Basic Array Iteration in Ruby

Question

What's a better way to traverse an array while iterating through another array? For example, if I have two arrays like the following:

names = [ "Rover", "Fido", "Lassie", "Calypso"]
breeds = [ "Terrier", "Lhasa Apso", "Collie", "Bulldog"]

Assuming the arrays correspond with one another - that is, Rover is a Terrier, Fido is a Lhasa Apso, etc. - I'd like to create a dog class, and a new dog object for each item:

class Dog
  attr_reader :name, :breed

  def initialize(name, breed)
    @name = name
    @breed = breed
  end
end

I can iterate through names and breeds with the following:

index = 0

names.each do |name|
  Dog.new("#{name}", "#{breeds[index]}")
  index = index.next
end

However, I get the feeling that using the index variable is the wrong way to go about it. What would be a better way?

Answer 1

dogs = names.zip(breeds).map { |name, breed| Dog.new(name, breed) }

Array#zip interleaves the target array with elements of the arguments, so

irb> [1, 2, 3].zip(['a', 'b', 'c'])
 #=> [ [1, 'a'], [2, 'b'], [3, 'c'] ]

You can use arrays of different lengths (in which case the target array determines the length of the resulting array, with the extra entries filled in with nil).

irb> [1, 2, 3, 4, 5].zip(['a', 'b', 'c'])
 #=> [ [1, 'a'], [2, 'b'], [3, 'c'], [4, nil], [5, nil] ]
irb> [1, 2, 3].zip(['a', 'b', 'c', 'd', 'e'])
 #=> [ [1, 'a'], [2, 'b'], [3, 'c'] ]

You can also zip more than two arrays together:

irb> [1,2,3].zip(['a', 'b', 'c'], [:alpha, :beta, :gamma])
 #=> [ [1, 'a', :alpha], [2, 'b', :beta], [3, 'c', :gamma] ]

Array#map is a great way to transform an array, since it returns an array where each entry is the result of running the block on the corresponding entry in the target array.

irb> [1,2,3].map { |n| 10 - n }
 #=> [ 9, 8, 7 ]

When using iterators over arrays of arrays, if you give a multiple parameter block, the array entries will be automatically broken into those parameters:

irb> [ [1, 'a'], [2, 'b'], [3, 'c'] ].each { |array| p array }
[ 1, 'a' ]
[ 2, 'b' ]
[ 3, 'c' ]
#=> nil
irb> [ [1, 'a'], [2, 'b'], [3, 'c'] ].each do |num, char| 
...>   puts "number: #{num}, character: #{char}" 
...> end
number 1, character: a
number 2, character: b
number 3, character: c
#=> [ [1, 'a'], [2, 'b'], [3, 'c'] ]

Like Matt Briggs mentioned, #each_with_index is another good tool to know about. It iterates through the elements of an array, passing a block each element in turn.

irb> ['a', 'b', 'c'].each_with_index do |char, index| 
...>   puts "character #{char} at index #{index}"
...> end
character a at index 0
character b at index 1
character c at index 2
#=> [ 'a', 'b', 'c' ]

When using an iterator like #each_with_index you can use parentheses to break up array elements into their constituent parts:

irb> [ [1, 'a'], [2, 'b'], [3, 'c'] ].each_with_index do |(num, char), index| 
...>   puts "number: #{num}, character: #{char} at index #{index}" 
...> end
number 1, character: a at index 0
number 2, character: b at index 1
number 3, character: c at index 2
#=> [ [1, 'a'], [2, 'b'], [3, 'c'] ]

Answer 2

each_with_index leaps to mind, it is a better way to do it the way you are doing it. rampion has a better overall answer though, this situation is what zip is for.

Answer 3

This is adapted from Flanagan and Matz, "The Ruby Programming Language", 5.3.5 "External Iterators", Example 5-1, p. 139:

++++++++++++++++++++++++++++++++++++++++++

require 'enumerator'  # needed for Ruby 1.8

names = ["Rover", "Fido", "Lassie", "Calypso"]  
breeds = ["Terrier", "Lhasa Apso", "Collie", "Bulldog"]

class Dog  
    attr_reader :name, :breed  

    def initialize(name, breed)  
        @name = name  
        @breed = breed  
    end  
end

def bundle(*enumerables)  
    enumerators = enumerables.map {|e| e.to_enum}  
    loop {yield enumerators.map {|e| e.next} }  
end  

bundle(names, breeds) {|x| p Dog.new(*x) }  

+++++++++++++++++++++++++++++++++++++++++++

Output:

#<Dog:0x10014b648 @name="Rover", @breed="Terrier">  
#<Dog:0x10014b0d0 @name="Fido", @breed="Lhasa Apso">  
#<Dog:0x10014ab80 @name="Lassie", @breed="Collie">  
#<Dog:0x10014a770 @name="Calypso", @breed="Bulldog">  

which I think is what we wanted!

Answer 4

As well as each_with_index (mentioned by Matt), there's each_index. I sometimes use this because it makes the program more symmetrical, and therefore wrong code will look wrong.

names.each_index do |i|
  name, breed = dogs[i], breeds[i] #Can also use dogs.fetch(i) if you want to fail fast
  Dog.new(name, breed)
end