Everything is an object mental model in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the 'Everything is an object' mental model mean in Ruby?

In Ruby, everything you work with is an object. Numbers, strings, methods, and even classes themselves are objects. This means they all have methods and properties you can use.

beginner

How can you call a method on a number in Ruby?

Since numbers are objects, you can call methods on them like this: 5.next which returns 6.

intermediate

Are classes in Ruby objects too?

Yes! Classes in Ruby are objects of the class Class. This means you can call methods on classes themselves.

beginner

What is the class of a string like "hello" in Ruby?

The class of "hello" is String. You can check this by calling <code>"hello".class</code> which returns <code>String</code>.

intermediate

Why is the 'Everything is an object' model helpful?

It helps you understand that you can use methods on any value, making Ruby very consistent and flexible. It also means you can extend or change behavior by adding methods to objects.

In Ruby, what is the class of the number 10?

A Integer

B Number

C Fixnum

D Object

In modern Ruby, 10 is an Integer object. Fixnum was used in older versions but now Integer covers all whole numbers.

Which of these is NOT true about Ruby's 'Everything is an object' model?

A Classes themselves are objects.

B Only strings and arrays are objects.

C You can call methods on numbers.

D Even true and false are objects.

In Ruby, everything is an object, not just strings and arrays.

What will "hello".class return in Ruby?

A Class

B Object

C Text

D String

The class of a string literal is String.

Which class do all Ruby classes belong to?

A Class

B Object

C Module

D String

All classes in Ruby are instances of the Class class.

What does calling 5.next return in Ruby?

A 7

B 5

C 6

D Error

Since 5 is an object, calling the next method returns the next integer, 6.

Explain the 'Everything is an object' mental model in Ruby and give two examples.

Think about how you can call methods on numbers and strings.

Why does Ruby treat classes as objects? How does this affect how you use classes?

Consider how classes themselves can have methods.

Practice

1. In Ruby, which of the following is not considered an object?

easy

A. Numbers like 5

B. Strings like "hello"

C. Booleans like true

D. Variables themselves

Solution

1. Step 1: Understand Ruby's object model

   In Ruby, everything you work with directly, like numbers, strings, and booleans, are objects.

2. Step 2: Differentiate variables from objects

   Variables are names or references pointing to objects, but they themselves are not objects.

3. Final Answer:

   Variables themselves are not objects -> Option D

4. Quick Check:

   Variables are references, not objects [OK]

Hint: Remember: variables point to objects, they aren't objects themselves [OK]

Common Mistakes:

• Thinking variables are objects
• Confusing object instances with variable names
• Assuming booleans are not objects

2. Which of the following is the correct way to call the method upcase on a string object name in Ruby?

easy

A. name.upcase

B. upcase(name)

C. name->upcase()

D. upcase.name()

Solution

1. Step 1: Recall Ruby method call syntax

   In Ruby, methods are called on objects using dot notation: object.method.

2. Step 2: Apply to the string object

   For the string variable name, calling upcase is done as name.upcase.

3. Final Answer:

   name.upcase -> Option A

4. Quick Check:

   Method call syntax = object.method [OK]

Hint: Use dot (.) to call methods on objects in Ruby [OK]

Common Mistakes:

• Using function-style calls like upcase(name)
• Using arrow (->) notation from other languages
• Placing method after dot incorrectly

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

num = 10
puts num.class

medium

A. Integer

B. Fixnum

C. Numeric

D. Object

Solution

1. Step 1: Understand the class method

   The class method returns the class of the object it is called on.

2. Step 2: Identify the class of the number 10 in Ruby 3.12+

   In modern Ruby versions, integers are instances of the Integer class (Fixnum and Bignum were unified).

3. Final Answer:

   Integer -> Option A

4. Quick Check:

   10.class = Integer [OK]

Hint: Use .class to find an object's type in Ruby [OK]

Common Mistakes:

• Choosing Fixnum (old Ruby versions)
• Confusing Numeric with Integer
• Thinking class returns Object always

4. The following Ruby code prints nothing. What is the problem?

text = "HELLO"
puts text.upcase!

medium

A. Missing parentheses for method call

B. Using upcase! on a string literal causes error

C. The method upcase! returns nil if no changes made

D. Strings are not objects, so methods can't be called

Solution

1. Step 1: Understand upcase! behavior

   The upcase! method modifies the string in place and returns nil if no changes were made.

2. Step 2: Analyze the code's output

   Since "HELLO" is uppercase, upcase! makes no changes and returns nil, so puts prints nothing.

3. Step 3: Identify the error cause

   The return value nil is printed when no mutation occurs.

4. Final Answer:

   upcase! returns nil if no changes made -> Option C

5. Quick Check:

   Bang methods can return nil if no change [OK]

Hint: Bang (!) methods may return nil if no change happens [OK]

Common Mistakes:

• Thinking bang methods always return a string
• Assuming strings are not objects
• Missing that upcase! modifies in place

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

class NumberWrapper
  def initialize(value)
    @value = value
  end

  def to_s
    "Value is: #{@value.to_s}"
  end
end

num = NumberWrapper.new(42)
puts num.to_s

hard

A. 42

B. Value is: 42

C. Value is: @value

D. Error: undefined method to_s

Solution

1. Step 1: Understand the class and method definitions

   The class NumberWrapper stores a value and defines a to_s method that returns a string including the value.

2. Step 2: Analyze the to_s method call

   The method returns the string "Value is: " plus the string version of @value, which is 42.

3. Step 3: Check the output of puts num.to_s

   It prints the string returned by to_s, which is "Value is: 42".

4. Final Answer:

   Value is: 42 -> Option B

5. Quick Check:

   Custom to_s returns formatted string [OK]

Hint: to_s returns string representation of any object [OK]

Common Mistakes:

• Expecting just the number without text
• Confusing instance variable with string literal
• Thinking to_s is undefined