Iterating Through Hashes in Ruby

In my career, I have only seen Ruby hashes being iterated through by declaring both the key and value. However, there are several ways of approaching hash iterations.

Note that at the time of this writing I’m running Ruby 3.1.2.

Iterating With Key and Value

The most common way I’ve seen hash iterations is by declaring both the key and value variables in the loop. The result is as expected:

items = {
  'A': 1,
  'B': 2,
  'C': 3
}

items.each do |key, value|
  puts "key: #{key}, value: #{value}"
end

The code above has the following output:

key: A, value: 1
key: B, value: 2
key: C, value: 3

Iterating With a Single Variable

When iterating through a hash and declaring only a single variable, the variable is an array containing the key and value of the hash as elements.

items = {
  'A': 1,
  'B': 2,
  'C': 3
}

items.each do |item|
  puts "#{item}"
end

The code above has the following output:

[:A, 1]
[:B, 2]
[:C, 3]

In this case, the item variable is an array of 2 elements. The keys are Symbols while the values retain their original type.

There is also the .each_pair method, which works exactly the same way:

items = {
  'A': 1,
  'B': 2,
  'C': 3
}

items.each_pair do |pair|
  puts "#{pair}"
end

The output is exactly the same as before:

[:A, 1]
[:B, 2]
[:C, 3]

Iterating Through Keys

The keys of a Ruby hash can be retrieved with .keys:

items = {
  'A': 1,
  'B': 2,
  'C': 3
}

puts items.keys

The code above prints out the following:

A
B
C

If we want to iterate over each key, we can use .keys in a loop:

items = {
  'A': 1,
  'B': 2,
  'C': 3
}

items.keys.each do |key|
  puts key
end

And the output is exactly the same:

A
B
C

Finally, there is also the .each_key method that works the same way and is recommended by Rubocop:

items = {
  'A': 1,
  'B': 2,
  'C': 3
}

items.each_key do |key|
  puts key
end

Once again, the output is the same:

A
B
C

Iterating Through Values

Like with keys, the values of a hash can be retrieved with .values:

items = {
  'A': 1,
  'B': 2,
  'C': 3
}

puts items.values

The code above outputs the following:

1
2
3

We can use .values to iterate through the values of a hash:

items = {
  'A': 1,
  'B': 2,
  'C': 3
}

items.values.each do |value|
  puts value
end

The output is the same:

1
2
3

There is also the .each_value method that achieves the same result and is also recommended by Rubocop:

items = {
  'A': 1,
  'B': 2,
  'C': 3
}

items.each_value do |value|
  puts value
end

Once again, the output is exactly the same as before:

1
2
3