From: "Brian Schröder" <ruby.brian@...>
Date: 2005-08-13T17:04:41+09:00
Subject: Re: cartesian product - next to last version

On 12/08/05, walter a kehowski <wkehowski@cox.net> wrote:
> Hello,
> 
> Yielding the values is certainly a good idea, but it makes the code a
> lot bigger. Anyhow, here is the recursive, yielding version.
> 
> def cartprod(base, *others)
>   if block_given?
>     if others.empty?
>       base.each{|a| yield [a]}
>     else
>       base.each do | a |
> cartprod(*others) do | b |
>   yield [a, *b]
> end
>       end
>     end
>     nil # <--Why?
>   else
>     return base.map{|a|[a]} if others.empty?
>     others = cartprod(*others)
>     base.inject([]) { | r, a | others.inject(r) { | r, b | r << ([a,
> *b]) } }
>   end
> end
> 
> ## Question: Why the nil?
> 

because otherwise the function would return the base array when
invoked with a block. And that would not make any sense for method
chaining. Nil will invoke an exception if a chained method is called
upon it.

regards,

Brian



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