July 10, 2011, 11:57 a.m.
posted by jazzjack
Writing an Iterator Over a Data StructureProblemYou'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. SolutionComplex 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): t = Tree.new("Parent") child1 = t << "Child 1" child1 << "Grandchild 1.1" child1 << "Grandchild 1.2" child2 = t << "Child 2" child2 << "Grandchild 2.1" A simple treeHow 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 DiscussionThe 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.length1 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 See Also

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