Proc creation and call in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a Proc in Ruby?

A Proc is an object that holds a block of code which can be stored in a variable and called later.

beginner

How do you create a Proc in Ruby?

You create a Proc by using Proc.new { ... } or the shorthand proc { ... }.

beginner

How do you call a Proc once it is created?

You call a Proc using proc_object.call(arguments).

intermediate

What happens if you call a Proc with the wrong number of arguments?

A Proc is flexible and does not raise an error if called with fewer or more arguments than expected. Missing arguments become nil.

intermediate

Difference between Proc and lambda in Ruby?

A Proc is more lenient with arguments and returns from the calling method, while a lambda checks arguments strictly and returns from itself.

How do you create a Proc object in Ruby?

A proc = function() { puts 'Hello' }

B def proc { puts 'Hello' }

C create Proc { puts 'Hello' }

D Proc.new { puts 'Hello' }

You create a Proc using Proc.new with a block, like Proc.new { puts 'Hello' }.

How do you execute the code inside a Proc named 'p'?

A p.call()

B execute p()

C call p()

D p.run()

You call a Proc using the call method: p.call().

What happens if you call a Proc with fewer arguments than it expects?

A Missing arguments become nil

B It raises an error

C It ignores extra arguments

D It converts arguments to strings

A Proc assigns nil to missing arguments instead of raising an error.

Which of these is true about Procs compared to lambdas?

A Procs check argument count strictly

B Procs return from the calling method

C Procs cannot be stored in variables

D Procs are not objects

Procs return from the calling method, unlike lambdas which return from themselves.

Which syntax creates a Proc using shorthand in Ruby?

A lambda { puts 'Hi' }

B Proc.new do puts 'Hi' end

C proc { puts 'Hi' }

D def proc; puts 'Hi'; end

The shorthand to create a Proc is using proc { ... }.

Explain how to create and call a Proc in Ruby with an example.

Think about how you store a block of code and run it later.

Describe the difference between a Proc and a lambda in Ruby.

Focus on how they treat arguments and return statements.

Practice

1. What does a Proc in Ruby do?

easy

A. Stores a block of code to run later

B. Creates a new variable

C. Defines a class

D. Starts a loop

Solution

1. Step 1: Understand what a Proc is and match description

   A Proc is a way to save some code to run later, like a recipe you keep for cooking. Only Stores a block of code to run later describes storing code to run later, which matches what a Proc does.

2. Final Answer:

   Stores a block of code to run later -> Option A

3. Quick Check:

   Proc = stored code block [OK]

Hint: Proc saves code to run later, like a saved recipe [OK]

Common Mistakes:

• Thinking Proc creates variables
• Confusing Proc with class definition
• Assuming Proc starts loops automatically

2. Which of the following is the correct way to create a Proc in Ruby?

easy

A. my_proc = Proc.new { |x| x * 2 }

B. my_proc = new Proc { |x| x * 2 }

C. my_proc = Proc.create { |x| x * 2 }

D. my_proc = Proc.call { |x| x * 2 }

Solution

1. Step 1: Recall Proc creation syntax and check options

   In Ruby, you create a Proc using Proc.new or proc with a block. Only my_proc = Proc.new { |x| x * 2 } uses the correct syntax Proc.new { |x| x * 2 }. Others use invalid methods like new Proc, Proc.create, or Proc.call.

2. Final Answer:

   my_proc = Proc.new { |x| x * 2 } -> Option A

3. Quick Check:

   Proc.new + block = correct Proc creation [OK]

Hint: Use Proc.new or proc with a block to create a Proc [OK]

Common Mistakes:

• Using 'new Proc' instead of 'Proc.new'
• Trying to call Proc.create or Proc.call to create
• Missing the block braces {}

3. What is the output of this Ruby code?

my_proc = proc { |x| x + 3 }
puts my_proc.call(4)

medium

A. 4

B. Error

C. 3

D. 7

Solution

1. Step 1: Trace the Proc definition and execution

   The Proc adds 3 to the input number: x + 3. Calling my_proc.call(4) computes 4 + 3 = 7.

2. Final Answer:

   7 -> Option D

3. Quick Check:

   4 + 3 = 7 [OK]

Hint: Proc.call runs code with given argument [OK]

Common Mistakes:

• Confusing Proc.call with Proc.new
• Forgetting to pass argument to call
• Expecting output without call

4. Identify the error in this code snippet:

my_proc = Proc.new { |x| x * 2 }
puts my_proc.call

medium

A. Proc.new syntax is incorrect

B. Missing argument when calling the Proc

C. Cannot call Proc with call method

D. Block should not have parameters

Solution

1. Step 1: Analyze Proc definition and call for errors

   The Proc expects one argument x to multiply by 2. The call is made without any argument, which causes an error because x is missing.

2. Final Answer:

   Missing argument when calling the Proc -> Option B

3. Quick Check:

   Proc needs argument when called [OK]

Hint: Always pass required arguments when calling a Proc [OK]

Common Mistakes:

• Calling Proc without required arguments
• Thinking Proc.new syntax is wrong
• Believing call method is invalid

5. Given this code, what will be the output?

def create_proc
  Proc.new { |x| x.nil? ? 'No value' : x * 10 }
end

my_proc = create_proc
puts my_proc.call(nil)
puts my_proc.call(5)

hard

A. nil\n50

B. Error\n50

C. No value\n50

D. No value\n5

Solution

1. Step 1: Analyze the Proc logic and trace both calls

   The Proc checks if x is nil; if yes, returns 'No value', else returns x * 10. Calling with nil returns 'No value'. Calling with 5 returns 5 * 10 = 50.

2. Final Answer:

   No value\n50 -> Option C

3. Quick Check:

   nil -> 'No value', 5 -> 50 [OK]

Hint: Proc can handle nil with conditional inside block [OK]

Common Mistakes:

• Expecting error when passing nil
• Forgetting ternary condition inside Proc
• Assuming multiplication by 10 applies to nil