From: Sam Duncan <sduncan@...>
Date: 2011-12-15T03:51:44+09:00
Subject: Re: How to pass a list of conditions as parameters

On 14/12/11 21:27, Robert Klemme wrote:
> On Wed, Dec 14, 2011 at 3:05 AM, Sam Duncan<sduncan@wetafx.co.nz>  wrote:
>> On 14/12/11 14:28, Thescholar Thescholar wrote:
>>> I�m unsure how to explain this one so I'll do my best.
>>>
>>> Technically, I would like to pass an array of numbers and an array of
>>> conditions as parameters in a method. The tricky part is that I would
>>> like to evaluate each number of that array to each condition of that
>>> other array.
>>>
>>> The reason I would like to achieve something like this is because I plan
>>> to change the list of rules/conditions each time I want to call this
>>> method.
> But what is the desired output?  That's crucial for the solution (see below).
>
>>> Here�s the unworking code so far:
>>>
>>> (see attachment)
>>>
>>> Thank you in advance for your valuable support! Any help or hyperlinks
>>> with explanations is greatly appreciated!
>>>
>>> Attachments:
>>> http://www.ruby-forum.com/attachment/6838/array_of_conditions.txt
>>>
>> Nothing pretty or fancy here, but seems to do it? Note, I think the hash
>> interpolation is a 1.9.x thing.
>>
>>
>> array_of_numbers = [0, 2, 4, 7, 10, 25]
>>
>> # these could easily come from a config file/ db/ whatevs
>> array_of_eval_conditions = [ "(%{number} == 2)", "(%{number} == 5 and
>> %{number} == 5)", "(%{number} != 1)"]
>>
>> # these could too, but would require a few more hoops
>> array_of_lambda_conditions = [lambda {|x| x == 2}, lambda {|x| x == 5 and x
>> == 5}, lambda {|x| x != 1}]
> Why "more hoops"?  There is no point in having conditions in Strings
> if they are constant in the script.  There is no need for eval here.
>
> Please note that an more recent versions of Ruby (at least 1.9.3)
> lamba implements operator === which is reasonable because then you can
> use a lambda as condition in a case expression and with
> Enumerable#grep:
>
> irb(main):004:0>  pos = lambda {|x| x>= 0}
> =>  #<Proc:0x100e300c@(irb):4 (lambda)>
> irb(main):005:0>  vals = [-10,-5,0,5,10]
> =>  [-10, -5, 0, 5, 10]
>
> irb(main):007:0>  vals.each {|v| p v, pos[v], pos === v; case v;when
> pos;puts "pos";else puts "not" end}
> -10
> false
> false
> not
> -5
> false
> false
> not
> 0
> true
> true
> pos
> 5
> true
> true
> pos
> 10
> true
> true
> pos
> =>  [-10, -5, 0, 5, 10]
>
> There do exists ways to do this already, e.g. assuming that conditions
> are in an array of lambdas:
>
> # get all which satisfy all conditions
> items.select {|x| conditions.all? {|c| c[v]}}
>
> # get first which satisfies all conditions
> items.find {|x| conditions.all? {|c| c[v]}}
>
> # get all which satisfy any condition
> items.select {|x| conditions.any? {|c| c[v]}}
>
> # get first which satisfies any condition
> items.find {|x| conditions.any? {|c| c[v]}}
>
> See also Enumerable#reject etc.
>
> Kind regards
>
> rober
>
Having the expressions as strings means you can very easily load them 
from a data store or config file - the fact they are constant in this 
/example/ is incidental =]
This is not so easy with lambdas which was my point. It also lends 
itself to a simple boolean expression syntax which can conceivably be 
configured by a non-coder. The OP does not have 1.9.x by the way.

Sam