Default values for missing keys in Ruby - Mini Project: Build & Apply

Default values for missing keys

📖 Scenario: Imagine you run a small library. You keep track of how many books each member has borrowed using a Ruby hash. Sometimes, a member might not have borrowed any books yet, so their name won't be in the hash. You want to make sure that when you check the number of books borrowed, you get 0 instead of nil for those members.

🎯 Goal: You will create a Ruby hash with some members and their borrowed book counts. Then, you will set a default value so that if you ask for a member not in the hash, it returns 0 instead of nil.

📋 What You'll Learn

Create a hash called borrowed_books with exact entries: 'Alice' => 3, 'Bob' => 5, 'Charlie' => 0

Set the default value of borrowed_books to 0

Access the borrowed book count for 'Diana' (a member not in the hash)

Print the borrowed book count for 'Diana'

💡 Why This Matters

🌍 Real World

Setting default values in hashes helps avoid errors when looking up data that might not exist yet, like tracking user activity or inventory counts.

💼 Career

Many programming jobs require handling data collections safely. Knowing how to set default values in hashes is a common and useful skill in Ruby development.

1

Create the initial hash

Create a hash called borrowed_books with these exact entries: 'Alice' => 3, 'Bob' => 5, 'Charlie' => 0

Ruby

# Create the borrowed_books hash with the given entries
# Your code here

Hint

Use curly braces {} to create a hash and separate keys and values with =>.

2

Set the default value for missing keys

Set the default value of the hash borrowed_books to 0 so that missing keys return 0 instead of nil

Ruby

borrowed_books = { 'Alice' => 3, 'Bob' => 5, 'Charlie' => 0 }
# Set default value to 0 for missing keys
# Your code here

Hint

Use hash.default = value to set a default value for missing keys.

3

Access a missing key

Access the borrowed book count for the member 'Diana' from the hash borrowed_books and store it in a variable called diana_books

Ruby

borrowed_books = { 'Alice' => 3, 'Bob' => 5, 'Charlie' => 0 }
borrowed_books.default = 0
# Get the borrowed book count for 'Diana'
# Your code here

Hint

Use hash[key] to get the value for a key.

4

Print the result

Print the value of the variable diana_books to show the borrowed book count for 'Diana'

Ruby

borrowed_books = { 'Alice' => 3, 'Bob' => 5, 'Charlie' => 0 }
borrowed_books.default = 0
diana_books = borrowed_books['Diana']
# Print the borrowed book count for 'Diana'
# Your code here

Hint

Use puts variable_name to print the value.

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