Spaceship operator (<=>) in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this spaceship operator comparison?

Consider the following Ruby code using the spaceship operator (<=>). What will be printed?

Ruby

puts 5 <=> 10

A 1

B 0

C nil

D -1

💡 Hint

The spaceship operator returns -1 if the left side is less than the right side.

The spaceship operator returns -1 when the left value is smaller than the right value, 0 if equal, and 1 if greater.

❓ Predict Output

intermediate

What does this spaceship operator expression return?

What is the output of this Ruby code snippet?

Ruby

result = 'apple' <=> 'apple'
puts result

A -1

B 0

C 1

D nil

💡 Hint

The spaceship operator returns 0 when both sides are equal.

Since both strings are the same, the operator returns 0.

❓ Predict Output

advanced

What is the output of this spaceship operator with arrays?

What will this Ruby code print?

Ruby

puts [1, 2, 3] <=> [1, 2, 4]

A -1

B 0

C 1

D nil

💡 Hint

The spaceship operator compares arrays element by element.

The first differing element is 3 vs 4, and since 3 < 4, the result is -1.

❓ Predict Output

advanced

What happens when comparing incompatible types with <=>?

What is the output of this Ruby code?

Ruby

puts 10 <=> '10'

A nil

B 1

C -1

D 0

💡 Hint

The spaceship operator returns nil when the two objects are not comparable.

Since an integer and a string cannot be compared, the operator returns nil.

🧠 Conceptual

expert

How does the spaceship operator behave in custom class sorting?

Given a Ruby class with a custom <=> method, what will be the output of sorting an array of its objects? class Box attr_reader :volume def initialize(volume) @volume = volume end def <=>(other) volume <=> other.volume end end boxes = [Box.new(30), Box.new(10), Box.new(20)] sorted_volumes = boxes.sort.map(&:volume) puts sorted_volumes.inspect

A [30, 20, 10]

B Raises an error

C [10, 20, 30]

D [20, 10, 30]

💡 Hint

The spaceship operator defines how objects are compared during sorting.

The custom <=> compares volumes, so sorting orders boxes by volume ascending.

Practice

1.

What does the Ruby spaceship operator <=> return when comparing two values?

easy

A. true if left is smaller, false if equal, nil if left is bigger

B. -1 if left is smaller, 0 if equal, 1 if left is bigger

C. The sum of both values

D. A boolean indicating if values are equal

Solution

1. Step 1: Understand the spaceship operator purpose

   The spaceship operator compares two values and returns an integer indicating their order.

2. Step 2: Recall the return values

   It returns -1 if the left value is smaller, 0 if both are equal, and 1 if the left is bigger.

3. Final Answer:

   -1 if left is smaller, 0 if equal, 1 if left is bigger -> Option B

4. Quick Check:

   Spaceship operator returns -1, 0, or 1 [OK]

Hint: Remember: -1 smaller, 0 equal, 1 bigger [OK]

Common Mistakes:

• Confusing return values with booleans
• Thinking it returns a sum or boolean
• Mixing up which side is compared

2.

Which of the following is the correct syntax to compare a and b using the spaceship operator in Ruby?

?

easy

A. a => b

B. a =< b

C. a <= b

D. a <=> b

Solution

1. Step 1: Identify the spaceship operator syntax

   The spaceship operator is written as <=> between two values.

2. Step 2: Match the correct syntax

   Only a <=> b uses the correct operator symbol and order.

3. Final Answer:

   a <=> b -> Option D

4. Quick Check:

   Correct operator is <=> [OK]

Hint: Look for <=> exactly between variables [OK]

Common Mistakes:

• Using => which is a hash rocket
• Using <= or =< which are comparison operators
• Swapping operator symbols incorrectly

3.

What is the output of the following Ruby code?

puts 5 <=> 10
puts 10 <=> 10
puts 15 <=> 10

medium

A. false
true
false

B. 1
0
-1

C. -1
0
1

D. 10
10
10

Solution

1. Step 1: Evaluate each comparison

   5 <=> 10 returns -1 because 5 is smaller than 10.
   10 <=> 10 returns 0 because both are equal.
   15 <=> 10 returns 1 because 15 is bigger than 10.

2. Step 2: Confirm output lines

   Each puts prints the result on a new line, so output is three lines: -1, 0, 1.

3. Final Answer:

   -1 0 1 -> Option C

4. Quick Check:

   5 < 10 = -1, equal = 0, 15 > 10 = 1 [OK]

Hint: Remember: smaller = -1, equal = 0, bigger = 1 [OK]

Common Mistakes:

• Mixing up the order of output
• Expecting boolean true/false instead of -1,0,1
• Confusing <=> with < or > operators

4.

Identify the error in this Ruby code snippet using the spaceship operator:

result = 7 <=>
3
puts result

medium

A. Syntax error due to line break in operator

B. Returns nil because 7 and 3 are not comparable

C. Returns 1 as expected

D. Runtime error due to undefined variable

Solution

1. Step 1: Check operator usage across lines

   The spaceship operator <=> must be on the same line between operands; splitting it causes syntax error.

2. Step 2: Understand Ruby syntax rules

   Ruby does not allow breaking operators across lines without proper continuation, so this code raises a syntax error.

3. Final Answer:

   Syntax error due to line break in operator -> Option A

4. Quick Check:

   Operators must not be split across lines [OK]

Hint: Keep <=> on one line between values [OK]

Common Mistakes:

• Splitting operator across lines
• Assuming nil return for invalid syntax
• Confusing runtime errors with syntax errors

5.

Given the array arr = [3, 1, 4, 1, 5], which code snippet correctly sorts the array in descending order using the spaceship operator?

hard

A. arr.sort { |a, b| b <=> a }

B. arr.sort { |a, b| a == b }

C. arr.sort_by { |x| x <=> 0 }

D. arr.sort { |a, b| a <=> b }

Solution

1. Step 1: Understand sorting with spaceship operator

   The block compares two elements a and b. To sort descending, compare b to a.

2. Step 2: Analyze each option

   arr.sort { |a, b| a <=> b } sorts ascending (a <=> b).
   arr.sort { |a, b| b <=> a } sorts descending (b <=> a).
   arr.sort_by { |x| x <=> 0 } misuses sort_by with spaceship operator.
   arr.sort { |a, b| a == b } returns boolean, invalid for sorting.

3. Final Answer:

   arr.sort { |a, b| b <=> a } -> Option A

4. Quick Check:

   Reverse order uses b <=> a [OK]

Hint: Reverse order: swap a and b in <=> [OK]

Common Mistakes:

• Using a <=> b for descending sort
• Misusing sort_by with spaceship operator
• Returning boolean instead of integer in block