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Enumerable has first:
(3..5).to_enum.first
# => 3
but it does not have last:
(3..5).to_enum.last
# => NoMethodError: undefined method `last' for #<Enumerator: 3..5:each>
Why is that?
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Enumeration refers to traversing over objects. In Ruby, we call an object enumerable when it describes a set of items and a method to loop over each of them.
Enumerable methods work by giving them a block. In that block you tell them what you want to do with every element. For example: [1,2,3].map { |n| n * 2 } Gives you a new array where every number has been doubled.
Some Ruby classes include Enumerable: Array. Dir. Hash.
It is because not all enumerable objects have the last element.
The simplest example would be:
[1, 2, 3].cycle
# (an example of what cycle does)
[1,2,3].cycle.first(9) #=> [1, 2, 3, 1, 2, 3, 1, 2, 3]
Even if the enumerator elements are finite, there is no easy way to get the last element other than iterating through it to the end, which would be extremely inefficient.
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