Method objects with method() in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of calling a method object

What is the output of this Ruby code?

Ruby

class Greeter
  def hello
    "Hello, world!"
  end
end

g = Greeter.new
m = g.method(:hello)
puts m.call

A Hello, world!

B NoMethodError

C nil

D ArgumentError

💡 Hint

Remember that method(:name) returns a method object that you can call.

The method(:hello) returns a method object bound to g. Calling m.call runs hello and returns "Hello, world!".

🧠 Conceptual

intermediate

Understanding method objects and binding

Which statement about Ruby method objects created by method() is true?

A They automatically convert to procs when assigned to variables.

B They cannot be called more than once.

C They are bound to the object they were created from and remember its state.

D They are unbound and can be called on any object of the same class.

💡 Hint

Think about what happens when you call method(:name) on an object.

Method objects are bound to the specific object they come from, so they keep that object's state when called.

🔧 Debug

advanced

Identify the error when calling an unbound method

What error does this Ruby code produce?

Ruby

class Person
  def greet
    "Hi!"
  end
end

m = Person.instance_method(:greet)
m.call

A ArgumentError

B TypeError

C NameError

D UnboundMethodError

💡 Hint

Consider what instance_method returns and how it differs from method.

instance_method returns an unbound method that must be bound to an object before calling. Calling m.call without binding raises an ArgumentError because no receiver is given.

📝 Syntax

advanced

Correct way to convert a method object to a proc

Which option correctly converts a method object to a proc and calls it?

Ruby

class Calculator
  def double(x)
    x * 2
  end
end

calc = Calculator.new
m = calc.method(:double)
# call the method object as a proc with argument 5

A p = m.to_proc(5); puts p.call

B p = m.proc; puts p.call(5)

C p = m.to_proc; puts p.call

D p = m.to_proc; puts p.call(5)

💡 Hint

Check the correct method name to convert a method object to a proc and how to call it with arguments.

to_proc converts the method object to a proc. Then you can call it with arguments like p.call(5).

🚀 Application

expert

Using method objects to implement a callback system

You want to store multiple method objects from different instances and call them later. Which code snippet correctly stores and calls these method objects?

Ruby

class Worker
  def initialize(name)
    @name = name
  end
  def work
    "#{@name} is working"
  end
end

w1 = Worker.new("Alice")
w2 = Worker.new("Bob")
callbacks = []
# store method objects from w1 and w2
callbacks << w1.method(:work)
callbacks << w2.method(:work)

# call all stored callbacks and collect results
results = callbacks.map { |m| m.call }
puts results

A ["is working", "is working"]

B ["Alice is working", "Bob is working"]

C ["Alice", "Bob"]

D RuntimeError

💡 Hint

Think about how method objects keep their binding to the original instance.

Each method object remembers the instance it came from, so calling them later returns the correct string with the instance's name.

Practice

1. What does the Ruby expression obj.method(:foo) return?

easy

A. A Method object representing the method foo of obj

B. The result of calling foo on obj

C. A symbol :foo

D. An error because method is not a valid method

Solution

1. Step 1: Understand method usage

   The method method on an object returns a Method object representing the named method.

2. Step 2: Identify the return type

   Calling obj.method(:foo) returns a Method object, not the method's result or a symbol.

3. Final Answer:

   A Method object representing the method foo of obj -> Option A

4. Quick Check:

   obj.method(:foo) returns Method object [OK]

Hint: Remember: method returns a Method object, not the call result [OK]

Common Mistakes:

• Thinking it calls the method immediately
• Confusing with symbol :foo
• Assuming it returns a string or error

2. Which of the following is the correct syntax to get a Method object for the method greet on object person?

easy

A. person.send(:greet)

B. person.method[b greet]

C. person.method(greet)

D. person.method(:greet)

Solution

1. Step 1: Check argument type for method

   The method method expects a symbol representing the method name.

2. Step 2: Validate options

   person.method(:greet) uses a symbol :greet, which is correct. person.send(:greet) calls the method directly, returning its result instead of a Method object. person.method(greet) passes an undefined variable greet, causing error. person.method[b greet] uses brackets, which is invalid syntax.

3. Final Answer:

   person.method(:greet) -> Option D

4. Quick Check:

   Use symbol argument with method [OK]

Hint: Use a symbol like :greet inside method() [OK]

Common Mistakes:

• Confusing method() with send()
• Using brackets instead of parentheses
• Passing undefined variable instead of symbol

3. What is the output of the following Ruby code?

class Calculator
  def add(x, y)
    x + y
  end
end
calc = Calculator.new
adder = calc.method(:add)
puts adder.call(5, 7)

medium

A. 5 7

B. 12

C. add

D. NoMethodError

Solution

1. Step 1: Understand method object creation

   The method(:add) returns a Method object for add on calc.

2. Step 2: Calling the Method object

   Calling adder.call(5, 7) executes add(5, 7), which returns 12.

3. Final Answer:

   12 -> Option B

4. Quick Check:

   Method object call returns sum 12 [OK]

Hint: Use call on Method object to run method with arguments [OK]

Common Mistakes:

• Expecting method name or arguments printed
• Confusing output with error
• Not using call to invoke method

4. Identify the error in this Ruby code snippet:

class Person
  def name
    "Alice"
  end
end
p = Person.new
m = p.method(name)
m.call

medium

A. Missing quotes around method name in method(name)

B. Cannot call method on an object

C. Method name does not exist

D. Using call on Method object is invalid

Solution

1. Step 1: Check argument to method

   The argument name is used as a variable, but it should be a symbol or string representing the method name.

2. Step 2: Correct usage

   It should be p.method(:name) or p.method('name') to get the Method object.

3. Final Answer:

   Missing quotes around method name in method(name) -> Option A

4. Quick Check:

   Method name must be symbol or string [OK]

Hint: Always pass method name as symbol or string to method() [OK]

Common Mistakes:

• Passing bare method name without quotes or colon
• Thinking method is invalid on objects
• Confusing call usage

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

class Greeter
  def initialize(name)
    @name = name
  end
  def greet
    "Hello, #{@name}!"
  end
end

g1 = Greeter.new("Bob")
g2 = Greeter.new("Sue")
m1 = g1.method(:greet)
m2 = g2.method(:greet)

puts m1.call
puts m2.call

hard

A. Hello, Sue!\nHello, Sue!

B. Hello, Bob!\nHello, Bob!

C. Hello, Bob!\nHello, Sue!

D. Error: undefined method greet

Solution

1. Step 1: Understand instance variables and method objects

   Each Greeter instance stores its own @name. The method(:greet) returns a Method object bound to that instance.

2. Step 2: Calling Method objects

   Calling m1.call runs greet on g1, printing "Hello, Bob!". Similarly, m2.call prints "Hello, Sue!".

3. Final Answer:

   Hello, Bob! Hello, Sue! -> Option C

4. Quick Check:

   Method objects keep instance context [OK]

Hint: Method objects remember their object, so output matches instance data [OK]

Common Mistakes:

• Assuming method objects lose instance context
• Expecting same output for both calls
• Confusing method object with class method