From: Robert Klemme <bob.news@...>
Date: 2006-01-08T03:08:04+09:00
Subject: Re: Iterator Fu Failing Me

Bob Hutchison <hutch@recursive.ca> wrote:
> On Jan 7, 2006, at 11:13 AM, Robert Klemme wrote:
>
>> Bob Hutchison <hutch@recursive.ca> wrote:
>>> On Jan 6, 2006, at 11:36 PM, James Edward Gray II wrote:
>>>
>>>> That works.  Now can anyone give me a version of the middle section
>>>> that doesn't require I call parse?() 50 times?  I want something
>>>> close to:
>>>>
>>>>   elements = tokens.map do |token|
>>>>     [ClassA, ClassB, ClassC].find { |kind| kind.parse?(token) }
>>>>   end
>>>>
>>>> Except that I want the return result of parse?(), instead of the
>>>> class that took it.
>>>
>>> Why not:
>>>
>>> elements = tokens.map do |token|
>>>   result = nil
>>>   [ClassA, ClassB, ClassC].find { |kind| result = kind.parse?
>>> (token) }
>>> end
>>
>> Because this won't return the proper value from the block.  You
>> need at least to add a line with "result" after the #find. :-)
>
> cut'n'past-o... oops
>
>>   And then it's much more inelegant than using #detect. :-)
>
> You mean #detect or #select?

#detect - because it stops as soon as it has a hit while #select will return 
an array.  We need just the first hit.

>> [nil,nil,nil,"w",2,3].find {|x|puts x;x}
nil
nil
nil
w
=> "w"
>> [nil,nil,nil,"w",2,3].select {|x|puts x;x}
nil
nil
nil
w
2
3
=> ["w", 2, 3]

> elements = tokens.select do |token|
>   result = nil
>   [ClassA, ClassB, ClassC].find { |kind| results << kind.parse?
> (token) }
>   result
> end

You don't define results and you never assign result another value than 
nil...

> And this is kind of handy too...
>
> elements = tokens.map do |token|
>   result = nil
>   [ClassA, ClassB, ClassC].find { |kind| result = [token, kind.parse?
> (token)] }
>   result
> end

There's no point in storing token in result because it's unchanged and 
available as block parameter.

Say what you want, find/detect is the most elegant solution.

    robert