Quiz - May

Question 1

What does the following code print?

def add(x, y)
  return(x + y)
end

result = add(4, 5) do
puts “Hey Ruby”
end

p result

Answer 1

9

The add() method is passed a code block, but it doesn’t require the code block. When methods are passed a code block but don’t require the code block, the code block is simply ignored.

Question 2

is this valid code?

class Learning
  def i_learn
    x = 'ruby'
    y = 'rails'
    z = 'sqlite3'

end
end

b = Learning.new.i_learn
p eval('"The #{self.class}, the #{x}, the #{y}, the #{z}"', b)

How can we get this output?
output
“The Learning, the ruby, the rails, the sqlite3”

Answer 2

Binding objects can be used as the second argument of eval() to set the scope. The string of code passed as the first argument in eval() is evaluated in the context of the binding passed in the second argument.

Question 3

Add code to the following class so

Motivation.new.speak returns “Go speed racer!!!”

first = 'speed'
second = 'racer'
class Motivation
  def speak
    # add code here
  end
end

Answer 3

The TOPLEVEL_BINDING constant refers to the top level object. Note that this code will not work if it’s pasted into the IRB console, this code must be run from a file with the normal Ruby interpreter.

Question 4

What does the following code print? Explain.

class A
  private
  def method_missing(name, *args)
    args.unshift(name.to_s).join
  end
end

p A.new.meta(‘programming’, ‘ruby’)

Answer 4

“metaprogrammingruby”

The name parameter is assigned to the method name (:meta) and args is assigned to an array of the arguments ([‘programming’, ‘ruby’]).

Question 5

What does the following code print? Explain.

class Human
  def Human.about; end
  def self.generation; end
  def hi; end
  instance_eval do
    def bye; end
  end
end

p Human.singleton_methods

Answer 5

[:about, :generation, :bye]

There are several ways to define singleton methods. Notice that the Human#hi method is a regular instance method and is not a singleton method.

Question 6

What does the following code print? Explain.

class Soccer
  @fun = 'woo hoo'
  def initialize
    @goal = 'score'
  end
end

s = Soccer.new
p s.instance_variables

Answer 6

[:@goal]

The @goal instance variable is bound to instances of the Soccer class. The @fun instance variable is bound to the Soccer class object itself, not instances of the Soccer class.

Question 7

Compare eql? and edual? methods

Answer 7

eql? – Checks if the value and type of two operands are the same (as opposed to the == operator which compares values but ignores types). For example, 1 == 1.0 evaluates to true, whereas 1.eql?(1.0) evaluates to false.

equal? – Compares the identity of two objects; i.e., returns true if both operands have the same object id (i.e., if they both refer to the same object). Note that this will return false when comparing two identical copies of the same object.

Question 8

How do you remove nil values in array using ruby?

> > [nil,123,nil,”test”]
=> [123, “test”]

Answer 8

> > [nil,123,nil,”test”].compact

=> [123, “test”]

Question 9

if false
value = ‘test’
end

What will be the value of:

defined? value
defined? none

Answer 9

defined? value
#=> "local-variable"
defined? bar
#=> nil

Question 10

Can you call a private method outside a Ruby class using its object?

Given the class Test:

class Test
   private
       def method
         p "I am a private method"
      end
end

Answer 10

> > Test.new.send(:method)
“I am a private method”

Yes, with the help of the send method.

Question 11

What does the following code print? Explain.

class Pencil; end

p Pencil.class
p Pencil.superclass

Answer 11

Class
Object

All class objects (objects that can be used to instantiate other objects) are instances of the Class class. Hash.class, Array.class, String.class, and Pencil.class all return Class.

Question 12

What does the following code print? Explain.

p BasicObject.superclass

Answer 12

nil
Every class object (i.e. instance of the Class class) has a superclass, except for BasicObject. BasicObject is at the top of the class hierarchy and does not have a superclass.

Question 13

What does the following code print? Explain.

p BasicObject.singleton_class.superclass

Answer 13

Class
BasicObject's singleton class inherits from Class.

Question 14

versions = {:ruby=>”3”}

What is the difference between the following lines of code?

versions[:rails]
versions.fetch(:rails)

Answer 14

versions.fetch(:rails) raises an exception because the :railskey is not present in the versionshash (if a second argument was added to the fetch() method, the default value would be returned instead of raising an exception).

versions[:rails] returns nil because the :rails key is not present in the versions hash.

Question 15

What is the output of this code?

def dubSplat(a, *b, **c)
p b
p c
end

dubSplat(1,2,3, 4, a: 40, b: 50)

Answer 15

=> [2, 3, 4]
You can also use the splat operator(*) to grab any segment of an array

The double splat operator (**) can be used for hashes!
=> {:a=>40, :b=>50}

Question 16

What is Spaceship Operator in Ruby? How it is working?

Answer 16

a <=> b :=
  if a < b then return -1
  if a = b then return  0
  if a > b then return  1
  if a and b are not comparable then return nil

Question 17

What is the name of this operator? How it will work?

===

Answer 17

subsumption operator.

“If a described a set, would b be a member of that set?”

(1. .5) === 3 # => true
(1. .5) === 6 # => false

Integer === 42 # => true
Integer === ‘fourtytwo’ # => false

Question 18

True or False?

> count ||= 0

count ||= 0 gives count the value 0 if count is nil or false.

Answer 18

True

a ||= b
The assignment statement supports a set of shortcuts: a op= b is the same
as a = a op b. This works for most operators:
count += 1 # same as count = count + 1
price *= discount # price = price * discount
count ||= 0 # count = count || 0
So, count ||= 0 gives count the value 0 if count is nil or false.

Question 19

string = “Ruby” + “ 3”

string = “Ruby” “3”

string = “Ruby” &laquo_space;” 3.”

string = “Ruby”.concat(“3”)

puts string

“Ruby 3”

Which one is false?

Answer 19

All are true

there are 4 ways to do string concatenation

Question 20

What does the following code return?

book = ["intelligent investor"]
great_book = ["intelligent investor"]

book.object_id == great_book.object_id

Answer 20

false

The book and great_book arrays have the same content, but are different instances of the Array class and therefore have different object_ids. Array equality comparisons in Ruby check if arrays have the same content in the same order, not if the arrays are the same exact object.