From: Ken Bloom <kbloom@...>
Date: 2008-05-16T02:40:01+09:00
Subject: Re: Contionnal sum

Xavier Noria <fxn@hashref.com> wrote:
> On Thu, May 15, 2008 at 6:26 PM, Fernando Perez <pedrolito@lavache.com> wrote:
> 
>> I am wondering if it is possible to create a conditionnal sum.
>>
>> For instance I would like to do something like this:
>>
>> @total_amount = @orders.sum { |order| order.amount if order.paid == true
>> }
> 
> sum... is that Rails? If that's the case you could filter them before:
> 
>   # untested
>   @total_sum = @orders.select(&:paid).sum(&:amount)
> 
> Although AR has another SQL-based #sum that may be worth exploring if
> @orders are ARs.
> 
> That being said, perhaps you weren't aware of the fact that
> Enumerable#sum is defined by Active Support, but let me point out that
> those extensions are discussed in the very Rails mailing list.

Enumerable#sum is also in facets.
The .select method is a good way, but you can use less memory by doing

@orders.sum {|order| order.paid? order.amount : 0 }

Facets' #sum doesn't skip nils, so the method you proposed won't work.

--Ken

-- 
Ken (Chanoch) Bloom. PhD candidate. Linguistic Cognition Laboratory.
Department of Computer Science. Illinois Institute of Technology.
http://www.iit.edu/~kbloom1/