Default values for missing keys in Ruby - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a hash with a default value of 0.

Ruby

counts = Hash.new([1])

Drag options to blanks, or click blank then click option'

A''

Bfalse

Cnil

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using nil as default value returns nil for missing keys, not 0.

Using empty string '' sets default to a string, not a number.

Using Hash.new(0) sets the default value to 0 for any missing key.

2fill in blank

medium

Complete the code to create a hash with a default value of an empty array.

Ruby

groups = Hash.new([1])

Drag options to blanks, or click blank then click option'

A{}

B[]

Cnil

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using curly braces {} creates an empty hash, not an array.

Using nil means missing keys return nil, not an array.

Using Hash.new([]) sets the default value to the same empty array for all missing keys.

3fill in blank

hard

Fix the error in the code to correctly set a default value that is a new empty array for each missing key.

Ruby

groups = Hash.new { |hash, key| hash[[1]] = [] }

Drag options to blanks, or click blank then click option'

Akey

Bhash

Cgroups

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'hash' instead of 'key' as the index causes an error.

Using 'value' is incorrect because the block only has hash and key parameters.

The block receives the hash and the missing key. Assigning hash[key] = [] creates a new empty array for each missing key.

4fill in blank

hard

Fill both blanks to create a hash that counts occurrences of words.

Ruby

counts = Hash.new([1])
words.each do |word|
  counts[word] = counts[word] [2] 1
end

Drag options to blanks, or click blank then click option'

A0

B+

C-

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-' subtracts instead of adding.

Using '*' multiplies, which is incorrect for counting.

Setting default to 0 allows counting. Using '+' adds 1 to the current count.

5fill in blank

hard

Fill all three blanks to create a hash with default value as a new array and add elements to it.

Ruby

groups = Hash.new { |hash, [1]| hash[[2]] = [3] }
groups[:fruits] << 'apple'

Drag options to blanks, or click blank then click option'

Akey

C[]

D{}

Attempts:

3 left

💡 Hint

Common Mistakes

Using '{}' assigns an empty hash instead of an array.

Using different names for the key parameter causes errors.

The block takes the missing key as 'key'. Assigning hash[key] = [] creates a new empty array for each missing key, allowing elements to be added.

Practice

1. What does setting a default value for a Ruby hash do when you access a key that does not exist?
h = Hash.new(0)

easy

A. Returns the default value instead of nil for missing keys

B. Raises an error for missing keys

C. Adds the missing key with nil value to the hash

D. Returns nil for missing keys as usual

Solution

1. Step 1: Understand default value behavior

   When a hash is created with Hash.new(0), it sets 0 as the default value for any missing key.

2. Step 2: Accessing a missing key

   Accessing a key not present in the hash returns the default value 0 instead of nil.

3. Final Answer:

   Returns the default value instead of nil for missing keys -> Option A

4. Quick Check:

   Default value = 0 returned for missing keys [OK]

Hint: Default value replaces nil for missing keys [OK]

Common Mistakes:

• Thinking missing keys raise errors
• Assuming missing keys return nil always
• Believing missing keys get added automatically

2. Which of the following is the correct syntax to create a Ruby hash with a default value of an empty array for missing keys?

easy

A. h = Hash.new { |key| [] }

B. h = Hash.new { [] }

C. h = Hash.new([])

D. h = Hash.new { |hash, key| hash[key] = [] }

Solution

1. Step 1: Understand default block usage

   Using a block with Hash.new allows dynamic default values. To assign a new array to each missing key, the block must assign it to the hash.

2. Step 2: Identify correct block syntax

   The block receives hash and key parameters and assigns [] to hash[key]. This ensures each missing key gets its own array.

3. Final Answer:

   h = Hash.new { |hash, key| hash[key] = [] } -> Option D

4. Quick Check:

   Block assigns new array to missing keys [OK]

Hint: Use block with two params to assign default arrays [OK]

Common Mistakes:

• Using Hash.new([]) shares one array for all keys
• Using block without assigning to hash[key]
• Using wrong block parameters

3. What is the output of the following Ruby code?

h = Hash.new { |hash, key| hash[key] = [] }
h[:a] << 1
h[:b] << 2
p h

medium

A. {:a=>[1], :b=>[2], :c=>[]}

B. {:a=>[1], :b=>[2], nil=>[]}

C. {:a=>[1], :b=>[2]}

D. {}

Solution

1. Step 1: Understand default block behavior

   The hash is created with a block that assigns an empty array to missing keys when accessed.

2. Step 2: Modify arrays for keys :a and :b

   Appending 1 to h[:a] creates :a => [1]. Similarly, appending 2 to h[:b] creates :b => [2].

3. Step 3: Print the hash

   Only keys :a and :b have values. No other keys exist.

4. Final Answer:

   {:a=>[1], :b=>[2]} -> Option C

5. Quick Check:

   Default arrays assigned and modified correctly [OK]

Hint: Default block assigns new array per missing key [OK]

Common Mistakes:

• Expecting keys not accessed to appear
• Confusing default value with nil
• Thinking default array is shared

4. Identify the error in this Ruby code that tries to set a default value of 0 for missing keys:

h = Hash.new
h.default = 0
puts h[:missing]

medium

A. h.default=0 sets default but does not affect missing keys

B. No error; code works and prints 0

C. h.default=0 should be h.default = 0 (space required)

D. Syntax error in setting default value

Solution

1. Step 1: Check default value assignment

   Using h.default = 0 correctly sets the default value for missing keys after hash creation.

2. Step 2: Access missing key

   Accessing h[:missing] returns 0 as expected, no error occurs.

3. Final Answer:

   No error; code works and prints 0 -> Option B

4. Quick Check:

   Setting default after creation works [OK]

Hint: Use h.default = value to set default after creation [OK]

Common Mistakes:

• Thinking default= is a syntax error
• Confusing default= with default_proc
• Expecting missing keys to raise errors

5. You want a Ruby hash that counts how many times each word appears in an array. Which code correctly uses default values to do this?

words = ['apple', 'banana', 'apple', 'orange', 'banana', 'apple']
counter = ???
words.each { |w| counter[w] += 1 }
puts counter

hard

A. counter = Hash.new(0)

B. counter = Hash.new { |h, k| [] }

C. counter = Hash.new([])

D. counter = {}

Solution

1. Step 1: Understand counting with default values

   To count words, missing keys must return 0 so counter[w] += 1 works. The += reads the default 0, increments to 1, and assigns back, storing the key.

2. Step 2: Compare options

   Hash.new(0) returns 0 for missing keys (no store); += 1 stores the result. Perfect for counting.
   Hash.new { |h, k| [] } returns []; [] += 1 TypeError.
   Hash.new([]) returns shared []; TypeError.
   {} returns nil; error.

3. Final Answer:

   counter = Hash.new(0) -> Option A

4. Quick Check:

   Hash.new(0) enables counter[w] += 1 [OK]

Hint: Hash.new(0) for numeric counting [OK]

Common Mistakes:

• Using block or Hash.new([]) returns array; TypeError on += 1
• Using {} with no default; nil error
• Wrong block parameters or no assignment