Rails - Jul 2006 - Can anyone explain this code? It uses Inject

def webs
    @webs ||= Web.find(:all).inject({}) { |webs, web| 
webs.merge(web.address => web) }
  end

-- 
Posted via http://www.ruby-forum.com/.

Chris wrote:
>   def webs
>     @webs ||= Web.find(:all).inject({}) { |webs, web| 
> webs.merge(web.address => web) }
>   end
> 
Turns an array into a hash with the web address as a key.

Eric

On 7/25/06, Chris <cadams1342@hotmail.com> wrote:
>
>   def webs
>     @webs ||= Web.find(:all).inject({}) { |webs, web|
> webs.merge(web.address => web) }
>   end
>
> --
> Posted via http://www.ruby-forum.com/.
> _______________________________________________
> Rails mailing list
> Rails@lists.rubyonrails.org
> http://lists.rubyonrails.org/mailman/listinfo/rails
>
@webs ||= # if @webs has a vaule do nothing, if it''s nil then put
whatever
is on the right into @webs

Web.find(:all).inject({}){ |webs, web| webs.merge(web.address => web)}

Inject can be a bit tricky.  basically It might be easier to abstract it.

Inject replaces the cumulative with whatever the block evaluates to each
iteration starting with the inject parameter.

[1,2,3].inject({}) { |cumulative, element| cumulative.merge( element.to_s =>
element ) }

Walking through this one

Element 1
  cumulative = {} (the param in inject
  element = 1 (The first element to be iterated over)

  evaluate block and replace the cumulative
  cumulative = cumulative.merge( element.to_s => element) #=> gives {
''1'' =>
1 }
end iteration

Element 2
  cumulative = { ''1'' => 1 } - The result of the last
iteration
  element = 2

  evaluate block and replace the cumulative
  cumulative = cumulative.merge( element.to_s => element) #=> gives {
''1'' =>
1, ''2'' => 2 }
end iteration

Element 3
  cumulative = { ''1'' => 1, ''2'' => 2 }
- The result of the last iteration
  element = 3

  evaluate block and replace the cumulative
  cumulative = cumulative.merge( element.to_s => element) #=> gives {
''1'' =>
1, ''2'' =>2, ''3'' => 3 }
end iteration

and so on.

so in the example you would be building a hash of all the web objects of the
form
web.address => web

Hope that helps
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Thanks... that helped

-- 
Posted via http://www.ruby-forum.com/.

One other thing to note about the code is that it creates quite a few 
hashes, 1 initial hash and one for each element in the array. Using the 
more abstract example and irb:

irb(main):001:0> [1,2,3].inject({}) { |cumulative, element| puts 
cumulative.object_id; cumulative.merge( element.to_s => element ) }
23135864
23135732
23135648
=> {"1"=>1, "2"=>2, "3"=>3}

Here you can see the object_id is different each time, indicating that 
cumulative is different on each interation and therefore a new object. 
This can be avoided by using the merge! method that will modify the 
cumulative object instead of returning a new Hash on each iteration.

irb(main):002:0> [1,2,3].inject({}) { |cumulative, element| puts 
cumulative.object_id; cumulative.merge!( element.to_s => element ) }
23116412
23116412
23116412
=> {"1"=>1, "2"=>2, "3"=>3}

russ.

-- 
Posted via http://www.ruby-forum.com/.

On 7/25/06, Chris <cadams1342@hotmail.com> wrote:
>   def webs
>     @webs ||= Web.find(:all).inject({}) { |webs, web|
> webs.merge(web.address => web) }
>   end
FYI in edge rails:

 def webs
   @webs ||= Web.find(:all).index_by &:address
 end


-- 
Rick Olson
http://techno-weenie.net

Rick Olson wrote:
> On 7/25/06, Chris <cadams1342@hotmail.com> wrote:
>>   def webs
>>     @webs ||= Web.find(:all).inject({}) { |webs, web|
>> webs.merge(web.address => web) }
>>   end
> 
> FYI in edge rails:
> 
>  def webs
>    @webs ||= Web.find(:all).index_by &:address
>  end
Does the ampersand mean anything, or is it a typo?

Joe

-- 
Posted via http://www.ruby-forum.com/.

On 7/27/06, Joe <joe@yahoo.com> wrote:
>
> Rick Olson wrote:
> > On 7/25/06, Chris <cadams1342@hotmail.com> wrote:
> >>   def webs
> >>     @webs ||= Web.find(:all).inject({}) { |webs, web|
> >> webs.merge(web.address => web) }
> >>   end
> >
> > FYI in edge rails:
> >
> >  def webs
> >    @webs ||= Web.find(:all).index_by &:address
> >  end
>
> Does the ampersand mean anything, or is it a typo?
>
> Joe
>
--
> Posted via http://www.ruby-forum.com/.
> _______________________________________________
> Rails mailing list
> Rails@lists.rubyonrails.org
> http://lists.rubyonrails.org/mailman/listinfo/rails
>
The ampersand is a Symbol#to_proc.  It''s a nice shorcut and basically
says
use the address method on the object passed into the block.

The equivelant is

@webs || Web.find(:all).index_by {  |web| web.address }
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On 27 Jul 2006, at 07:03, Joe wrote:
> Rick Olson wrote:
>> On 7/25/06, Chris <cadams1342@hotmail.com> wrote:
>>>   def webs
>>>     @webs ||= Web.find(:all).inject({}) { |webs, web|
>>> webs.merge(web.address => web) }
>>>   end
>>
>> FYI in edge rails:
>>
>>  def webs
>>    @webs ||= Web.find(:all).index_by &:address
>>  end
>
> Does the ampersand mean anything, or is it a typo?
It converts the argument to a Proc. You can find everything you need  
to know at http://eli.thegreenplace.net/2006/04/18/understanding-ruby- 
blocks-procs-and-methods/
(search for "The ampersand (&)" in the article).

Best regards

Peter De Berdt

>
> Does the ampersand mean anything, or is it a typo?
Explanation:

http://blogs.pragprog.com/cgi-bin/pragdave.cgi/Tech/Ruby/ToProc.rdoc


Regards,
Rimantas
--
http://rimantas.com/