Symbol type and immutability in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Ruby code using symbols?

Consider the following Ruby code snippet. What will it print?

Ruby

sym1 = :hello
sym2 = :hello
puts sym1.object_id == sym2.object_id

A SyntaxError

B true

C false

D nil

💡 Hint

Symbols with the same name share the same object in memory.

In Ruby, symbols are immutable and unique. Two symbols with the same name point to the same object, so their object_ids are equal.

❓ Predict Output

intermediate

What happens when you try to modify a symbol?

What error will this Ruby code produce?

Ruby

sym = :example
sym[0] = 'E'

A NoMethodError: undefined method `[]=' for Symbol

B RuntimeError: can't modify symbol

C TypeError: no implicit conversion of String into Integer

D SyntaxError

💡 Hint

Symbols do not support string-like modification methods.

Symbols are immutable and do not have methods like []= to modify them. Trying to call []= on a symbol raises NoMethodError.

🧠 Conceptual

advanced

Why are symbols considered immutable in Ruby?

Which of the following best explains why symbols are immutable?

A Because Ruby automatically freezes all objects of type Symbol

B Because symbols are stored as strings internally and strings are immutable

C Because symbols are unique and shared, changing one would affect all references

D Because symbols are garbage collected immediately after creation

💡 Hint

Think about what would happen if you changed a symbol's value.

Symbols are immutable because they are unique and shared across the program. Changing one symbol would change all references, which is not allowed.

❓ Predict Output

advanced

What is the output of this code comparing symbols and strings?

What will this Ruby code print?

Ruby

puts :ruby == 'ruby'
puts :ruby.eql?('ruby')

A

false
false

B

true
true

C

false
true

D

true
false

💡 Hint

Symbols and strings are different types even if they look similar.

Symbols and strings are different objects and types in Ruby. Comparing them with == or eql? returns false.

❓ Predict Output

expert

How many unique objects are created in this Ruby code?

How many unique objects are created after running this code?

Ruby

arr = [:a, :b, :a, :c, :b]
unique = arr.uniq
puts unique.size

A 2

B 5

C 4

D 3

💡 Hint

uniq removes duplicates from the array.

The array has symbols :a, :b, :a, :c, :b. The unique symbols are :a, :b, and :c, so size is 3.

Practice

1.

What is a key characteristic of a Ruby Symbol?

easy

A. It is immutable and unique for the same name

B. It can be changed after creation

C. It stores large amounts of data

D. It behaves like a string with spaces

Solution

1. Step 1: Understand what a Symbol is

   A Symbol in Ruby is a name or label that is immutable, meaning it cannot be changed once created.

2. Step 2: Recognize uniqueness of Symbols

   Two Symbols with the same name always refer to the same object, making them unique and memory efficient.

3. Final Answer:

   It is immutable and unique for the same name -> Option A

4. Quick Check:

   Symbol = immutable and unique [OK]

Hint: Symbols never change and are unique by name [OK]

Common Mistakes:

• Thinking symbols can be modified like strings
• Confusing symbols with regular strings
• Assuming symbols store large data
• Believing symbols allow spaces like strings

2.

Which of the following is the correct way to write a Ruby symbol?

?

easy

A. 'my_symbol'

B. "my_symbol"

C. my_symbol

D. :my_symbol

Solution

1. Step 1: Identify symbol syntax in Ruby

   Symbols are written with a colon : followed by a name without quotes.

2. Step 2: Compare options

   Only :my_symbol uses the correct symbol syntax; others are strings or invalid.

3. Final Answer:

   :my_symbol -> Option D

4. Quick Check:

   Symbol syntax = colon + name [OK]

Hint: Symbols start with a colon : followed by name [OK]

Common Mistakes:

• Using quotes around symbols
• Writing symbols without colon
• Confusing symbols with strings
• Using invalid characters in symbol names

3.

What will be the output of the following Ruby code?

sym1 = :apple
sym2 = :apple
puts sym1.object_id == sym2.object_id

medium

A. false

B. nil

C. true

D. SyntaxError

Solution

1. Step 1: Understand symbol object identity

   Symbols with the same name share the same object_id because they are unique and immutable.

2. Step 2: Compare object_ids of sym1 and sym2

   Both :apple symbols point to the same object, so their object_ids are equal, making the comparison true.

3. Final Answer:

   true -> Option C

4. Quick Check:

   Same symbol = same object_id [OK]

Hint: Same symbol names share object_id, so comparison is true [OK]

Common Mistakes:

• Assuming different symbols have different object_ids
• Confusing symbols with strings which differ
• Expecting false because of separate variables
• Thinking symbols are mutable objects

4.

Find the error in this Ruby code snippet:

sym = :hello
sym << " world"
puts sym

medium

A. Symbols can be changed with << operator

B. Symbols cannot be concatenated with <<

C. Syntax error in symbol declaration

D. Missing quotes around symbol

Solution

1. Step 1: Check symbol mutability

   Symbols are immutable, so you cannot change them or add to them with operators like <<.

2. Step 2: Identify the error in code

   The line sym << " world" tries to modify the symbol, which causes a runtime error.

3. Final Answer:

   Symbols cannot be concatenated with << -> Option B

4. Quick Check:

   Symbols are immutable, no << allowed [OK]

Hint: Symbols cannot be changed or concatenated [OK]

Common Mistakes:

• Trying to modify symbols like strings
• Assuming symbols can be concatenated
• Confusing syntax errors with runtime errors
• Using << operator on immutable objects

5.

Given the following code, what will be the output?

hash = { apple: 1, banana: 2 }
key = :apple
hash[key] = 3
puts hash[:apple]

Consider the immutability and identity of symbols in your answer.

hard

A. 3

B. 1

C. nil

D. KeyError

Solution

1. Step 1: Understand symbol keys in hash

   Symbols used as keys are immutable and unique, so :apple and key (which is :apple) refer to the same key.

2. Step 2: Update hash value for :apple

   Assigning hash[key] = 3 updates the value for the :apple key from 1 to 3.

3. Step 3: Output the updated value

   puts hash[:apple] prints the updated value 3.

4. Final Answer:

   3 -> Option A

5. Quick Check:

   Symbol keys are unique, so value updates correctly [OK]

Hint: Symbol keys are unique; updating key changes value [OK]

Common Mistakes:

• Thinking key is different because of variable
• Expecting original value 1 instead of updated 3
• Confusing symbols with strings as keys
• Assuming symbols can be duplicated in hash keys