Spaceship operator (<=>) in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the spaceship operator (<=>) do in Ruby?

It compares two values and returns -1 if the left is less, 0 if equal, and 1 if the left is greater.

beginner

What is the return value of 5 <=> 10 in Ruby?

It returns -1 because 5 is less than 10.

intermediate

How can the spaceship operator help in sorting arrays?

It provides a simple way to define custom comparison logic for sorting by returning -1, 0, or 1.

beginner

What will be the result of 'apple' <=> 'banana' in Ruby?

It returns -1 because 'apple' comes before 'banana' alphabetically.

advanced

Can the spaceship operator be used with custom objects in Ruby?

Yes, by defining the <=> method inside the object's class to specify how to compare instances.

What does the spaceship operator (<=>) return when the left value is greater than the right?

A 0

B -1

C nil

D 1

It returns 1 when the left value is greater than the right.

Which of these is a valid use of the spaceship operator in Ruby?

A 5 => 3

B 5 <=> 3

C 5 =< 3

D 5 ==< 3

Only '5 <=> 3' is the correct syntax for the spaceship operator.

What does the spaceship operator return if two values are equal?

A 0

B 1

C -1

D nil

It returns 0 when both values are equal.

How can the spaceship operator be used in sorting an array?

A By replacing the sort method

B By using it as a method to reverse the array

C By providing a block that returns <=> between elements

D By converting elements to strings

You use <=> inside a block to tell Ruby how to compare elements during sorting.

What happens if you use the spaceship operator on objects without defining <=> method?

A It returns nil

B It returns 0

C It raises an error

D It always returns 1

If <=> is not defined for objects, it returns nil indicating they can't be compared.

Explain how the spaceship operator (<=>) works in Ruby and give an example.

Think about how it tells if one value is less, equal, or greater than another.

Describe how you can use the spaceship operator to sort an array with custom objects.

Custom objects need a way to compare themselves for sorting.

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