Iterators

Ref. WGR Section 6.3, "Iterators and code blocks"

Loops as Methods

• An Iterator is a method that allows you to loop through all the members of a collection
• Works like "for" or "while" but without any extra language keywords
• It executes a block again and again
• Usually this block is the default block
  • this lets you define the block during the method call
  • concise and readable (arguably)

times is on your side

potatoes = nil
3.my_times do |i|
  potatoes = i+1
  puts "#{potatoes} potato"
end
puts potatoes + 1

implementing times using until

@@@ruby
class Integer
  def my_times
    i = 0
    until i == self
      yield i
      i += 1
    end
  end
end

to each his own

["apple", "banana", "cherry"].my_each do |fruit|
  puts "I love #{fruit}!"
end

warning: each returns the collection, not the value

def count_chars strings
  c = 0
  strings.each do |s|
    c += s.length
  end
  c
end

count_chars ["apple", "banana", "cherry"]
=> 17

implementing each using until

@@@ruby
class Array
  def my_each
    i = 0
    until i == size
      yield self[i]
      i += 1
    end
  end
end

the map is not the territory

["apple", "banana", "cherry"].map do |fruit|
  fruit.reverse
end
=> ["elppa", "ananab", "yrrehc"]

implementing map using each

@@@ruby
class Array
  def my_map
    a = []
    each do |item|
      a << yield(item)
    end
    a
  end
end

Other Awesome Iterators

• select
  • returns all items for which the block returns true(ish)
• reject
  • returns all items for which the block returns true(ish)
• collect (alias map)
  • makes a new array out of whatever the block returns
• detect (alias find)
  • returns a single item, not an array
  • returns the first item which makes the block return true(ish)
• inject
  • accumulates (huh?) -- more on this later

Ref. http://matthewcarriere.com/2008/06/23/using-select-reject-collect-inject-and-detect/

lethal injection

• inject is a really fun iterator, but it's really weird
• it passes a persistent "accumulator" to each iteration
• the return value of the block becomes the next accumulator

inject example

  @@@ruby
  class Array
    def sum
      inject(0) {|t,n| t + n}
    end
  end

  [1,2,3].sum #=> 6

• To help understand this, write out a table with the values of t, n, and the return value for each iteration.