Conditional assignment (||=) in Ruby - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to assign 10 to x only if x is nil or false.

Ruby

x [1] 10

Drag options to blanks, or click blank then click option'

A+=

B=

C&&=

D||=

Attempts:

3 left

💡 Hint

Common Mistakes

Using = assigns unconditionally.

Using &&= assigns only if variable is truthy.

Using += adds to the existing value.

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

2fill in blank

medium

Complete the code to set default value 'guest' for username if it is nil or false.

Ruby

username [1] 'guest'

Drag options to blanks, or click blank then click option'

A&&=

B=

C||=

D+=

Attempts:

3 left

💡 Hint

Common Mistakes

Using = overwrites the value always.

Using &&= assigns only if variable is truthy.

Using += tries to add strings.

The ||= operator sets username to 'guest' only if it is nil or false.

3fill in blank

hard

Fix the error in the code to assign 5 to count only if count is nil or false.

Ruby

count [1] 5

Drag options to blanks, or click blank then click option'

A||=

B=

C&&=

D+=

Attempts:

3 left

💡 Hint

Common Mistakes

Using = assigns unconditionally.

Using &&= assigns only if variable is truthy.

Using += adds to the existing value.

Using ||= ensures count is assigned 5 only if it is nil or false.

4fill in blank

hard

Fill both blanks to assign 'blue' to color only if color is nil or false, and then append ' sky' to it.

Ruby

color [1] 'blue'
color [2] ' sky'

Drag options to blanks, or click blank then click option'

A||=

B+=

C=

D&&=

Attempts:

3 left

💡 Hint

Common Mistakes

Using = instead of ||= causes unconditional assignment.

Using &&= instead of ||= won't assign if nil.

Using = instead of += won't append.

First, color ||= 'blue' assigns 'blue' if color is nil or false. Then, color += ' sky' appends ' sky' to the string.

5fill in blank

hard

Fill all three blanks to assign 0 to score if nil or false, then add 10, then assign 'done' to status if nil or false.

Ruby

score [1] 0
score [2] 10
status [3] 'done'

Drag options to blanks, or click blank then click option'

A||=

B+=

D=

Attempts:

3 left

💡 Hint

Common Mistakes

Using = instead of ||= causes unconditional assignment.

Using = instead of += won't add to score.

Mixing up the order of operations.

First, score ||= 0 sets score to 0 if nil or false. Then, score += 10 adds 10. Finally, status ||= 'done' sets status if nil or false.

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