Block given? check in Ruby - Step-by-Step Execution

Concept Flow - Block given? check

Method called

↓

Check if block given?

↓

Yield block

↓

Continue method

↓

End

The method checks if a block is given. If yes, it runs the block with yield; if no, it skips yield and continues.

Execution Sample

Ruby

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

greet { puts "Hello!" }

This code runs a method that prints 'Hello!' if a block is given, otherwise prints 'No block given'.

Execution Table

Step	Action	block_given? check	Branch Taken	Output
1	Call greet with block	true	Yes (yield block)	Hello!
2	Yield block execution	-	-	Hello! printed
3	Method ends	-	-	-

💡 Block was given, so yield executed and method ended.

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
block_given?	false	true	true	true

Key Moments - 2 Insights

Why does the method not print 'No block given' when a block is passed?

Because block_given? returns true (see execution_table step 1), so the method yields to the block instead of printing the else message.

What happens if we call greet without a block?

block_given? returns false, so the else branch runs and prints 'No block given' (not shown in this trace but implied).

Visual Quiz - 3 Questions

Look at the execution table, what is the value of block_given? at step 1?

A false

B true

C nil

D undefined

💡 Hint

Check the 'block_given? check' column in execution_table row for step 1.

At which step does the method yield to the block?

A Step 2

B Step 3

C Step 1

D No yield occurs

💡 Hint

Look at the 'Action' and 'Branch Taken' columns in execution_table.

If greet is called without a block, what would change in the execution_table?

A block_given? would be true and yield runs

B Method would raise an error

C block_given? would be false and else branch runs

D Output would be 'Hello!'

💡 Hint

Refer to key_moments about behavior when no block is given.

Concept Snapshot

Use block_given? inside a method to check if a block was passed.
If true, use yield to run the block.
If false, skip yield or run alternative code.
Prevents errors when yield is called without a block.
Syntax:
  if block_given?
    yield
  else
    # no block code
  end

Full Transcript

This example shows how Ruby methods can check if a block is given using block_given?. When the method greet is called with a block, block_given? returns true, so the method yields to the block and prints 'Hello!'. If no block is given, block_given? returns false and the method prints 'No block given'. This prevents errors from calling yield without a block. The execution table traces each step: calling the method, checking block_given?, yielding the block, and ending the method. The variable tracker shows block_given? changing from false to true when the block is present. Key moments clarify why the else branch is skipped when a block exists and what happens if no block is passed. The quiz tests understanding of block_given? values, when yield runs, and behavior without a block. The snapshot summarizes how to safely use yield with block_given? in Ruby methods.