Boolean Enumerators

Objectives

1. Understand return values for enumerators.

2. Use true/false expressions within a block.

3. Use the #all? enumerator to create a true/false return value.

4. Use the #none? enumerator to create a true/false return value.

5. Use the #any? enumerator to create a true/false return value.

6. Use the #include? enumerator to create a true/false return value.

Overview

When we are iterating over objects in a collection like with #each we generally don't care about the return values.

["Red", "Yellow", "Blue"].each do |color|
  puts "There are #{color.length} letters in #{color}"
end #=> ["Red", "Yellow", "Blue"]

If you run this code in IRB, you'll see:

001:0 > ["Red", "Yellow", "Blue"].each do |color|
?>        puts "There are #{color.length} letters in #{color}"
003:1 > end

There are 3 letters in Red
There are 6 letters in Yellow
There are 4 letters in Blue
=> ["Red", "Yellow", "Blue"]

You can see the block did what we intended it to do, it printed our output. You'll notice the last line also indicates that the #each method also returned a value. All expressions in ruby must return a value. When you use #each on a collection, the return value is always the original collection. Nothing you do inside the block you pass #each will ever change the return value. But that's not always the case. With other enumerator methods, the return value of the method is very much dependent on the block.

#all?

Imagine wanting to know if all the numbers in an array are odd. You could use each with something like:

all_odd = true
[1,2,3].each do |number|
  if number.even? # Will evaluate to false for 1, true for 2, false for 3
    all_odd = false
  end
end
all_odd #=> false

That works, the end value of all_odd will be false because 2 flipped the all_odd variable to false. However, something so simple - checking if all the elements in this array are odd - isn't being expressed clearly. Worse than our code not expressing our intention is that our code requires us to maintain variable state, all_odd, which can easily lead to errors (say if some other piece of code accidentally changes that variable value).

Consider the following example using #all?:

all_odd = [1,3].all? do |number|
  number.odd? # Will evaluate to true for 1, true for 3
end #=> true
all_odd #=> true

The rule for the #all? enumerator is that the block passed to it must return true for every iteration for the entire #all? expression or method to return true. If we introduce an even number to the collection, the return value will change.

all_odd = [1,2,3].all? do |number|
  number.odd? # Will evaluate to true for 1, false for 2, true for 3
end #=> false
all_odd #=> false

That's the rule for #all? - every iteration, every loop of the block must return true. When the block encounters the value 2 for number, it will run the expression 2.odd? which will return false. Because there was at least one iteration of the block that had a false return value, the entire #all? expression returns false.

#none?

Imagine the opposite of #all?, a method #none?, where we are interested in none of the elements in a collection producing a true expression within the block passed to #none?.

[1,3].none?{|i| i.even?} #=> true

The entire expression #none? returns true because none of those numbers will produce a true expression when asked within the block if they are even. Compare the code above to the code required to test that condition using #each:

none_even = true
[1,3].each do |i|
  if i.even?
    none_even = false
  end
end #=> [1,3] because `#each` always returns the original collection
none_even #=> true

These high-level boolean enumerators like #all? and #none? are way cleaner for evaluating elements in a collection for true/false conditions.

To recap, if any of the elements in the collection evaluate to true when passed to the block, #none? will return false. If none of the elements evaluate to true, #none? will return true.

#any?

Sometimes you want to be a bit more forgiving than #all? or #none? and just ensure that at least one element in a collection will create a true expression within the block passed. #any? is perfect for this. The #any? enumerator will return true if at least one iteration of the block evaluates to true, but false if none of them do.

[1,2,100].any?{|i| i > 99} #=> true

The #any? expression above will return true because at least one element, 100, will produce a true evaluation in the block.

#include?

Whereas #any? is useful for evaluating the truthiness of the logic of a block, #include? is helpful if you'd like to merely compare actual contents of a known value.

#include? will return true if the given element exists in the object. If it doesn't find a match, it will return false.

the_numbers = [4,8,15,16,23,42]
the_numbers.include?(42)   #=> true
the_numbers.include?(6)   #=> false

The #include? expression first returns true because the_numbers[5] == 42. When it is run with 6, it will evaluate to false since that item is not present in the array.

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