Hash creation with symbols and strings in Ruby - Mini Project: Build & Apply

Hash creation with symbols and strings

📖 Scenario: You are creating a simple contact list where each contact has a name and a phone number. You will use a Ruby hash to store this information.

🎯 Goal: Build a Ruby hash that uses both symbols and strings as keys to store contact names and their phone numbers.

📋 What You'll Learn

Create a hash with symbol keys and string keys

Use exact keys and values as specified

Access and print the hash content

💡 Why This Matters

🌍 Real World

Storing contact information in hashes is common in many Ruby applications like address books or user profiles.

💼 Career

Understanding how to use symbols and strings as hash keys is important for Ruby developers working with data structures and APIs.

1

Create a hash with symbol keys

Create a Ruby hash called contacts with these exact entries: :alice with value "123-4567", and :bob with value "987-6543".

Ruby

# Create the contacts hash with symbol keys
# Your code here

Hint

Use the Ruby hash syntax with symbols as keys like alice: "123-4567".

2

Add string keys to the hash

Add two more entries to the contacts hash with string keys: "carol" with value "555-1234", and "dave" with value "555-5678".

Ruby

contacts = { alice: "123-4567", bob: "987-6543" }
# Add string keys "carol" and "dave" with their phone numbers
# Your code here

Hint

Use square brackets to add string keys like contacts["carol"] = "555-1234".

3

Access and print all contacts

Use a contacts.each loop with variables key and value to iterate over the contacts hash and create a new hash called formatted_contacts where keys are strings (convert symbols to strings) and values are the phone numbers.

Ruby

contacts = { alice: "123-4567", bob: "987-6543" }
contacts["carol"] = "555-1234"
contacts["dave"] = "555-5678"

formatted_contacts = {}
# Use contacts.each to fill formatted_contacts with string keys
# Your code here

Hint

Use key.to_s to convert symbol keys to strings inside the loop.

4

Print the formatted contacts hash

Write puts formatted_contacts to display the formatted_contacts hash.

Ruby

contacts = { alice: "123-4567", bob: "987-6543" }
contacts["carol"] = "555-1234"
contacts["dave"] = "555-5678"

formatted_contacts = {}
contacts.each do |key, value|
  formatted_contacts[key.to_s] = value
end

# Print the formatted_contacts hash
# Your code here

Hint

Use puts to print the hash. The output will show keys as strings.

Practice

1. Which of the following is a valid way to create a Ruby hash with symbol keys?

easy

A. { name => "Alice", age => 30 }

B. { "name": "Alice", "age": 30 }

C. { :name => "Alice", "age": 30 }

D. { name: "Alice", age: 30 }

Solution

1. Step 1: Understand symbol key syntax

   In Ruby, symbol keys can be written as name: value without the rocket (=>) syntax.

2. Step 2: Check each option's syntax

   { name: "Alice", age: 30 } uses symbol keys with colon syntax correctly. { "name": "Alice", "age": 30 } uses string keys with colon, which is invalid. { :name => "Alice", "age": 30 } uses rocket syntax for the symbol key correctly but attempts colon syntax after a string key, which is invalid. { name => "Alice", age => 30 } uses bare words without colon or rocket, which is invalid.

3. Final Answer:

   { name: "Alice", age: 30 } -> Option D

4. Quick Check:

   Symbol keys use colon syntax = { name: "Alice", age: 30 } [OK]

Hint: Symbol keys use colon after name, not quotes or bare words [OK]

Common Mistakes:

• Using quotes around symbol keys with colon syntax
• Omitting colon or rocket for keys
• Mixing string and symbol keys without proper syntax

2. Which of the following hash definitions will cause a syntax error in Ruby?

easy

A. { name => "Bob", age => 25 }

B. { :name => "Bob", :age => 25 }

C. { "name" => "Bob", "age" => 25 }

D. { name: "Bob", age: 25 }

Solution

1. Step 1: Review valid key syntax

   Symbol keys can be written as :name => or name:. String keys must be quoted and use rocket syntax.

2. Step 2: Identify invalid syntax

   { name => "Bob", age => 25 } uses bare words as keys without colon or rocket, which is invalid and causes syntax error.

3. Final Answer:

   SyntaxError -> Option A

4. Quick Check:

   Bare words as keys need colon or rocket [OK]

Hint: Keys must be symbols with colon or strings with quotes and rocket [OK]

Common Mistakes:

• Using bare words as keys without colon or rocket
• Mixing string keys without quotes
• Confusing colon placement for symbols

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

person = { name: "Carol", "age" => 28 }
puts person[:name]
puts person["age"]

medium

A. Carol
28

B. Carol

C.
28

D. nil
nil

Solution

1. Step 1: Understand hash keys and access

   The hash has a symbol key :name and a string key "age". Accessing with matching key types returns values.

2. Step 2: Evaluate each puts statement

   person[:name] returns "Carol". person["age"] returns 28. Both print on separate lines.

3. Final Answer:

   Carol 28 -> Option A

4. Quick Check:

   Match key types to get values = Carol 28 [OK]

Hint: Use matching symbol or string keys to access hash values [OK]

Common Mistakes:

• Accessing string key with symbol or vice versa
• Expecting mixed keys to be interchangeable
• Forgetting keys are type-sensitive

4. Identify the error in this Ruby hash creation:

data = { :city => "Paris", country: "France", "population" => 2148327, state } 

medium

A. Hash keys cannot mix symbols and strings

B. Incorrect symbol syntax for :city

C. Missing value for key 'state'

D. String keys must use colon syntax

Solution

1. Step 1: Check each key-value pair

   Keys :city, country, and "population" have values. The key 'state' is written without a value, which is invalid.

2. Step 2: Identify syntax error

   Every key in a hash must have a corresponding value. Missing value for 'state' causes syntax error.

3. Final Answer:

   Missing value for key 'state' -> Option C

4. Quick Check:

   All keys need values in hash = Missing value for key 'state' [OK]

Hint: Every key must have a value; check for missing pairs [OK]

Common Mistakes:

• Leaving keys without values
• Confusing symbol and string key syntax
• Assuming mixed keys cause errors

5. Given the array arr = [["name", "Dave"], [:age, 40], ["city", "NY"]], which code correctly converts it into a hash with symbol keys where possible?

hard

A. hash = arr.to_h

B. hash = arr.map { |k, v| [k.to_sym, v] }.to_h

C. hash = arr.map { |k, v| [k, v] }.to_h

D. hash = arr.map { |k, v| [k.to_s, v] }.to_h

Solution

1. Step 1: Understand the array structure

   The array has pairs with keys as strings or symbols. We want all keys as symbols where possible.

2. Step 2: Convert keys to symbols

   hash = arr.map { |k, v| [k.to_sym, v] }.to_h converts each key to symbol using k.to_sym before creating the hash, ensuring keys are symbols.

3. Step 3: Check other options

   hash = arr.to_h keeps keys as is (mixed). hash = arr.map { |k, v| [k, v] }.to_h keeps keys as is without conversion. hash = arr.map { |k, v| [k.to_s, v] }.to_h converts all keys to strings, not symbols.

4. Final Answer:

   hash = arr.map { |k, v| [k.to_sym, v] }.to_h -> Option B

5. Quick Check:

   Use to_sym on keys to get symbol keys [OK]

Hint: Use map with to_sym on keys to get symbol keys [OK]

Common Mistakes:

• Not converting string keys to symbols
• Using to_s instead of to_sym
• Assuming to_h converts keys automatically