Welcome to this lesson on Hash Tables in Ruby, where we’ll explore the Hash class — a core data structure that provides efficient solutions for many algorithmic problems.

Hash tables store collections of key-value pairs and offer fast access, insertion, and deletion. This makes them invaluable for tasks where quick lookups are essential, significantly improving time complexity over other approaches in certain scenarios.

A Ruby Hash uses a hash function to store data based on a unique key, allowing rapid access by simply referencing the key.

For example, say we have a list of integers and a target sum, and we need to find two numbers that add up to this target. A brute-force approach would involve comparing each number with every other, resulting in a quadratic time complexity. However, by using a hash table, we can store each number as we encounter it, along with its index, and check if the required complement (target minus the current number) is already in the hash. This method drastically reduces computation time.

Here’s how the solution might look:

Ruby
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1def two_sum(nums, target)
2  hash = {}
3  nums.each_with_index do |num, i|
4    complement = target - num
5    return [hash[complement], i] if hash.key?(complement)
6    hash[num] = i
7  end
8  []
9end
10
11# Test
12puts two_sum([2, 7, 11, 15], 9).inspect  # Output: [0, 1]

Now that we’ve introduced Hash Tables through Ruby's Hash class, our exercises will dive deeper, helping you build familiarity with this powerful data structure and apply it to solve complex problems efficiently.

Mastering hashes will open up new possibilities in your problem-solving toolkit. Let’s get started!