From: Rob Biedenharn <Rob@...>
Date: 2008-09-20T03:41:01+09:00
Subject: Re: Hash default value question

On Sep 19, 2008, at 1:40 PM, Glenn wrote:
> Hi,
>
> Can anyone explain why hash_of_indexes1 in the code below works, but  
> hash_of_index2 doesn't?  (I got the first method as a response to a  
> previous post.)
>
> What I'd like to get is a hash with a key value for each unique  
> value in the receiver array, and I'd like the value for each key to  
> be an array of integers corresponding to the index values of the  
> elements in the original array.  The first method works, but the  
> second one doesn't.  I figured if I created the hash in the second  
> method with a default value of an empty array, h[e.to_f] in the each  
> method of the hash would return an empty array the first time it  
> sees each key.  But what actually happens is that the values for all  
> of the keys are the same when the hash's each method finishes --  
> each value is an array with all of the index values in the receiver  
> array.
>
> class Array
>   def hash_of_indexes1
>     h = Hash.new
>     each_with_index { |e, i| h[e.to_f] = Array( h[e.to_f] ) << i }
>     h
>   end
>
>   def hash_of_indexes2
>     h = Hash.new([])
>     each_with_index { |e, i| h[e.to_f] = h[e.to_f] << i }
>     h
>   end
> end
>
> [1, 2, 2, 3].hash_of_indexes1.inspect ## --> returns {1.0=>[0],  
> 3.0=>[3], 2.0=>[1, 2]}
>
> [1, 2, 2, 3].hash_of_indexes2.inspect ## --> returns {1.0=>[0, 1, 2,  
> 3], 3.0=>[0, 1, 2, 3], 2.0=>[0, 1, 2, 3]}
>
> Thanks for your help.

The Hash.new([]) causes a single array to be used (shared) for the  
default value.  Using the block form, Hash.new {[]}, would produce the  
behavior you want.  It's common to see this used as:
   Hash.new {|h,k| h[k] = [] }
if you want the key to exist in the hash when referenced (as you seem  
to expect here).

class Array
   def hash_of_indexes3
     h = Hash.new {|h,k| h[k] = []}
     each_with_index {|e,i| h[e] << i }
     h
   end
end
irb> [1, 2, 2, 3].hash_of_indexes3
=> {1=>[0], 2=>[1, 2], 3=>[3]}

I'm not sure why you wanted to call .to_f on each element, I left that  
out.

-Rob

> ----- Original Message ----
> From: William James <w_a_x_man@yahoo.com>
> To: ruby-talk ML <ruby-talk@ruby-lang.org>
> Sent: Sunday, September 14, 2008 4:02:54 PM
> Subject: Re: Passing a method call into a method
>
> On Sep 14, 8:18 am, Glenn <glenn_r...@yahoo.com> wrote:
>> [Note:  parts of this message were removed to make it a legal post.]
>>
>> Hi,
>>
>> I am trying to figure out how to pass a method call into a method.
>>
>> I wrote a generic method that call be called on an array of  
>> numbers, and returns an array of the index values in the receiver  
>> that meet a specified condition:
>>
>> class Array
>>   def hash_of_indexes
>>     if self.empty?
>>       'The array is empty.'
>>     else
>>       h = {}
>>       self.each_with_index { |e, i| h.include?(e.to_f) ? h[e.to_f]  
>> << i : h[e.to_f] = [i] }
>>       h
>>     end
>>   end
>
>
> class Array
>   def hash_of_indexes
>     h = {}
>     each_with_index{ |e,i|
>       e = e.to_f
>       h[e] = Array( h[e] ) << i }
>     h
>   end
> end

Rob Biedenharn		http://agileconsultingllc.com
Rob@AgileConsultingLLC.com
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Skype:	rob.biedenharn