Iterators in Ruby

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In this second part of a three-part article, we continue our discussion of code blocks and begin focusing on iterators as well. It is excerpted from chapter eight of the Ruby Cookbook, written by Lucas Carlson and Leonard Richardson (O'Reilly, 2006; ISBN: 0596523696). Copyright © 2006 O'Reilly Media, Inc. All rights reserved. Used with permission from the publisher. Available from booksellers or direct from O'Reilly Media.

7.4 Blocks as Closures: Using Outside Variables Within a Code Block

Problem

You want to share variables between a method, and a code block defined within it.

Solution

Just reference the variables, and Ruby will do the right thing. Here’s a method that adds a certain number to every element of an array:

  def add_to_all(array, number)
    array.collect { |x| x + number }
  end

  add_to_all([1, 2, 3], 10)   # => [11, 12, 13]

Enumerable#collectcan’t accessnumberdirectly, but it’s passed a block that can access it, sincenumberwas in scope when the block was defined.

Discussion

A Ruby block is a closure: it carries around the context in which it was defined. This is useful because it lets you define a block as though it were part of your normal code, then tear it off and send it to a predefined piece of code for processing.

A Ruby block contains references to the variable bindings, not copies of the values. If the variable changes later, the block will have access to the new value:

  tax_percent = 6
  position = lambda do
    "I have always supported a #{tax_percent}% tax on imported limes."
  end
  position.call
  # => "I have always supported a 6% tax on imported limes."

  tax_percent = 7.25
  position.call
  # => "I have always supported a 7.25% tax on imported limes."

This works both ways: you can rebind or modify a variable from within a block.

  counter = 0
  4.times { counter += 1; puts "Counter now #{counter}"}
  # Counter now 1
  # Counter now 2
  # Counter now 3
  # Counter now 4
  counter                            # => 4

This is especially useful when you want to simulateinjectorcollect in conjunction with a strange iterator. You can create a storage object outside the block, and add things to it from within the block. This code simulatesEnumerable#collect, but it collects the elements of an array in reverse order:

  accumulator = []
  [1, 2, 3].reverse_each { |x| accumulator << x + 1 }

  accumulator               # => [4, 3, 2]

Theaccumulatorvariable is not within the scope ofArray#reverse_each, but it is within the scope of the block.

Next: 7.5 Writing an Iterator Over a Data Structure >>
 

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