Hash creation with symbols and strings in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a hash in Ruby?

A hash is a collection of key-value pairs, like a dictionary, where you can look up a value using its key.

beginner

How do you create a hash with symbol keys in Ruby?

You can create it using curly braces and symbol keys with colons, like: { name: "Alice", age: 30 }.

beginner

How do you create a hash with string keys in Ruby?

Use curly braces with strings as keys and hash rockets, like: { "name" => "Alice", "age" => 30 }.

intermediate

What is the difference between symbol keys and string keys in Ruby hashes?

Symbol keys are immutable and often faster, while string keys are mutable. Symbols are written without quotes and strings with quotes.

intermediate

Show an example of a Ruby hash mixing symbol and string keys.

Example: { name: "Alice", "age" => 30 }. You can mix keys but be careful when accessing values.

Which of these is a valid Ruby hash with symbol keys?

A { name: "Bob", city: "NY" }

B { "name" => "Bob", "city" => "NY" }

C [ name: "Bob", city: "NY" ]

D ( name: "Bob", city: "NY" )

Option A uses symbol keys with colons inside curly braces, which is the correct syntax for symbol-key hashes.

How do you access the value for key :name in hash { name: "Eve" }?

A hash["name"]

B hash[:name]

C hash.name

D hash->name

You access symbol keys using the symbol itself inside square brackets, like hash[:name].

Which symbol key is equivalent to the string key "age" in a Ruby hash?

A :age

B "age"

C age

D age:

The symbol :age is a different type from the string "age" but often used as a key in hashes.

What happens if you mix symbol and string keys in a Ruby hash?

A They are treated as the same key

B The string key overrides the symbol key

C They are treated as different keys

D Ruby throws an error

Symbol and string keys are different objects, so they are treated as different keys in a hash.

Which syntax creates a hash with string keys?

A { color: "red", size: "M" }

B ( color: "red", size: "M" )

C [ "color" => "red", "size" => "M" ]

D { "color" => "red", "size" => "M" }

Using strings as keys requires the hash rocket => syntax inside curly braces.

Explain how to create a Ruby hash using symbol keys and how to access its values.

Think about keys without quotes and how you get values.

Describe the difference between using string keys and symbol keys in Ruby hashes.

Consider how keys look and behave.

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