Block given? check in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Ruby code using block_given??

Consider the following Ruby method that checks if a block is given and yields to it if so. What will be printed when calling greet with and without a block?

Ruby

def greet
  if block_given?
    yield
  else
    puts "No block given"
  end
end

greet

greet { puts "Hello from block!" }

A

No block given
No block given

B

No block given
Hello from block!

C

Hello from block!
No block given

D

Hello from block!
Hello from block!

💡 Hint

Remember that block_given? returns true only if a block is passed to the method.

The first call to greet has no block, so it prints "No block given". The second call has a block, so it yields and prints "Hello from block!".

🧠 Conceptual

intermediate

What does block_given? return inside a method?

In Ruby, what does the method block_given? return when called inside a method?

A Raises an error if no block is given

B True if any method in the program has a block, false otherwise

C True if the method was called with a block, false otherwise

D Always true, because blocks are always available

💡 Hint

Think about whether the method itself received a block or not.

block_given? returns true only if the current method was called with a block.

🔧 Debug

advanced

Why does this code raise an error when using yield without block_given??

Look at this Ruby method:

def run
  yield
end

run

What error will this code raise and why?

A No error, it prints nothing

B NoMethodError because yield is not a method

C SyntaxError because yield is used incorrectly

D LocalJumpError because yield is called without a block

💡 Hint

What happens if you call yield but no block is given?

Calling yield without a block causes a LocalJumpError because Ruby expects a block to execute.

❓ Predict Output

advanced

What is the output of this method using block_given? and yield?

What will this Ruby code print?

def check
  if block_given?
    yield(5)
  else
    puts "No block"
  end
end

check { |x| puts x * 2 }
check

A

10
No block

B

5
No block

C

No block
10

D

No block
No block

💡 Hint

The block receives the number 5 and multiplies it by 2.

The first call passes a block that prints 5 * 2 = 10. The second call has no block, so it prints "No block".

🧠 Conceptual

expert

How does block_given? behave with nested method calls?

Given these Ruby methods:

def outer
  inner
end

def inner
  block_given?
end

result = outer { puts "Hi" }

What is the value of result after this code runs?

A false

B true

C nil

D Raises an error

💡 Hint

Does the block passed to outer get passed automatically to inner?

The block is given to outer, but inner does not receive it unless explicitly passed. So block_given? inside inner returns false.

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