Closures and variable binding in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a closure in Ruby?

A closure is a block, proc, or lambda that captures variables from its surrounding context, allowing it to access those variables even when executed outside that context.

intermediate

How does variable binding work inside a Ruby closure?

Variables used inside a closure capture the variables from the surrounding scope by reference, so the closure sees their updated values even if the variables change later.

intermediate

What is the difference between a Proc and a lambda in Ruby closures?

Both are closures, but lambdas check the number of arguments and return control to the calling method, while Procs are more lenient with arguments and return from the enclosing context.

advanced

Explain with an example how a closure can remember a variable's value after the variable changes.

Example:
x = 10
closure = -> { x }
x = 20
closure.call # returns 20 because closure captures variable by reference, not by value.

beginner

Why are closures useful in Ruby programming?

Closures let you write flexible code by capturing context, enabling callbacks, delayed execution, and maintaining state without global variables.

What does a Ruby closure capture from its surrounding environment?

A Only global variables

B Variables and their current values

C Only method names

D Nothing, closures have no context

Closures capture variables and their current values from the surrounding environment.

In Ruby, what happens if you change a variable after creating a closure that uses it?

A The closure breaks and raises an error

B The closure keeps the old value forever

C The closure sees the updated variable value

D The closure ignores the variable

Ruby closures capture variables by reference, so they see the updated value when called.

Which Ruby closure type checks the number of arguments strictly?

A Lambda

B Proc

C Block

D Method

Lambdas check the number of arguments strictly, unlike Procs.

What is a common use case for closures in Ruby?

A To create global variables

B To perform file I/O

C To define classes

D To delay execution and maintain state

Closures are often used to delay execution and maintain state without global variables.

Which of these is NOT a closure in Ruby?

A Class

B Proc

C Lambda

D Block

Classes are not closures; blocks, procs, and lambdas are closures.

Explain what a closure is in Ruby and how it handles variable binding.

Think about how blocks or lambdas remember variables from outside.

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

Focus on argument checking and return behavior.

Practice

1. What is a closure in Ruby?

easy

A. A variable that changes its value automatically

B. A method that can only be called once

C. A function that remembers variables from its creation context

D. A class that cannot be inherited

Solution

1. Step 1: Understand the definition of closure

   A closure is a function or block that keeps access to variables from where it was created, even if called later.

2. Step 2: Match the definition to options

   A function that remembers variables from its creation context correctly describes this behavior. Other options describe unrelated concepts.

3. Final Answer:

   A function that remembers variables from its creation context -> Option C

4. Quick Check:

   Closure = function remembering variables [OK]

Hint: Closures remember variables where they were made [OK]

Common Mistakes:

• Thinking closures are just methods
• Confusing closures with classes
• Believing closures change variable values automatically

2. Which of the following is the correct way to create a closure using a lambda in Ruby?

easy

A. closure = lambda { |x| x * 2 }

B. closure = def(x) x * 2 end

C. closure = function(x) { x * 2 }

D. closure = -> x x * 2

Solution

1. Step 1: Identify Ruby lambda syntax

   In Ruby, lambdas can be created with lambda { |args| ... } or ->(args) { ... }.

2. Step 2: Check each option

   closure = lambda { |x| x * 2 } uses lambda { |x| x * 2 }, which is valid. closure = def(x) x * 2 end uses invalid syntax for a lambda. closure = function(x) { x * 2 } uses JavaScript-like syntax. closure = -> x x * 2 misses parentheses in the arrow syntax, which is invalid.

3. Final Answer:

   closure = lambda { |x| x * 2 } -> Option A

4. Quick Check:

   Correct lambda syntax = closure = lambda { |x| x * 2 } [OK]

Hint: Use lambda { |args| ... } for Ruby closures [OK]

Common Mistakes:

• Using JavaScript or Python syntax
• Omitting parentheses in arrow lambdas
• Trying to define lambdas with def keyword

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

def make_closures
  closures = []
  3.times do |i|
    closures << lambda { i }
  end
  closures
end

closures = make_closures
closures.each { |c| puts c.call }

medium

A. 2
2
2

B. 0
1
2

C. 0
0
0

D. Error: undefined variable i

Solution

1. Step 1: Understand variable binding in block

   Ruby lambdas capture block variables like i by value in this context, so each lambda remembers its own i value.

2. Step 2: Evaluate each lambda call

   The loop sets i to 0, 1, 2 sequentially. Each lambda captures the current i value, so outputs are 0, 1, 2 respectively.

3. Final Answer:

   0 1 2 -> Option B

4. Quick Check:

   Lambdas capture distinct values per iteration [OK]

Hint: Ruby block vars in closures captured by value per iteration [OK]

Common Mistakes:

• Thinking lambdas capture shared variable (all 2)
• Thinking last value is 3 (off-by-one)
• Expecting error or all zeros

4. Find the error in this Ruby code that tries to create closures:

closures = []
3.times do |i|
  closures << lambda { j }
  j = i
end
closures.each { |c| puts c.call }

medium

A. Variable j is used before assignment inside the lambda

B. Lambda syntax is incorrect

C. times method cannot be used with blocks

D. closures array is not initialized

Solution

1. Step 1: Identify variable usage inside lambda

   The lambda uses variable j before it is assigned any value in the block, causing an error when called.

2. Step 2: Understand variable binding timing

   Since j is assigned after lambda creation, the lambda captures j as nil or undefined at call time, causing an error.

3. Final Answer:

   Variable j is used before assignment inside the lambda -> Option A

4. Quick Check:

   Use variables after assignment in closures [OK]

Hint: Assign variables before using them in closures [OK]

Common Mistakes:

• Assuming lambda syntax is wrong
• Thinking times method is invalid
• Ignoring variable initialization order

5. How can you modify this Ruby code to create closures that each remember a different value of i even if i changes later?

closures = []
3.times do |i|
  closures << lambda { i }
end
closures.each { |c| puts c.call }

hard

A. Call closures inside the times block

B. Change lambda to proc

C. Use global variable $i instead of i

D. Use a local variable inside the block to capture i: val = i; closures << lambda { val }

Solution

1. Step 1: Understand variable capture in closures

   Closures capture variables by reference, so if i changes later, all lambdas see the last value.

2. Step 2: Use a local variable to capture current value

   Assign val = i inside the block, then create lambda using val. This captures the value at that iteration.

3. Final Answer:

   Use a local variable inside the block to capture i: val = i; closures << lambda { val } -> Option D

4. Quick Check:

   Capture current value with local variable [OK]

Hint: Assign loop variable to local var inside block for closure [OK]

Common Mistakes:

• Thinking proc changes variable capture behavior
• Using global variables which share state
• Calling closures too early inside loop