Predicate methods (ending with ?) in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a predicate method in Ruby?

A predicate method is a method that ends with a question mark (?) and returns a boolean value (true or false). It answers a yes/no question about an object.

beginner

Why do predicate methods in Ruby end with a question mark (?)?

The question mark at the end signals that the method returns a boolean value, making it clear the method answers a yes/no question.

beginner

Example: What does the Ruby method `empty?` do when called on an array?

The `empty?` method returns true if the array has no elements, otherwise it returns false.

intermediate

How would you define a custom predicate method in Ruby?

Define a method ending with `?` that returns true or false based on a condition. For example:

def adult?(age)
  age >= 18
end

intermediate

Can predicate methods return values other than true or false?

Technically yes, but by convention predicate methods should return only true or false to clearly indicate a yes/no answer.

What does a predicate method in Ruby always return?

A true or false

B a string

C an integer

D nil

Predicate methods always return true or false to answer yes/no questions.

Which of these is a valid predicate method name in Ruby?

A valid?

B check

C run!

D start

Predicate methods end with a question mark (?), like 'valid?'.

What does the Ruby method `nil?` check?

A If an object is empty

B If an object is a number

C If an object is true

D If an object is nil

`nil?` returns true if the object is nil, otherwise false.

How do you call a predicate method named `empty?` on an array named `arr`?

A arr.empty

B empty?(arr)

C arr.empty?

D empty.arr?

Predicate methods are called like any other method: object.method_name?.

Why is it helpful to use predicate methods?

A They speed up the program

B They make code easier to read by clearly showing yes/no checks

C They return strings

D They create new objects

Predicate methods improve readability by signaling boolean checks.

Explain what a predicate method is and why it ends with a question mark in Ruby.

Think about how Ruby shows methods that answer true or false questions.

Write a simple custom predicate method in Ruby that checks if a number is even.

Use the modulo operator (%) to check even numbers.

Practice

1. What does a predicate method in Ruby always return?

easy

A. A string value

B. A boolean value (true or false)

C. An integer value

D. An array

Solution

1. Step 1: Understand predicate method purpose

   Predicate methods in Ruby are designed to answer yes/no questions about an object or data.

2. Step 2: Identify return type of predicate methods

   They always return true or false, which are boolean values.

3. Final Answer:

   A boolean value (true or false) -> Option B

4. Quick Check:

   Predicate methods return true/false [OK]

Hint: Predicate methods always answer yes/no questions [OK]

Common Mistakes:

• Thinking predicate methods return strings or numbers
• Confusing predicate methods with regular methods
• Ignoring the question mark at the end

2. Which of the following is the correct way to define a predicate method in Ruby?

easy

A. def valid?; true; end

B. def valid; true; end

C. def valid!; true; end

D. def valid: true; end

Solution

1. Step 1: Recognize predicate method naming

   Predicate methods must end with a question mark (?).

2. Step 2: Check method syntax

   def valid?; true; end defines a method named valid? correctly with def and end keywords.

3. Final Answer:

   def valid?; true; end -> Option A

4. Quick Check:

   Method name ends with ? and syntax is correct [OK]

Hint: Predicate methods end with ? and use def ... end [OK]

Common Mistakes:

• Omitting the question mark in method name
• Using invalid method name characters like ! or :
• Incorrect method definition syntax

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

def empty_string?(str)
  str.empty?
end

puts empty_string?("")
puts empty_string?("hello")

medium

A. false\ntrue

B. true\ntrue

C. true\nfalse

D. false\nfalse

Solution

1. Step 1: Understand the method behavior

   The method empty_string? calls str.empty? which returns true if str is an empty string.

2. Step 2: Evaluate the calls

   empty_string?("") returns true because "" is empty; empty_string?("hello") returns false because "hello" is not empty.

3. Final Answer:

   true\nfalse -> Option C

4. Quick Check:

   Empty string check returns true, non-empty returns false [OK]

Hint: empty? returns true only for empty strings [OK]

Common Mistakes:

• Assuming non-empty strings return true
• Confusing empty? with nil? method
• Ignoring the question mark in method names

4. Identify the error in this Ruby code:

def positive?(num)
  if num > 0
    true
  else
    false
  end
end

puts positive?("5")

medium

A. ArgumentError because "5" is a string, not a number

B. SyntaxError due to missing end

C. No error, outputs true

D. Returns nil instead of true/false

Solution

1. Step 1: Analyze method input and comparison

   The method compares num > 0, but "5" is a string, not a number.

2. Step 2: Understand Ruby type comparison

   Comparing string "5" with integer 0 causes an ArgumentError at runtime.

3. Final Answer:

   ArgumentError because "5" is a string, not a number -> Option A

4. Quick Check:

   Comparing string and integer causes ArgumentError [OK]

Hint: Check input types before numeric comparisons [OK]

Common Mistakes:

• Passing strings instead of numbers
• Assuming no error on mixed type comparisons
• Missing end keyword (not the case here)

5. You want to create a predicate method valid_name? that returns true only if a string is not empty and contains only letters. Which implementation is correct?

hard

A. def valid_name?(name)
name.nil? && name.match?(/[a-z]/)
end

B. def valid_name?(name)
name.empty? || name.match?(/\d+/)
end

C. def valid_name?(name)
name.length > 0 && name.include?(' ')
end

D. def valid_name?(name)
!name.empty? && name.match?(/^[a-zA-Z]+$/)
end

Solution

1. Step 1: Understand the requirements

   The method must return true only if the string is not empty and contains only letters.

2. Step 2: Evaluate each option

   def valid_name?(name) !name.empty? && name.match?(/^[a-zA-Z]+$/) end checks that name is not empty and matches only letters using regex /^[a-zA-Z]+$/. This meets the requirements.

3. Final Answer:

   def valid_name?(name) !name.empty? && name.match?(/^[a-zA-Z]+$/) end -> Option D

4. Quick Check:

   Not empty and only letters = valid_name? true [OK]

Hint: Use regex and empty? to validate strings in predicates [OK]

Common Mistakes:

• Using || instead of && for conditions
• Checking for digits instead of letters
• Ignoring empty string check