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Hashes in Ruby

Key‑value maps with flexible key types; symbols are common as keys.

Basics

Create hashes with symbol keys or string keys; symbols are fast, immutable identifiers.

h1 = { name: "Matz", lang: "ruby" }
h2 = { "name" => "Matz", "lang" => "ruby" }
h1[:name]    # => "Matz"
h2["lang"]  # => "ruby"

Defaults and default procs

Setting a default value returns the same object each time—mutations will be shared. Prefer a block for per‑key defaults.

h = Hash.new([])
h[:a] << 1
h[:b] << 2
h[:a]  # => [1, 2]  (same default array!)

# Preferred: default block
h = Hash.new { |hash, key| hash[key] = [] }
h[:a] << 1
h[:b] << 2
h[:a]  # => [1]

Access: [], fetch, and dig

[] returns nil for missing keys; fetch can supply a default or raise; dig walks nested structures safely.

h = { user: { name: "Ava" } }
h[:missing]                # => nil
h.fetch(:missing, 0)       # => 0
h.fetch(:missing) { 42 }   # => 42 (block default)
h.dig(:user, :name)        # => "Ava"

Update and merge

Assign directly or merge hashes; later keys overwrite earlier ones.

h[:lang] = "rb"
updated = h.merge(platform: :unix)   # new hash
h.merge!(extra: true)                # mutate h

Transform and filter

Change keys/values in bulk or select specific pairs.

h = { a: 1, b: 2, c: 3 }
h.transform_keys! { |k| k.to_s.upcase }
h.transform_values! { |v| v * 10 }

h.select { |k, v| v > 10 }  # filtering
h.reject { |k, v| v == 20 }

Iteration and order

Hashes preserve insertion order. Iterate pairs, keys, or values.

h.each { |k, v| puts "#{k}=#{v}" }
h.keys      # => ["A", "B", "C"]
h.values    # => [10, 20, 30]

Nested hashes and autovivification

Use a default proc to build nested hashes on demand.

nested = Hash.new { |h, k| h[k] = Hash.new(&h.default_proc) }
nested[:users][:by_id][1] = { name: "Ava" }

Key checks and utilities

h.key?(:A)
h.include?("B")
h.has_value?(30)
h.fetch_values("A", "C")  # => [10, 30]

Summary

  • Prefer symbol keys for identifiers; strings interoperate with external APIs
  • Use block defaults to avoid shared‑object pitfalls
  • Reach for fetch, dig, and merge/merge! for robust access and updates
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