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each_index 

        # Array.each_index

(from ruby core)
---
    array.each_index {|index| ... } -> self
    array.each_index -> Enumerator

---

Iterates over array indexes.

When a block given, passes each successive array index to the block;
returns `self`:
    a = [:foo, 'bar', 2]
    a.each_index {|index|  puts "#{index} #{a[index]}" }

Output:
    0 foo
    1 bar
    2 2

Allows the array to be modified during iteration:
    a = [:foo, 'bar', 2]
    a.each_index {|index| puts index; a.clear if index > 0 }

Output:
    0
    1

When no block given, returns a new Enumerator:
    a = [:foo, 'bar', 2]
    e = a.each_index
    e # => #<Enumerator: [:foo, "bar", 2]:each_index>
    a1 = e.each {|index|  puts "#{index} #{a[index]}"}

Output:
    0 foo
    1 bar
    2 2

Related: #each, #reverse_each.

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