Object identity (equal? vs ==) in Ruby - Practice Questions

Challenge - 5 Problems

❓ Predict Output

intermediate

Understanding == vs equal? with strings

What is the output of this Ruby code?

Ruby

a = "hello"
b = "hello"
puts a == b
puts a.equal?(b)

A

false
true

B

true
false

C

true
true

D

false
false

💡 Hint

Remember that == checks value equality, while equal? checks if both variables point to the same object.

The == operator compares the content of the strings, so it returns true because both strings have the same characters.

The equal? method checks if both variables refer to the exact same object in memory. Since a and b are two different string objects with the same content, equal? returns false.

❓ Predict Output

intermediate

Object identity with integers

What will this Ruby code print?

Ruby

x = 1000
y = 1000
puts x == y
puts x.equal?(y)

A

true
false

B

true
true

C

false
false

D

false
true

💡 Hint

Check how Ruby handles integers and object identity for large numbers.

The == operator compares the values, so it returns true because both x and y are 1000.

The equal? method checks if both variables point to the same object. For integers larger than a certain range, Ruby creates separate objects, so equal? returns false.

❓ Predict Output

advanced

Comparing symbols with == and equal?

What is the output of this Ruby code?

Ruby

sym1 = :ruby
sym2 = :ruby
puts sym1 == sym2
puts sym1.equal?(sym2)

A

true
false

B

false
false

C

false
true

D

true
true

💡 Hint

Symbols are unique and immutable in Ruby.

Symbols with the same name are the same object in Ruby.

Therefore, both == and equal? return true.

❓ Predict Output

advanced

Effect of dup on object identity

What does this Ruby code print?

Ruby

arr1 = [1, 2, 3]
arr2 = arr1.dup
puts arr1 == arr2
puts arr1.equal?(arr2)

A

true
false

B

true
true

C

false
false

D

false
true

💡 Hint

dup creates a shallow copy of the object.

The == operator compares the content of the arrays, which are the same, so it returns true.

The equal? method checks if both variables point to the same object. Since dup creates a new object, equal? returns false.

❓ Predict Output

expert

Mutable objects and identity after modification

What is the output of this Ruby code?

Ruby

str1 = "hello"
str2 = str1
str2 << " world"
puts str1
puts str2
puts str1.equal?(str2)

A

hello
hello
true

B

hello
hello world
false

C

hello world
hello world
true

D

hello world
hello world
false

💡 Hint

Remember that modifying str2 affects str1 if they reference the same object.

Both str1 and str2 point to the same string object.

Appending " world" to str2 modifies the object itself, so str1 also shows the change.

equal? returns true because both variables reference the same object.

Practice

1. In Ruby, what does the method equal? check between two objects?

easy

A. If both objects respond to the same methods

B. If both objects have the same content or value

C. If both variables point to the exact same object in memory

D. If both objects are of the same class

Solution

1. Step 1: Understand equal? method purpose

   The equal? method checks if two variables reference the exact same object in memory, not just if they look alike.

2. Step 2: Compare with other methods

   Unlike ==, which compares content, equal? is about object identity.

3. Final Answer:

   If both variables point to the exact same object in memory -> Option C

4. Quick Check:

   equal? checks object identity = D [OK]

Hint: Remember: equal? means same object, not just same value [OK]

Common Mistakes:

• Confusing equal? with ==
• Thinking equal? compares content
• Assuming equal? checks class type

2. Which of the following Ruby code snippets correctly uses equal? to check if two variables refer to the same object?

easy

A. a.equal?(b)

B. a == b

C. a.equals(b)

D. equal?(a, b)

Solution

1. Step 1: Recall correct syntax for equal?

   The method equal? is called on one object and passed another as an argument, like a.equal?(b).

2. Step 2: Identify incorrect options

   a == b uses content equality, a.equals(b) is not a Ruby method, and equal?(a, b) is invalid syntax.

3. Final Answer:

   a.equal?(b) -> Option A

4. Quick Check:

   Correct equal? syntax = B [OK]

Hint: Use object.equal?(other_object) syntax [OK]

Common Mistakes:

• Using == instead of equal?
• Writing equals instead of equal?
• Calling equal? as a standalone function

3. What will be the output of the following Ruby code?

a = "hello"
b = "hello"
puts a == b
puts a.equal?(b)

medium

A. true
true

B. true
false

C. false
true

D. false
false

Solution

1. Step 1: Evaluate a == b

   Both a and b are strings with the same content "hello", so a == b returns true.

2. Step 2: Evaluate a.equal?(b)

   Though content is same, a and b are different string objects, so equal? returns false.

3. Final Answer:

   true false -> Option B

4. Quick Check:

   == compares content, equal? compares object identity = C [OK]

Hint: == checks value, equal? checks same object [OK]

Common Mistakes:

• Assuming equal? returns true for identical strings
• Confusing output order
• Thinking == checks object identity

4. Consider this Ruby code snippet:

arr1 = [1, 2, 3]
arr2 = arr1
arr3 = [1, 2, 3]

puts arr1.equal?(arr2)
puts arr1.equal?(arr3)

What is the output and why?

medium

A. false
false

B. false
true

C. true
true

D. true
false

Solution

1. Step 1: Analyze arr1.equal?(arr2)

   Since arr2 is assigned directly from arr1, both point to the same object, so equal? returns true.

2. Step 2: Analyze arr1.equal?(arr3)

   arr3 is a new array with the same content but a different object, so equal? returns false.

3. Final Answer:

   true false -> Option D

4. Quick Check:

   Same object = true, same content but different object = false [OK]

Hint: Assignment copies object reference; new array creates new object [OK]

Common Mistakes:

• Assuming arrays with same content are the same object
• Confusing == with equal?
• Thinking assignment creates a new object

5. You want to check if two variables x and y refer to the same object, but x might be nil. Which Ruby code safely performs this check without errors?

hard

A. x.equal?(y)

B. y.equals(x)

C. x&.equal?(y)

D. x == y

Solution

1. Step 1: Check safety of equal? on nil

   In Ruby, nil is an object and responds to equal?, so x.equal?(y) is safe even if x is nil. It returns false unless y is also nil.

2. Step 2: Eliminate distractors

   x&.equal?(y) returns nil if x is nil, which is not a proper boolean result. y.equals(x) calls a non-existent method. x == y checks content, not identity.

3. Final Answer:

   x.equal?(y) -> Option A

4. Quick Check:

   nil.equal?(y) safe boolean check = C [OK]

Hint: Even nil.equal?(other) works without error [OK]

Common Mistakes:

• Using &. which returns nil instead of boolean
• Calling non-existent equals method
• Using == which checks content, not identity