Predicate methods (ending with ?) in Ruby - Step-by-Step Execution

Concept Flow - Predicate methods (ending with ?)

Call method ending with ?

↓

Evaluate condition inside method

↓

Return true or false

↓

Use result in if/else or assignment

Predicate methods check a condition and return true or false, helping decisions in code.

Execution Sample

Ruby

def even?(num)
  num % 2 == 0
end

puts even?(4)
puts even?(5)

Defines a predicate method to check if a number is even, then prints true or false.

Execution Table

Step	Action	Input	Condition Evaluated	Return Value	Output
1	Call even?(4)	4	4 % 2 == 0	true	true
2	Print result	true	-	-	true
3	Call even?(5)	5	5 % 2 == 0	false	false
4	Print result	false	-	-	false
5	End	-	-	-	-

💡 All calls completed, program ends.

Variable Tracker

Variable	Start	After Step 1	After Step 3	Final
num	-	4	5	-
return_value	-	true	false	-

Key Moments - 2 Insights

Why does the method name end with a question mark?

In Ruby, methods ending with ? are predicate methods that return true or false, as shown in steps 1 and 3 of the execution_table.

What does the method return when the condition is false?

The method returns false, as seen in step 3 where even?(5) returns false.

Visual Quiz - 3 Questions

Look at the execution_table, what is the return value when calling even?(4)?

A true

B false

C 4

D nil

💡 Hint

Check step 1 in the execution_table where even?(4) is called.

At which step does the method return false?

A Step 4

B Step 2

C Step 3

D Step 1

💡 Hint

Look at the return value column in the execution_table.

If we call even?(6), what would be the expected return value?

A false

B true

C 6

D nil

💡 Hint

Refer to the pattern in the execution_table for even numbers returning true.

Concept Snapshot

Predicate methods end with ? and return true or false.
They check a condition inside and return the result.
Used for clear yes/no questions in code.
Example: def even?(num); num % 2 == 0; end
Call with even?(4) returns true.

Full Transcript

Predicate methods in Ruby are special methods that end with a question mark. They always return true or false based on a condition. For example, the method even?(num) checks if a number is even by using the condition num % 2 == 0. When called with 4, it returns true, and with 5, it returns false. These methods help make code easier to read by clearly showing they answer yes/no questions. The execution table shows each step: calling the method, evaluating the condition, returning true or false, and printing the result.

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