Conditional assignment (||=) in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Ruby code using ||= operator?

Consider the following Ruby code snippet. What will be printed when it runs?

Ruby

value = nil
value ||= 10
puts value

A 0

B nil

C Error

D 10

💡 Hint

Remember ||= assigns the value only if the variable is nil or false.

The ||= operator assigns the right side value only if the left side is nil or false. Since value starts as nil, it gets assigned 10.

❓ Predict Output

intermediate

What will be the value of variable after conditional assignment?

What is the value of count after this code runs?

Ruby

count = 5
count ||= 10
puts count

A nil

B 10

C 5

D Error

💡 Hint

The ||= operator does not overwrite if the variable is already truthy.

Since count is already 5 (which is truthy), count ||= 10 does not change it.

🔧 Debug

advanced

Why does this code raise an error with ||= operator?

This Ruby code raises an error. Which option explains the cause?

Ruby

def example
  @var ||= 5
  @var = nil
  @var ||= 10
end
example

A The code runs fine and returns 10

B The code raises a SyntaxError due to misplaced ||= operator

C The second ||= is ignored because @var is nil

D Using ||= on an uninitialized instance variable causes a NameError

💡 Hint

Instance variables default to nil if uninitialized.

Instance variables in Ruby default to nil if not set. The first ||= assigns 5, then @var is set to nil, so the second ||= assigns 10. The code runs fine and returns 10.

🧠 Conceptual

advanced

What does ||= do in Ruby?

Choose the best description of what the ||= operator does in Ruby.

A Assigns the right value only if the left variable is nil or false

B Always assigns the right value to the left variable

C Assigns the right value only if the left variable is truthy

D Assigns the right value only if the left variable is zero

💡 Hint

Think about when ||= skips assignment.

The ||= operator assigns the right side value only if the left side is nil or false (i.e., falsy). Otherwise, it keeps the original value.

❓ Predict Output

expert

What is the output of this Ruby code with ||= and method calls?

What will this Ruby code print?

Ruby

def get_value
  nil
end
result = get_value ||= 100
puts result

A 100

B Error

C nil

D false

💡 Hint

Consider operator precedence and what ||= applies to here.

The expression get_value ||= 100 tries to assign to the method get_value, which is not allowed. This causes a SyntaxError.

Practice

1. What does the Ruby operator ||= do when used like x ||= 10?

easy

A. Always assigns 10 to x, replacing any value

B. Assigns 10 to x only if x is true

C. Assigns 10 to x only if x is nil or false

D. Throws an error if x is already assigned

Solution

1. Step 1: Understand the ||= operator

   The operator ||= assigns the right side value only if the left side variable is nil or false.

2. Step 2: Apply to example x ||= 10

   If x is nil or false, it becomes 10; otherwise, it keeps its current value.

3. Final Answer:

   Assigns 10 to x only if x is nil or false -> Option C

4. Quick Check:

   Conditional assignment = Assign if nil/false [OK]

Hint: ||= sets value only if variable is nil or false [OK]

Common Mistakes:

• Thinking ||= always assigns the value
• Confusing ||= with simple assignment =
• Assuming ||= assigns when variable is true

2. Which of the following is the correct syntax to assign 5 to variable a only if a is nil or false?

easy

A. a ||= 5

B. a =|| 5

C. a =| 5

D. a =|= 5

Solution

1. Step 1: Identify correct operator syntax

   The correct Ruby operator for conditional assignment is ||=, not any variation with single or misplaced symbols.

2. Step 2: Match syntax to options

   Only a ||= 5 uses ||= correctly as a ||= 5.

3. Final Answer:

   a ||= 5 -> Option A

4. Quick Check:

   Correct syntax = a ||= 5 [OK]

Hint: Remember ||= has two vertical bars before equals [OK]

Common Mistakes:

• Using single | or misplaced symbols
• Confusing with assignment =
• Typing extra or missing characters

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

a = nil
b = 20
a ||= 10
b ||= 30
puts a
puts b

medium

A. 10\n30

B. nil\n30

C. nil\n20

D. 10\n20

Solution

1. Step 1: Evaluate a ||= 10 when a is nil

   Since a is nil, a ||= 10 assigns 10 to a.

2. Step 2: Evaluate b ||= 30 when b is 20

   Since b is already 20 (truthy), b ||= 30 does not change b.

3. Final Answer:

   10 20 -> Option D

4. Quick Check:

   nil replaced, truthy kept = 10 and 20 [OK]

Hint: ||= changes only nil or false variables [OK]

Common Mistakes:

• Assuming b changes to 30
• Thinking nil stays nil after ||= 10
• Mixing up output order

4. Find the error in this Ruby code snippet:

count = 0
count ||= 5
puts count

medium

A. No error; output is 0

B. Syntax error due to ||= usage

C. Output is 5 instead of 0

D. Runtime error because count is zero

Solution

1. Step 1: Understand value of count before ||=

   count is 0, which is not nil or false in Ruby (0 is truthy).

2. Step 2: Apply count ||= 5

   Since count is truthy (0), count ||= 5 does not change it.

3. Final Answer:

   No error; output is 0 -> Option A

4. Quick Check:

   0 is truthy, ||= keeps original value [OK]

Hint: Remember 0 is truthy in Ruby, ||= won't overwrite [OK]

Common Mistakes:

• Thinking 0 is false and gets replaced
• Expecting syntax or runtime error
• Confusing Ruby truthiness with other languages

5. Given this code:

settings = { theme: nil, font_size: 12 }
settings[:theme] ||= 'dark'
settings[:font_size] ||= 14
puts settings[:theme]
puts settings[:font_size]

What is the output and why?

hard

A. nil\n14 because theme is nil and font_size is replaced

B. dark\n12 because theme was nil and font_size was truthy

C. dark\n14 because both keys were nil or false

D. nil\n12 because ||= does not work on hashes

Solution

1. Step 1: Check initial values in settings hash

   settings[:theme] is nil, settings[:font_size] is 12 (truthy).

2. Step 2: Apply conditional assignment

   settings[:theme] ||= 'dark' sets theme to 'dark' because it was nil.
   settings[:font_size] ||= 14 does not change font_size because 12 is truthy.

3. Final Answer:

   dark 12 -> Option B

4. Quick Check:

   Nil replaced, truthy kept = dark and 12 [OK]

Hint: ||= sets defaults only if current value is nil or false [OK]

Common Mistakes:

• Assuming 12 is replaced by 14
• Thinking ||= doesn't work on hash values
• Confusing nil with false values