Yield to execute blocks in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the yield keyword do in Ruby?

yield pauses the method and runs the block of code given to the method, then returns to the method.

beginner

How do you pass a block to a method in Ruby?

You write the method call with a block using { ... } or do ... end after the method name.

intermediate

What happens if a method calls yield but no block is given?

Ruby raises a LocalJumpError because it expects a block to run but none was provided.

intermediate

How can you check if a block was given before calling yield?

Use the method block_given? which returns true if a block is passed, otherwise false.

beginner

Write a simple Ruby method that uses yield to run a block.

def greet
  puts "Hello"
  yield
  puts "Goodbye"
end

greet { puts "Nice to meet you!" }

What does yield do inside a Ruby method?

A Raises an error always

B Returns the method immediately

C Defines a new block

D Runs the block passed to the method

yield runs the block given to the method at that point.

How do you write a block when calling a method in Ruby?

A Using <code>{ ... }</code> or <code>do ... end</code> after the method call

B Inside parentheses after the method name

C Using square brackets <code>[ ... ]</code>

D Blocks are not allowed in Ruby

Blocks are written after the method call using curly braces or do-end.

What method checks if a block was given to a Ruby method?

A yield?

B block_given?

C has_block?

D block?

block_given? returns true if a block was passed.

What error occurs if yield is called without a block?

A SyntaxError

B NoMethodError

C LocalJumpError

D ArgumentError

Ruby raises LocalJumpError when yield has no block.

Which is a correct way to use yield in a method?

A def example; yield; end

B def example; yield?; end

C def example; yield(); end

D def example; yield block; end

yield is called simply as yield inside the method.

Explain how yield works in Ruby methods and why it is useful.

Think about how a method can run extra code given from outside.

Describe how to safely use yield when a block might not be given.

How to prevent errors if no block is passed?

Practice

1. What does the yield keyword do in a Ruby method?

easy

A. It runs the block of code passed to the method.

B. It returns the method's value immediately.

C. It defines a new method inside the current method.

D. It pauses the method execution permanently.

Solution

1. Step 1: Understand the role of yield

   The yield keyword is used inside a method to call the block that was passed to that method.

2. Step 2: Differentiate from other keywords

   Unlike return or method definitions, yield specifically executes the block given to the method.

3. Final Answer:

   It runs the block of code passed to the method. -> Option A

4. Quick Check:

   Yield executes block = A [OK]

Hint: Yield always runs the block given to the method [OK]

Common Mistakes:

• Thinking yield returns method value
• Confusing yield with return
• Assuming yield defines a method

2. Which of the following is the correct syntax to use yield with an argument inside a method?

easy

A. def example; yield(5); end

B. def example; yield5; end

C. def example; yield = 5; end

D. def example; yield: 5; end

Solution

1. Step 1: Recall yield syntax with arguments

   To pass arguments to the block, yield must be called with parentheses around the arguments, like yield(5).

2. Step 2: Check other options for syntax errors

   Options B, C, and D are incorrect because they either miss parentheses or use invalid syntax.

3. Final Answer:

   def example; yield(5); end -> Option A

4. Quick Check:

   Yield with args uses parentheses = A [OK]

Hint: Use parentheses to pass arguments with yield [OK]

Common Mistakes:

• Omitting parentheses when passing arguments
• Using assignment (=) with yield
• Using colon (:) instead of parentheses

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

def greet
  yield("Alice")
end

greet { |name| puts "Hello, #{name}!" }

medium

A. Error: no block given

B. Hello, Alice!

C. Hello, !

D. Hello, name!

Solution

1. Step 1: Understand yield with argument

   The method greet calls yield with the string "Alice" as argument, passing it to the block.

2. Step 2: Evaluate the block execution

   The block receives "Alice" as name and prints "Hello, Alice!".

3. Final Answer:

   Hello, Alice! -> Option B

4. Quick Check:

   Yield passes "Alice" to block = C [OK]

Hint: Yield passes arguments to block parameters [OK]

Common Mistakes:

• Ignoring the argument passed to yield
• Assuming block is not called
• Confusing variable names in block

4. Identify the error in the following Ruby code:

def run_block
  yield if block_given
end

run_block { puts "Block executed" }

medium

A. yield cannot be used without arguments

B. Syntax error in method definition

C. Block is not passed to the method

D. Missing question mark (?) in block_given? method call

Solution

1. Step 1: Check usage of block_given?

   The method to check if a block is given is block_given?, note the question mark.

2. Step 2: Identify the error in code

   The code uses block_given without the question mark (?), which causes a NameError because block_given is not a valid method.

3. Final Answer:

   Missing question mark (?) in block_given? method call -> Option D

4. Quick Check:

   Use block_given? with question mark [OK]

Hint: Always use block_given? (with ?) [OK]

Common Mistakes:

• Forgetting question mark (?) on block_given?
• Assuming yield needs arguments always
• Not passing a block when expected

5. Given the method below, what will be the output?

def repeat(n)
  if block_given?
    n.times { |i| yield(i) }
  else
    puts "No block given"
  end
end

repeat(3) { |num| puts num * 2 }

hard

A. 0
1
2

B. 1
2
3

C. 0
2
4

D. No block given

Solution

1. Step 1: Analyze the method behavior

   The method repeat checks if a block is given. Since a block is passed, it runs n.times and yields the index i to the block.

2. Step 2: Evaluate the block execution

   The block multiplies each yielded number by 2 and prints it. For n=3, i goes 0,1,2, so outputs are 0, 2, 4.

3. Final Answer:

   0 2 4 -> Option C

4. Quick Check:

   Yield with times and block multiplies index = D [OK]

Hint: Yield passes each index to block in times loop [OK]

Common Mistakes:

• Confusing index values starting at 1
• Ignoring block_given? check
• Expecting output without block