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Building Up a Hash Using Injection
CodeExpectedActual
collection = [ [1, 'one'], [2, 'two'], [3, 'three'], 
               [4, 'four'], [5, 'five']
             ]
collection.inject({}) do |hash, value|
  hash[value.first] = value.last
  hash
end
 {5=>"five", 1=>"one", 2=>"two", 3=>"three", 4=>"four"} {5=>"five", 1=>"one", 2=>"two", 3=>"three", 4=>"four"} 
collection.dup.inject({}) { |hash, value| hash[value.first] = value.last }
 
IndexError: index 3 out of string
...
 
IndexError: index 3 out of string
	from (irb):8:in `[]='
	from (irb):8
	from (irb):8:in `inject'
	from (irb):8

Hash.new["key"] = "some value"
 "some value" "some value" 
collection.inject({}) do |hash, value|
  hash.update value.first => value.last
end
 {5=>"five", 1=>"ontwo", 2=>"two", 3=>"three", 4=>"four"} {5=>"five", 1=>"ontwo", 2=>"two", 3=>"three", 4=>"four"} 
class Array
  def to_h(default=nil)
    Hash[ *inject([]) { |a, value| a.push value, default || yield(value) } ]
  end
end
a = [1, 2, 3]
a.to_h(true)
 {1=>true, 2=>true, 3=>true} {1=>true, 2=>true, 3=>true} 
a.to_h { |value| [value * -1, value * 2] }
 {1=>[-1, 2], 2=>[-2, 4], 3=>[-3, 6]} {1=>[-1, 2], 2=>[-2, 4], 3=>[-3, 6]}