Ruby Hash/Array delete_if without a block

Question

According to Ruby Hash/Array documentation, the delete_if method returns an enumerator if no block is given. How is this useful? Can someone give an example to demonstrate this pattern?

Answer 1

There are some methods defined on Enumerator that give flexibility to iterators. One such method I often use is with_index.

p %w[a b c d e f].delete_if.with_index{|_, i| i.even?}
# => ["b", "d", "f"]

If this was to be done without Enumerator class, all kinds of methods have to be defined, including delete_if_with_index, and that is not a good thing.

Answer 2

The enumerator will just allow you to run the block later. For example, if you had a method that specifically handled the delete if for several different objects, you could pass it the enumerator.

In the example below, it will print 1, 3, 5

arr = [0,1,2,3,4,5]
enumerator = arr.delete_if
enumerator.each { |el| el.even? }
puts arr.join(', ')