Hash Iteration
each, map, select.
Hash Iteration is a free Ruby Academy lesson on CoddyKit. This is lesson 3 of 4. Any 3 lessons of this learning path are free to read in full — after that, CoddyKit PRO unlocks every lesson, plus hands-on practice with a built-in code editor and a 24/7 AI tutor. It is part of the Ruby Academy learning path, and your progress syncs across the web and the CoddyKit app. The Ruby Academy course includes 4 lessons in total.
Iterating with each
The each method visits every pair. The block receives the key and value.
scores = { math: 90, art: 80 }
scores.each do |subject, score|
puts "#{subject}: #{score}"
endeach_pair
each_pair is an alias of each for hashes. Use whichever reads clearer.
h = { a: 1, b: 2 }
h.each_pair { |k, v| puts "#{k}=#{v}" }Iterating Keys Only
each_key loops over keys when you do not need the values.
h = { name: "Ada", age: 36 }
h.each_key { |k| puts k }Iterating Values Only
each_value loops over values directly.
h = { a: 10, b: 20 }
total = 0
h.each_value { |v| total += v }
puts totalTransforming with map
map on a hash returns an array. Each pair is yielded as key and value.
prices = { pen: 2, book: 5 }
labels = prices.map { |item, p| "#{item}: $#{p}" }
puts labels.inspecttransform_values
transform_values returns a new hash with each value changed but keys kept.
prices = { pen: 2, book: 5 }
doubled = prices.transform_values { |p| p * 2 }
puts doubledtransform_keys
transform_keys changes keys while keeping values. Useful for converting strings to symbols.
h = { "a" => 1, "b" => 2 }
puts h.transform_keys(&:to_sym)Filtering with select
select (alias filter) keeps pairs where the block is true and returns a new hash.
scores = { a: 90, b: 50, c: 70 }
passing = scores.select { |k, v| v >= 70 }
puts passingRejecting Pairs
reject is the opposite of select. It drops pairs where the block is true.
scores = { a: 90, b: 50 }
puts scores.reject { |k, v| v < 60 }Finding and Counting
find returns the first matching pair as an array. count with a block counts matches.
h = { a: 1, b: 2, c: 3 }
puts h.find { |k, v| v == 2 }.inspect
puts h.count { |k, v| v.odd? }Summing Values
Combine values with sum for totals, or use each_with_object for custom aggregation.
cart = { apple: 3, pear: 5 }
puts cart.values.sumQuick Check
Check your hash iteration knowledge.
Recap
You can now iterate hashes:
each,each_key,each_valuemap,transform_values,transform_keysselect/rejectto filterfind,count, and summing values
Next: default values and merging.
h = { a: 1, b: 2, c: 3 }
puts h.select { |k, v| v.odd? }
Learn Ruby with an AI tutor — free
Write and run real code in your browser, get instant help from a 24/7 AI tutor, and pick up where you left off on the web or in the app.
- Courses
- 29
- Lessons
- 109
Frequently Asked Questions
Is the “Hash Iteration” lesson free?
Yes — any 3 lessons of the Ruby Academy learning path, including “Hash Iteration”, are free to read in full here on the web. After that, CoddyKit PRO unlocks every lesson, plus interactive practice with a built-in code editor and a 24/7 AI tutor. The Ruby Academy course includes 4 lessons in total.
What will I learn in “Hash Iteration”?
each, map, select. You practise Ruby Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Ruby Academy?
No prior experience is required. Ruby Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 3 of 4.
How long does the “Hash Iteration” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Ruby Academy lesson?
Yes. Every Ruby Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.