Hash methods (keys, values, each) in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of keys and values methods

What is the output of this Ruby code?

Ruby

h = {a: 1, b: 2, c: 3}
puts h.keys.inspect
puts h.values.inspect

A

[:a, :b, :c]
["1", "2", "3"]

B

["a", "b", "c"]
[1, 2, 3]

C

["a", "b", "c"]
["1", "2", "3"]

D

[:a, :b, :c]
[1, 2, 3]

💡 Hint

Remember that Ruby symbols are shown with a colon and strings with quotes.

The keys method returns an array of the hash's keys as symbols, and values returns an array of the values as they are stored (integers here).

❓ Predict Output

intermediate

Using each to iterate over a hash

What does this Ruby code print?

Ruby

h = {x: 10, y: 20}
h.each do |key, value|
  puts "#{key}:#{value}" 
end

A

10:x
20:y

B

x:10
y:20

C

[:x, 10]
[:y, 20]

D

x => 10
y => 20

💡 Hint

Look at how the block variables are used inside the string.

The each method yields each key and value pair. The code prints key and value separated by a colon on separate lines.

🧠 Conceptual

advanced

Behavior of keys method on empty hash

What is the result of calling keys on an empty hash in Ruby?

A An empty array []

B nil

C Raises an error

D An array with one nil element [nil]

💡 Hint

Think about what keys exist in an empty hash.

Calling keys on an empty hash returns an empty array because there are no keys.

❓ Predict Output

advanced

Effect of modifying hash inside each

What is the output of this Ruby code?

Ruby

h = {a: 1, b: 2}
h.each do |k, v|
  h[k] = v * 2
end
puts h.inspect

A {:a=>1, :b=>4}

B {:a=>1, :b=>2}

C {:a=>2, :b=>4}

D RuntimeError: can't modify hash during iteration

💡 Hint

Think about whether modifying values during iteration is allowed.

Modifying the values of existing keys during each is allowed. The values get doubled.

❓ Predict Output

expert

Output of nested each with keys and values

What is printed by this Ruby code?

Ruby

h = {one: [1, 2], two: [3, 4]}
h.each do |key, values|
  values.each do |v|
    puts "#{key}-#{v}"
  end
end

A

one-1
one-2
two-3
two-4

B

one-1
two-2
one-3
two-4

C

one-1
one-2
one-3
one-4

D

1-one
2-one
3-two
4-two

💡 Hint

Look carefully at the nested iteration and string interpolation.

The outer each iterates keys and arrays. The inner each iterates values, printing key-value pairs separated by a dash.

Practice

1. What does the keys method do when called on a Ruby hash?

easy

A. Deletes all keys from the hash

B. Returns an array of all the values in the hash

C. Returns an array of all the keys in the hash

D. Iterates over each key-value pair in the hash

Solution

1. Step 1: Understand the keys method

   The keys method extracts all keys from the hash and returns them as an array.

2. Step 2: Compare with other methods

   values returns values, each iterates pairs, so only keys returns keys array.

3. Final Answer:

   Returns an array of all the keys in the hash -> Option C

4. Quick Check:

   keys = keys array [OK]

Hint: Remember: keys method lists all keys as an array [OK]

Common Mistakes:

• Confusing keys with values method
• Thinking keys modifies the hash
• Mixing up keys with each method

2. Which of the following is the correct syntax to iterate over each key-value pair in a Ruby hash h?

easy

A. h.each(key, value) { puts key, value }

B. h.each { |key, value| puts key + value }

C. h.each do |key value| puts key, value end

D. h.each { key, value -> puts key + value }

Solution

1. Step 1: Identify correct block syntax for each

   The correct syntax uses curly braces or do-end with block parameters separated by a comma inside pipes: { |key, value| ... }.

2. Step 2: Check each option

   h.each { |key, value| puts key + value } uses correct syntax with pipes and comma. Others have syntax errors or wrong block format.

3. Final Answer:

   h.each { |key, value| puts key + value } -> Option B

4. Quick Check:

   Correct block syntax = h.each { |key, value| puts key + value } [OK]

Hint: Use pipes and commas inside block for key-value pairs [OK]

Common Mistakes:

• Omitting pipes around block parameters
• Missing comma between key and value
• Using arrow syntax incorrectly in blocks

3. What will be the output of the following Ruby code?

h = {a: 1, b: 2, c: 3}
h.keys

medium

A. [:1, :2, :3]

B. [1, 2, 3]

C. {a: 1, b: 2, c: 3}

D. [:a, :b, :c]

Solution

1. Step 1: Understand keys method on hash

   The keys method returns an array of all keys in the hash. Here keys are symbols :a, :b, :c.

2. Step 2: Match output with options

   [:a, :b, :c] matches the array of keys. [1, 2, 3] is values, C is the whole hash, D is invalid symbol array.

3. Final Answer:

   [:a, :b, :c] -> Option D

4. Quick Check:

   h.keys = [:a, :b, :c] [OK]

Hint: keys returns array of keys exactly as in hash [OK]

Common Mistakes:

• Confusing keys with values output
• Expecting hash instead of array
• Misreading symbols as strings

4. Identify the error in the following Ruby code snippet:

h = {x: 10, y: 20}
h.each do |k v|
  puts "#{k}: #{v}"
end

medium

A. Missing comma between block parameters k and v

B. Hash keys must be strings, not symbols

C. The each method cannot be used with do-end

D. The puts statement syntax is incorrect

Solution

1. Step 1: Check block parameter syntax

   Block parameters must be separated by a comma inside pipes: |k, v|. Here comma is missing.

2. Step 2: Verify other parts

   Keys as symbols are valid, each do-end is valid, and puts syntax is correct.

3. Final Answer:

   Missing comma between block parameters k and v -> Option A

4. Quick Check:

   Block params need comma [OK]

Hint: Always separate block parameters with commas inside pipes [OK]

Common Mistakes:

• Omitting comma between block variables
• Thinking symbols are invalid keys
• Confusing block syntax with method call

5. Given the hash h = {a: 1, b: 0, c: nil, d: 4}, which code snippet correctly returns an array of keys whose values are truthy (not false or nil)?

hard

A. h.select { |k, v| v }.keys

B. h.keys.select { |k| k }

C. h.values.select { |v| v }.keys

D. h.each { |k, v| k if v }.keys

Solution

1. Step 1: Understand filtering keys by truthy values

   We want keys where values are truthy (not false or nil). select filters pairs by value truthiness.

2. Step 2: Analyze each option

   h.select { |k, v| v }.keys filters hash pairs by truthy value, then gets keys -> [:a, :d] [check]. h.keys.select { |k| k } checks if key truthy (all symbols are), returns all keys. h.values.select { |v| v }.keys calls keys on array (NoMethodError). h.each { |k, v| k if v }.keys returns original hash, no filtering.

3. Final Answer:

   h.select { |k, v| v }.keys -> Option A

4. Quick Check:

   Filter pairs then get keys = h.select { |k, v| v }.keys [OK]

Hint: Use select on hash then call keys to filter by values [OK]

Common Mistakes:

• Trying to call keys on array of values
• Using each instead of select for filtering
• Selecting keys without checking values