What is the "right" way to iterate through an array in Ruby?

Solution 1:

This will iterate through all the elements:

array = [1, 2, 3, 4, 5, 6]
array.each { |x| puts x }

Prints:

This will iterate through all the elements giving you the value and the index:

array = ["A", "B", "C"]
array.each_with_index {|val, index| puts "#{val} => #{index}" }

Prints:

A => 0
B => 1
C => 2

I'm not quite sure from your question which one you are looking for.

Solution 2:

I think there is no one right way. There are a lot of different ways to iterate, and each has its own niche.

each is sufficient for many usages, since I don't often care about the indexes.
each_ with _index acts like Hash#each - you get the value and the index.
each_index - just the indexes. I don't use this one often. Equivalent to "length.times".
map is another way to iterate, useful when you want to transform one array into another.
select is the iterator to use when you want to choose a subset.
inject is useful for generating sums or products, or collecting a single result.

It may seem like a lot to remember, but don't worry, you can get by without knowing all of them. But as you start to learn and use the different methods, your code will become cleaner and clearer, and you'll be on your way to Ruby mastery.

Solution 3:

I'm not saying that Array -> |value,index| and Hash -> |key,value| is not insane (see Horace Loeb's comment), but I am saying that there is a sane way to expect this arrangement.

When I am dealing with hashes, I am often more focused on the keys than the values, and I am usually dealing with keys and values in that order, either key => value or hash[key] = value.

If you want duck-typing, then either explicitly use a defined method as Brent Longborough showed, or an implicit method as maxhawkins showed.

Ruby is all about accommodating the language to suit the programmer, not about the programmer accommodating to suit the language. This is why there are so many ways. There are so many ways to think about something. In Ruby, you choose the closest and the rest of the code usually falls out extremely neatly and concisely.

As for the original question, "What is the “right” way to iterate through an array in Ruby?", well, I think the core way (i.e. without powerful syntactic sugar or object oriented power) is to do:

for index in 0 ... array.size
  puts "array[#{index}] = #{array[index].inspect}"
end

But Ruby is all about powerful syntactic sugar and object oriented power, but anyway here is the equivalent for hashes, and the keys can be ordered or not:

for key in hash.keys.sort
  puts "hash[#{key.inspect}] = #{hash[key].inspect}"
end

So, my answer is, "The “right” way to iterate through an array in Ruby depends on you (i.e. the programmer or the programming team) and the project.". The better Ruby programmer makes the better choice (of which syntactic power and/or which object oriented approach). The better Ruby programmer continues to look for more ways.

Now, I want to ask another question, "What is the “right” way to iterate through a Range in Ruby backwards?"! (This question is how I came to this page.)

It is nice to do (for the forwards):

(1..10).each{|i| puts "i=#{i}" }

but I don't like to do (for the backwards):

(1..10).to_a.reverse.each{|i| puts "i=#{i}" }

Well, I don't actually mind doing that too much, but when I am teaching going backwards, I want to show my students a nice symmetry (i.e. with minimal difference, e.g. only adding a reverse, or a step -1, but without modifying anything else). You can do (for symmetry):

(a=*1..10).each{|i| puts "i=#{i}" }

and

(a=*1..10).reverse.each{|i| puts "i=#{i}" }

which I don't like much, but you can't do

(*1..10).each{|i| puts "i=#{i}" }
(*1..10).reverse.each{|i| puts "i=#{i}" }
#
(1..10).step(1){|i| puts "i=#{i}" }
(1..10).step(-1){|i| puts "i=#{i}" }
#
(1..10).each{|i| puts "i=#{i}" }
(10..1).each{|i| puts "i=#{i}" }   # I don't want this though.  It's dangerous

You could ultimately do

class Range

  def each_reverse(&block)
    self.to_a.reverse.each(&block)
  end

end

but I want to teach pure Ruby rather than object oriented approaches (just yet). I would like to iterate backwards:

without creating an array (consider 0..1000000000)
working for any Range (e.g. Strings, not just Integers)
without using any extra object oriented power (i.e. no class modification)

I believe this is impossible without defining a pred method, which means modifying the Range class to use it. If you can do this please let me know, otherwise confirmation of impossibility would be appreciated though it would be disappointing. Perhaps Ruby 1.9 addresses this.

(Thanks for your time in reading this.)

Solution 4:

Use each_with_index when you need both.

ary.each_with_index { |val, idx| # ...

What is the "right" way to iterate through an array in Ruby?

Solution 1:

Solution 2:

Solution 3:

Solution 4:

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