Writing an Iterator Over a Data Structure | Code Blocks and Iteration

Problem

You've created a custom data structure, and you want to implement an each method for it, or you want to implement an unusual way of iterating over an existing data structure.

Solution

Complex data structures are usually constructed out of the basic data structures: hashes, arrays, and so on. All of the basic data structures have defined the each method. If your data structure is composed entirely of scalar values and these simple data structures, you can write a new each method in terms of the each methods of its components.

Here's a simple tree data structure. A tree contains a single value, and a list of children (each of which is a smaller tree).

	class Tree
	 attr_reader :value
	 def initialize(value)
	 @value = value
	 @children = []
	 end

	 def <<(value)
	 subtree = Tree.new(value)
	 @children << subtree
	 return subtree
	 end
	end

Here's code to create a specific Tree (Figure 7-1):

	t = Tree.new("Parent")
	child1 = t << "Child 1"
	child1 << "Grandchild 1.1"
	child1 << "Grandchild 1.2"
	child2 = t << "Child 2"
	child2 << "Grandchild 2.1"

Figure 7-1. A simple tree

How can we iterate over this data structure? Since a tree is defined recursively, it makes sense to iterate over it recursively. This implementation of Tree#each yields the value stored in the tree, then iterates over its children (the children are stored in an array, which already supports each) and recursively calls Tree#each on every child tree.

	class Tree
	 def each
	 yield value
	 @children.each do |child_node|
	 child_node.each { |e| yield e }
	 end
	 end
	end

The each method traverses the tree in a way that looks right:

	t.each { |x| puts x }
	# Parent
	# Child 1
	# Grandchild 1.1
	# Grandchild 1.2
	# Child 2
	# Grandchild 2.1

Discussion

The simplest way to build an iterator is recursively: to use smaller iterators until you've covered every element in your data structure. But what if those iterators aren't there? More likely, what if they're there but they give you elements in the wrong order? You'll need to go down a level and write some loops.

Loops are somewhat declassé in Ruby because iterators are more idiomatic, but when you're writing an iterator you may have no choice but to use a loop. Here's a reprint of an iterator from Recipe 4.1, which illustrates how to use a while loop to iterate over an array from both sides:

	class Array
	 def each_from_both_sides()
	 front_index = 0
	 back_index = self.length-1
	 while front_index <= back_index
	 yield self[front_index]
	 front_index += 1
	 if front_index <= back_index
	 yield self[back_index]
	 back_index -= 1
	 end
	 end
	 end
	end

	%w{Curses! been again! foiled I've}.each_from_both_sides { |x| puts x }
	# Curses!
	# I've
	# been
	# foiled
	# again!

Here are two more simple iterators. The first one yields each element multiple times in a row:

	module Enumerable
	 def each_n_times(n)
	 each { |e| n.times { yield e } }
	 end
	end

	%w{Hello Echo}.each_n_times(3) { |x| puts x }
	# Hello
	# Hello
	# Hello
	# Echo
	# Echo
	# Echo

The next one returns the elements of an Enumerable in random order; see Recipe 4.10 for a more efficient way to do the shuffling.

	module Enumerable
	 def each_randomly
	 (sort_by { rand }).each { |e| yield e }
	 end
	end
	%w{Eat at Joe's}.each_randomly { |x| puts x }
	# Eat
	# Joe's
	# at