it.

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Thomas Wieczorek

8/24/2008 2:38:00 PM

0

On Sun, Aug 24, 2008 at 3:53 PM, Adam Akhtar <adamtemporary@gmail.com> wrote:
> ahh so the <=> thing is called implicitly, i dont have to call it
> automatically in my code. I just do sort!.

Yeah, it is. Enumerable#sort calls implicitly the <=> method of the elements.

>
> thanks very much, ill go away and fix it.
>

You're very welcome

Phlip

8/24/2008 2:53:00 PM

0

Adam Akhtar wrote:

> ahh so the <=> thing is called implicitly, i dont have to call it
> automatically in my code. I just do sort!.

Alternately, use sort_by to repurpose the implicit <=>. Examples:

.sort_by{rand} # a shuffle
.sort_by{|x| -x } # reverse order
.sort_by{|r| r.name.downcase } # ascibetic order by name

--
Phlip

Phlip

8/24/2008 3:00:00 PM

0

> .sort_by{|r| r.name.downcase } # ascibetic order by name

Actually, sorting by a downcase is not strictly an asciibetic sort. It's a cheap
form of "collation", where an expensive collation uses your locale to decipher
its dictionary order rules. Example: Packing McDonalds in with MacDonalds...

Adam Akhtar

8/24/2008 3:39:00 PM

0

yes ive seen sort_by a few times now. ill start using that in my code.
thanks everyone again.


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