Yield to execute blocks in Ruby

Introduction

Yield lets a method pause and run a block of code given to it. This helps make methods more flexible and reusable.

When you want a method to run some custom code given by the user.

When you want to repeat a task but allow small changes each time.

When you want to separate the main work from small details.

When you want to write cleaner code without passing many parameters.

When you want to create your own iterator or callback.

Syntax

Ruby

def method_name
  # some code
  yield
  # more code
end

The yield keyword runs the block passed to the method.

If no block is given, calling yield causes an error unless you check with block_given?.

Examples

This method says hello, runs the block, then says goodbye.

Ruby

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

greet { puts "Nice to meet you!" }

The method yields twice, sending different words to the block each time.

Ruby

def repeat
  yield("Ruby")
  yield("Blocks")
end

repeat { |word| puts "I love #{word}!" }

This method checks if a block exists before yielding to avoid errors.

Ruby

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

safe_yield
safe_yield { puts "Block here!" }

Sample Program

This program shows a method that yields two spices to a block, which then prints a message using them.

Ruby

def cooking
  puts "Start cooking"
  yield("salt", "pepper")
  puts "Done cooking"
end

cooking do |spice1, spice2|
  puts "Add #{spice1} and #{spice2} to the dish"
end

Important Notes

Use block_given? to check if a block was passed before calling yield.

You can pass values to the block by giving arguments to yield.

Blocks make your methods more flexible and let users add custom behavior.

Summary

Yield runs the block given to a method.

You can pass data to the block using yield with arguments.

Always check for a block with block_given? to avoid errors.

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