From: Joel VanderWerf <vjoel@...>
Date: 2007-02-11T05:23:38+09:00
Subject: Re: Jumping to "the next one" in something#each

Garance A Drosehn wrote:
> ARGV.each { |arg|
>    case arg
>    when "-f"
>        ...do something with the *next* value of |arg|...
>    end
> }

The following works with any Enumerable, doesn't use threads, 
continuations, or generators, doesn't construct temp arrays (except to 
hold the next value(s)) or do any shifting on the original argument, and 
has fairly nice syntax (but you have to remember not to fall off the end 
of your block with an integer value). It handles corner cases AFAICT.

Any suggestions for a better name than #each2?

module Enumerable
   class InsufficientDataError < StandardError; end

   def each2
     req = first = rest = nil

     each do |x|
       if req
         req -= 1
         rest << x
         next if req > 0

         req = yield first, rest
         first = nil
         rest = nil
       else
         req = yield x, nil
       end

       if req
         if req.kind_of?(Integer) and req > 0
           first = x
           rest = []
         else
           raise ArgumentError, "Bad request: next #{req.inspect}"
         end
       end
     end

     if req and req > 0
       raise InsufficientDataError,
         "Ran out of data fulfilling request: next #{req}"
     end
   end
end

(1..10).each2 do |x, rest|
   next 2 if (x == 3 or x == 7) and not rest
   puts "x = #{x}, rest = #{rest.inspect}"
   nil ## make sure not to request more entries by returning an integer
end

%w{ -f foo -b bar --copy src dst }.each2 do |arg, values|
   case arg
   when "-f", "-b"
     next 1 unless values
     puts "#{arg} #{values[0]}"

   when "--copy"
     next 2 unless values
     puts "#{arg} #{values.join(" ")}"
   end

   nil ## make sure not to request more entries by returning an integer
end

__END__

Output:

x = 1, rest = nil
x = 2, rest = nil
x = 3, rest = [4, 5]
x = 6, rest = nil
x = 7, rest = [8, 9]
x = 10, rest = nil
-f foo
-b bar
--copy src dst

-- 
       vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407