From: Robert Klemme <shortcutter@...>
Date: 2009-04-07T16:29:18+09:00
Subject: Re: how to dynamically create search term for array.find_all

On 07.04.2009 03:31, Adam Akhtar wrote:
> Ahh sorry Robert re: eval - categories are only half of the problem. Im 
> also allowing the user to filter by max and min stuff too.
> 
> So depending on what filtering options a user has set Ill have a varying 
> number of conditions in my code.
> 
> So one time it could be results.find_all{|x| x.rating <= some_parameter 
> && x.rating>= some_other_parameter && etc etc etc }
> 
> Since iterating over the array only once is best for performance I 
> thought it would be best to call find_all once and stuff all the 
> required conditions into it. This was where i was stumped on how ot do 
> that so i thought i could use eval
> 
> like so:
> 
> #conditions are collected from users choices and are stored in a hash
> some_conditions = {:max_rating => 10, :min_rating => 2, :categories => 
> [:horror, :scifi] }
> 
> def filter (some_films, some_conditions)
> 
> condition_string = []
> condition_string.push "x.rating <= #{some_conditions[max_rating]}" if 
> some_conditions[max_rating]
> condition_string.push "x.rating >= #{some_conditions[min_rating}" if 
> some_conditions[min_rating]
> 
> condition_string = condition_string.join(" && ")
> 
> filtered_results = some_films.find_all{|x| eval(condition_string)}

This approach is inefficient each eval is very costly: the string needs 
to be parsed and then executed.  You could rather do this:

condition_string = []
...
cond_block = eval("lambda {|x| #{condition_string.join(' && ')}}")
filtered_results = some_films.find_all(&cond_block)

> return filtered_results
> end
> 
> 
> The above code didnt filter by categories as its just for example 
> purposes but I was thinking of simply calling find_all again using the 
> code offered before rather than drafting up a complicated eval string 
> involving && and ||. Id love ot hear how i can better this code.

See above.  An alternative approach would be to implement fixed 
conditions and combine them, e.g.

CondLess = Struct.new :cmp_val do
   def ===(val)
     val < cmp_val
   end
end

conditions = []
conditions << CondLess.new(10) << CondMore.new(1)
...
filtered_results =
   some_films.find_all {|x| conditions.all? {|c| c === x} }

Note that #all? does short circuit so evaluation continues only to the 
first mismatch (if there is one) much the same as && and ||.

Kind regards

	robert