From: "Hal E. Fulton" <hal9000@...>
Date: 2002-08-06T05:20:12+09:00
Subject: Super-iterator? (long)

Here's an idea for you. I've worked on it a couple of days.

Please feel free to critique the concept and/or the code.

The code can definitely be improved. It's ugly and has 
limitations and probably has bugs.

Motivation:

1. Sometimes I want to know whether I'm in the first (or last)
iteration of an iterator block.
2. Sometimes I want to do things "alternately" (i.e., for every
other iteration).
3. I'm a tiny bit displeased by the difference between each and
each_with_index. I'd like them to be done by one thing.
4. Sometimes I want to change an item in an array, for instance.
each won't allow this; I have to use each_with_index and manually
index into the array.
5. Sometimes an ordinary iterator seems inadequate; I want something
that can grab the next entry on demand, without waiting for the 
loop to run around again.

The solution:

1. Create an IteratorVar class that stores all the information used
in the looping.
2. Give it first? and last? methods.
3. Give it odd? and even? methods.
4. Give it an accessor that can retrieve the value and is even able
to replace the value in the original list.
5. Give it a 'next' method that will update the counter and return
the relevant object.
6. Put 'iterator' into Enumerable (actually, at this point it should
probably NOT go into Enumerable) which wraps it all in a big while
loop that does a yield.
7. The yielded item will be an IteratorVar.

Bugs and limitations:

1. It pretty much only works for arrays now. The "list = self.entries" 
line created a new array, so I couldn't store items back into the array
with #value=. So I changed it to "list = self" -- meaning that the 
object *must* have [] and be indexed by an integer. So that's pretty 
much an Array. I've left it this way to illustrate the concept.
2. Obviously things like files are out of the question at the moment.

Code and output below. 

Cheers,
Hal Fulton



class IteratorVar
  attr_accessor :count_, :max_ 
  attr_reader :value
  attr_reader :skip?
  attr_writer :skip

  def initialize(index,max,list)
    @count_ = index
    @max_ = max
    @value = list[index]
    @skip = false
    @list_ = list
  end

  def first?
    @count_ == 0
  end

  def last?
    @count_ >= @max_
  end

  def odd?
    @count_ % 2 == 1
  end

  def even?
    @count_ % 2 == 0
  end

  def next
    @count_ += 1
    @value = @list_[@count_]
    @skip = true
    self.dup
  end

  def value=(val)
    @value = val
    @list_[@count_] = val
  end

  def inspect
    @value.inspect
  end

  def to_s
    @value.to_s
  end

  def to_str
    @value.to_str
  end

end

module Enumerable

  def iterate
    low = 0
    high = self.size-1
    list = self
    i = low
    x = IteratorVar.new(i,high,list)
    while i <= high 
      x.count_ = i
      x.value = list[i]
      yield x
      if x.skip?
 i = x.count_ 
 x.skip = false
      else 
 i = x.count_ += 1
      end
    end
  end

end


a = %w[zero one two three four]

a.iterate do |x|
  puts "Value is #{x} "
  puts "  Index is odd " if x.odd?
  puts "  Index is even" if x.even?
  puts "  First item!" if x.first?
  puts "  Last  item!" if x.last?
end

puts

# Same array a -- capitalize every other entry

a.iterate do |x|
  if x.odd? 
    x.value = x.value.upcase
  end
end

a.each {|x| puts x}

puts

# Problem: For a list of items, grab the "parent"
# and the fixed number of children following. A
# parent is a single letter -- if it's a vowel
# [aeiou] it has a single child; a consonant will
# have three children

puts

items = %w[ b 0 1 2 a 3 c 3 4 5 d 6 7 8 e 9 ]

items.iterate do |item|
  parent = item.value
  if parent =~ /[aeiou]/
    puts "#{parent} -> #{item.next}"
  else
    arr = []
    3.times { arr << item.next}
    puts "#{parent} -> #{arr}"
  end
end

# Output...
=begin
Value is zero 
  Index is even
  First item!
Value is one 
  Index is odd 
Value is two 
  Index is even
Value is three 
  Index is odd 
Value is four 
  Index is even
  Last  item!

zero
ONE
two
THREE
four


b -> 012
a -> 3
c -> 345
d -> 678
e -> 9
=end