Block given? check in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does block_given? do in Ruby?

block_given? checks if a block was passed to the current method. It returns true if a block is present, otherwise false.

beginner

How do you use block_given? inside a method?

Inside a method, you call block_given? to test if a block was passed. If true, you can safely call yield to execute the block.

intermediate

What happens if you call yield without a block and don't check block_given??

Ruby raises a LocalJumpError because yield expects a block to be given. Using block_given? prevents this error by checking first.

beginner

Example: What will this method print?

def greet
  if block_given?
    yield
  else
    puts 'Hello!'
  end
end

greet

This method prints Hello! because no block is given when calling greet. The block_given? check is false, so it runs the else part.

beginner

Why is block_given? useful in Ruby methods?

It lets methods behave differently depending on whether a block is passed. This makes methods flexible and safe by avoiding errors when no block is given.

What does block_given? return if a block is passed to the method?

A true

B false

C nil

D raises an error

block_given? returns true when a block is passed to the method.

What Ruby keyword is used to run the block inside a method?

A run

B call

C yield

D execute

yield runs the block passed to the method.

What error occurs if you use yield without a block and no block_given? check?

A LocalJumpError

B ArgumentError

C SyntaxError

D NoMethodError

Ruby raises LocalJumpError when yield is called without a block.

Which of these is a correct way to check if a block is given inside a method?

A if block_passed?

B if block_given?

C if has_block?

D if block?

block_given? is the correct Ruby method to check for a block.

What will this code print?

def test
  if block_given?
    yield
  else
    puts 'No block'
  end
end

test { puts 'Block here' }

A No block

B Error

C Nothing

D Block here

The block is given, so yield runs it and prints 'Block here'.

Explain how block_given? helps avoid errors when using blocks in Ruby methods.

Think about what happens if you call yield without a block.

Describe a simple Ruby method that behaves differently when a block is given versus when it is not.

Use an example like greeting with or without a block.

Practice

1. What does the Ruby method block_given? check inside a method?

easy

A. If the method is defined inside a class

B. If the method has any parameters

C. If the method returns a value

D. If a block was passed to the method

Solution

1. Step 1: Understand the purpose of block_given?

   This method returns true if the current method was called with a block passed to it.

2. Step 2: Differentiate from other method checks

   It does not check parameters, return values, or class definitions, only the presence of a block.

3. Final Answer:

   If a block was passed to the method -> Option D

4. Quick Check:

   block_given? checks block presence = A [OK]

Hint: block_given? tells if a block is passed to method [OK]

Common Mistakes:

• Confusing block_given? with method parameters
• Thinking it checks return values
• Assuming it checks class membership

2. Which of the following is the correct syntax to check if a block was given inside a Ruby method?

easy

A. if block_given then yield end

B. if block_given? () yield end

C. if block_given? then yield end

D. if block_given? yield () end

Solution

1. Step 1: Identify correct method call syntax

   block_given? is a method and should be called with a question mark and optionally parentheses. The most common and clear syntax is block_given? without parentheses.

2. Step 2: Confirm correct use of yield

   The yield keyword runs the block if it exists. The syntax if block_given? then yield end is correct and clear.

3. Final Answer:

   if block_given? then yield end -> Option C

4. Quick Check:

   Correct syntax uses block_given? and yield [OK]

Hint: Use block_given? with question mark and yield inside if [OK]

Common Mistakes:

• Omitting question mark in block_given?
• Using block_given without parentheses incorrectly
• Calling yield with parentheses unnecessarily

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

def greet
  if block_given?
    yield
  else
    puts "Hello!"
  end
end

greet { puts "Hi there!" }
greet

medium

A. Hi there!\nHello!

B. Hello!\nHello!

C. Hi there!\nHi there!

D. No output

Solution

1. Step 1: Analyze first greet call with block

   The first call passes a block that prints "Hi there!". Since block_given? is true, yield runs the block, printing "Hi there!".

2. Step 2: Analyze second greet call without block

   The second call has no block, so block_given? is false and the else branch runs, printing "Hello!".

3. Final Answer:

   Hi there!\nHello! -> Option A

4. Quick Check:

   Block runs first, then default message = A [OK]

Hint: Block runs if given; else default runs [OK]

Common Mistakes:

• Assuming yield runs even without block
• Thinking both calls print the same message
• Confusing order of outputs

4. Identify the error in this Ruby method:

def show
  yield if block_given
end

show { puts "Hello!" }

medium

A. Missing question mark in block_given method call

B. yield cannot be used without arguments

C. Method show requires a parameter

D. Block cannot be passed to show method

Solution

1. Step 1: Check block_given usage

   The method block_given? requires the question mark and should be called as block_given? or block_given?(). Here it is missing the question mark.

2. Step 2: Confirm yield usage

   The yield is correctly used and does not require arguments here. The method does not require parameters to accept a block.

3. Final Answer:

   Missing question mark in block_given method call -> Option A

4. Quick Check:

   block_given? needs question mark [OK]

Hint: Always use block_given? with question mark [OK]

Common Mistakes:

• Omitting question mark in block_given
• Thinking yield needs arguments always
• Assuming method needs parameters for blocks

5. You want to write a Ruby method process_items that takes an array and optionally a block. If a block is given, it should yield each item to the block; otherwise, it should return the array unchanged. Which code correctly implements this behavior?

hard

A. def process_items(arr)
arr.each { |item| yield item if block_given? }
end

B. def process_items(arr)
if block_given?
arr.each { |item| yield item }
else
arr
end
end

C. def process_items(arr)
if block_given
arr.each { |item| yield item }
else
arr
end
end

D. def process_items(arr)
arr.each { |item| yield item }
end

Solution

1. Step 1: Check block presence correctly

   def process_items(arr) if block_given? arr.each { |item| yield item } else arr end end uses block_given? correctly to check if a block is passed before yielding.

2. Step 2: Handle both cases properly

   If a block is given, it yields each item; otherwise, it returns the original array unchanged, matching the requirement.

3. Step 3: Review other options for errors

   def process_items(arr) arr.each { |item| yield item } end always yields without checking block presence, causing error if no block given. def process_items(arr) if block_given arr.each { |item| yield item } else arr end end misses question mark in block_given. def process_items(arr) arr.each { |item| yield item if block_given? } end yields conditionally inside each, but block_given? inside the each block always returns false so no yielding occurs, though it returns the array.

4. Final Answer:

   def process_items(arr) if block_given? arr.each { |item| yield item } else arr end end -> Option B

5. Quick Check:

   Check block_given? before yield and return array if no block [OK]

Hint: Check block_given? before yield; else return array [OK]

Common Mistakes:

• Not checking block_given? before yield
• Missing question mark in block_given
• Returning nil instead of array when no block