From: Florian Gross <flgr@...>
Date: 2004-09-05T22:10:13+09:00
Subject: Re: Style Question

Brian Schroeder wrote:

> Hello everybody,

Moin!

> I've got a stylistic question. I know that "the ruby way" to implement
> iterations is using .each and similar things, but sometimes I like to use
> 
> for i in 0...n
> 	for j in i...n
> 		do something with i and for
> 	end
> end
> 
> Thats nice.
> 
> But if I want to iterate downwards the only way I found is 10.downto(0) do
> | i | end

What about this:

for i in (0...n).to_a.reverse
   for j in (i...n).to_a.reverse
     p [i, j]
   end
end

The problem with that is that it will be slow for huge ranges.

Or maybe this:

require 'enumerator'
for i in (n - 1).enum_for(:downto, 0)
   for j in (n - 1).enum_for(:downto, i)
     p [i, j]
   end
end

Or this one:

class Range
   def reverse_each(&block)
     unless last.respond_to?(:downto)
       raise(TypeError, "cannot reverse iterate from #{last.class}")
     end

     is_first = true
     last.downto(first) do |item|
       if is_first and exclude_end?
         is_first = false
         next
       end

       block.call(item)
     end
   end
end

require 'enumerator'
for i in (0...n).enum_for(:reverse_each)
   for j in (i...n).enum_for(:reverse_each)
     p [i, j]
   end
end

And I think that Range should provide reverse_each in Standard Ruby... 
(There's no problem in it when using #downto -- it is only defined for 
Integers, not for Strings.)

Regards,
Florian Gross