each_with_index

Calls block with two arguments, the item and its index, for each item in enum. Given arguments are passed through to #each().

If no block is given, an enumerator is returned instead.

hash = Hash.new
%w(cat dog wombat).each_with_index {|item, index|
  hash[item] = index
}
hash   #=> {"cat"=>0, "dog"=>1, "wombat"=>2}