Conditional assignment (||=) in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the operator ||= do in Ruby?

It assigns a value to a variable only if that variable is currently nil or false. It means "assign if not already set."

beginner

Given name ||= "Guest", what happens if name is already "Alice"?

The value of name stays "Alice" because ||= only assigns if name is nil or false.

beginner

Why is ||= useful in Ruby programs?

It helps set default values without overwriting existing ones, making code cleaner and easier to read.

intermediate

What will be the output of this Ruby code?

count = nil
count ||= 10
count ||= 20
puts count

The output will be 10. The first ||= assigns 10 because count is nil. The second ||= does nothing because count is already 10 (not nil or false).

intermediate

Can ||= assign a value if the variable is false?

Yes. ||= treats both nil and false as "not set" and will assign the new value in either case.

What does variable ||= value do in Ruby?

A Assigns value only if variable is nil or false

B Always assigns value to variable

C Assigns value only if variable is true

D Assigns value only if variable is a number

||= assigns the value only if the variable is nil or false, otherwise it keeps the original value.

If score = 0, what will score ||= 10 do?

A Assign 10 to score

B Raise an error

C Keep score as 0

D Assign nil to score

Since 0 is neither nil nor false, score ||= 10 keeps score as 0.

Which values cause ||= to assign the new value?

A Only nil

B Both nil and false

C Any value

D Only false

||= assigns the new value if the variable is either nil or false.

What is a common use of ||= in Ruby?

A To set default values

B To multiply numbers

C To delete variables

D To compare strings

||= is often used to set default values if a variable is not already set.

What will this code print?

flag = false
flag ||= true
puts flag

A false

B nil

C error

D true

Since flag is false, flag ||= true assigns true to flag.

Explain how the ||= operator works in Ruby and give a simple example.

Think about setting a variable only if it doesn't have a useful value yet.

Describe a situation where using ||= can make your Ruby code cleaner.

Consider when you want to assign a value only if none exists.

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