Hash vs Array in Ruby: Key Differences and When to Use Each

In Ruby, a Hash stores key-value pairs allowing fast lookup by keys, while an Array holds ordered lists of elements accessed by index. Use Hash when you need to associate unique keys with values, and Array when order and position matter.

Quick Comparison

Here is a quick side-by-side look at the main differences between Hash and Array in Ruby.

Factor	Hash	Array
Data Structure Type	Key-value pairs	Ordered list of elements
Access Method	By unique key	By numeric index
Order	Preserves insertion order (Ruby 1.9+)	Always ordered
Duplicates	Keys are unique, values can repeat	Elements can repeat
Use Case	Lookup by key, dictionary-like	Sequence, list, stack, queue
Performance	Fast key lookup (average O(1))	Fast index access (O(1))

Key Differences

A Hash in Ruby is a collection of unique keys mapped to values. You use keys to quickly find the associated value. Keys can be symbols, strings, numbers, or any object that implements hash and eql?. Hashes preserve the order in which pairs are added starting from Ruby 1.9.

On the other hand, an Array is an ordered list of elements accessed by their position index starting at zero. Arrays allow duplicates and are best when order and sequence matter, like storing a list of items or iterating in order.

While both can store multiple items, the main difference is how you access them: Hash uses keys for direct lookup, making it ideal for dictionary-like data, whereas Array uses numeric indexes, making it ideal for ordered collections.

Code Comparison

ruby

person = { name: "Alice", age: 30, city: "NY" }
puts person[:name]
puts person[:age]

fruits = ["apple", "banana", "cherry"]
puts fruits[0]
puts fruits[2]

Output

Alice
30
apple
cherry

Array Equivalent

ruby

person = ["Alice", 30, "NY"]
puts person[0]  # name
puts person[1]  # age

Output

Alice
30

When to Use Which

Choose a Hash when you need to associate unique keys with values and want fast lookup by those keys, such as storing user attributes or configuration settings. Use an Array when you need to keep items in order, allow duplicates, or process elements by position, like a list of tasks or a sequence of events.

In short, use Hash for key-based access and Array for ordered collections.

Key Takeaways

Use Hash for fast key-value lookups with unique keys.

Use Array for ordered lists where position matters.

Hashes preserve insertion order and allow flexible key types.

Arrays allow duplicates and are accessed by numeric index.

Choose based on whether you need key-based access or ordered sequence.