Writing An Iterator Over a Data Structure In Ruby

Problem

Let's say we have created a custom data structure, and we want to implement "each" method for it, or we want to implement an unusual way of iterating over an existing data structure.

Solution

Usually, complex data structures are designed out of the basic data structures like arrays, hashes, and so on. And all of the basic data structures have defined the "each" method. 

Below is the simple tree data structure. A tree data structure contains a list of children and a single value. 

class Tree
attr_reader :value
def initialize(value)
@value = value
@children = []
end
def <<(value)
subtree = Tree.new(value)
@children << subtree
return subtree
end
end

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And, now below is the code to create a specific Tree:

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

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Now, the question is how we can use iterator over this data structure? As we can see, the tree is defined recursively, so it is valid to use an iterator over it recursively. The implementation of Tree#each yields the value stored in the tree, then iterates over its children 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

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Examples of an iterator

The effortless way to build an iterator is recursive, which means using smaller iterators until we have covered all the elements in a data structure. But there is no guarantee that those iterators are always there or they are present there but giving you elements in the wrong order, in such a case, we will need to write a loop ourselves.

In ruby, Traditional loops are outdated because enumerable iterators are more convenient, but when we are writing an iterator, we are left with no choice but to use a loop. Below is the example showing how to use a while loop to iterate 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!

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Now, there are two more simple iterators; first one submits each element numerous 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

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And the other returns the element of an Enumerable in random order:

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

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Frequently Asked Questions

How do we use iterator in Ruby?

Iterator is an idea that is used in object-oriented programming languages. Iteration simple means doing one thing multiple times like a loop. The simplest iterator is the loop method.

How do we iterate over the range of a number in Ruby?

For iterating over the range of a number in Ruby, each() method is used.Syntax: range1.each(|el| block).

How do we iterate an array in Ruby?

The simplest and most popular way to iterate individual items in the array is the Ruby Enumerable#each method.

What is an iteration in data structure?

Iteration is the process in which the set of instructions or structures are repeated several times until the conditions are met.

Conclusion

In this article, we have seen how to create an iterator over a data structure in ruby. We started with an introduction, saw the problem and its solution, and then saw some examples of iterators over a data structure in ruby.

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