Introduction

In the previous lesson, you learned about some really handy enumerable methods like #each, #map, and #select. In this lesson, we’re going to learn about even more enumerable methods! Woohoo! The party never ends here at The Odin Project!

This time, we’re focusing on a particular subset of enumerable methods: the predicate enumerable methods. You should recall from the Methods lesson that a predicate method is indicated by a question mark (?) at the end of the method name and returns either true or false. Again, we won’t be going through all of the predicate enumerable methods, so be sure to have a look at the Ruby docs to see what else Enumerable offers.

For all of the examples throughout this lesson, feel free to follow along in irb or any appropriate online REPL environment to get a better feel for how they work.

Lesson overview

This section contains a general overview of topics that you will learn in this lesson.

• Describe what a predicate enumerable method is.
• Explain how the #include? method works.
• Explain how the #any? method works.
• Explain how the #all? method works.
• Explain how the #none? method works.

The include? method

The #include? method works exactly like you think it should. If we want to know whether a particular element exists in an array, we can use the #include? method. This method will return true if the element you pass as an argument to #include? exists in the array or hash; otherwise, it will return false.

First, let’s explore how we would achieve this with the #each method:

numbers = [5, 6, 7, 8]
element = 6
result = false

numbers.each do |number|
  if number == element
    result = true
    break
  end
end

result
# => true

element = 3
result = false

numbers.each do |number|
  if number == element
    result = true
    break
  end
end

result
#=> false

Using #include?, this code can be greatly simplified:

numbers = [5, 6, 7, 8]

numbers.include?(6)
#=> true

numbers.include?(3)
#=> false

For another example, let’s return to the friends and invited_friends arrays from the previous lesson:

friends = ['Sharon', 'Leo', 'Leila', 'Brian', 'Arun']

invited_list = friends.select { |friend| friend != 'Brian' }

invited_list.include?('Brian')
#=> false

The any? method

You might also be able to guess what the #any? method does. It returns true if any elements in your array or hash match the condition within the block; otherwise, it will return false.

Let’s say we want to see if there is any number greater than 500 or less than 20 in an array of numbers. First, let’s see how we could achieve this using #each.

numbers = [21, 42, 303, 499, 550, 811]
result = false

numbers.each do |number|
  if number > 500
    result = true
    break
  end
end

result
#=> true

numbers = [21, 42, 303, 499, 550, 811]
result = false

numbers.each do |number|
  if number < 20
    result = true
    break
  end
end

result
#=> false

Using #any?, this code can be greatly simplified:

numbers = [21, 42, 303, 499, 550, 811]

numbers.any? { |number| number > 500 }
#=> true

numbers.any? { |number| number < 20 }
#=> false

The all? method

The all? method is also fairly intuitive. It only returns true if all the elements in your array or hash match the condition you set within the block; otherwise, it will return false.

Let’s say that we want to check whether all the words in our list are more than 3 characters or 6 characters long. First,let’s see how we could achieve this using #each:

fruits = ["apple", "banana", "strawberry", "pineapple"]
matches = []
result = false

fruits.each do |fruit|
  if fruit.length > 3
    matches.push(fruit)
  end
end

result = fruits.length == matches.length
result
#=> true

fruits = ["apple", "banana", "strawberry", "pineapple"]
matches = []
result = false

fruits.each do |fruit|
  if fruit.length > 6
    matches.push(fruit)
  end
end

result = fruits.length == matches.length
result
#=> false

Using #all?, this code can be greatly simplified:

fruits = ["apple", "banana", "strawberry", "pineapple"]

fruits.all? { |fruit| fruit.length > 3 }
#=> true

fruits.all? { |fruit| fruit.length > 6 }
#=> false

Special note to keep in mind while debugging: #all? will return true by default unless the block returns false or nil. So if you call #all? on an empty array or hash (i.e., there are no elements for the block to evaluate), it will return true.

The none? method

As you might expect, #none? returns true only if the condition in the block matches none of the elements in your array or hash; otherwise, it returns false.

First, let’s see how this could be achieved using #each. You’ll notice that this approach is very similar to what we did for #all?.

fruits = ["apple", "banana", "strawberry", "pineapple"]
result = false

fruits.each do |fruit|
  if fruit.length > 10
    result = false
    break
  end

  result = true
end

result
#=> true

fruits = ["apple", "banana", "strawberry", "pineapple"]
result = false

fruits.each do |fruit|
  if fruit.length > 6
    result = false
    break
  end

  result = true
end

result
#=> false

Using #none?, this can be greatly simplified:

fruits = ["apple", "banana", "strawberry", "pineapple"]

fruits.none? { |fruit| fruit.length > 10 }
#=> true

fruits.none? { |fruit| fruit.length > 6 }
#=> false

The one? method

Ruby has methods to check whether all, any, or none of the elements in an array match a condition. But what if you need exactly one match - no more, no less? Ruby has you covered with the #one? method, which returns true if the block evaluates to true for exactly one element.

fruits = ["apple", "banana", "strawberry", "pineapple"]

fruits.one? { |fruit| fruit.length > 9 }
#=> true

fruits.one? { |fruit| fruit.length > 2 }
#=> false

Assignment

Knowledge check

The following questions are an opportunity to reflect on key topics in this lesson. If you can’t answer a question, click on it to review the material, but keep in mind you are not expected to memorize or master this knowledge.

• Why is there a question mark after some method names?
• What does the #include? method do?
• What does the #any? method do?
• What does the #all? method do?
• What does the #none? method do?

Additional resources

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