you-ssk/gist:4044755

## gistfile1.rb
def format(a)
  a.inject([a.shift]){|r,e|
    if r[-1] == e-1
      r.pop(2) if r[-2] == '-'
      r << '-'
    else
      r << ','
    end
    r << e
  }.join.squeeze('-')
end

a = [1,2,3,5,6,7]
p format(a)
	def format(a)
	a.inject([a.shift]){\|r,e\|
	if r[-1] == e-1
	r.pop(2) if r[-2] == '-'
	r << '-'
	else
	r << ','
	end
	r << e
	}.join.squeeze('-')
	end

	a = [1,2,3,5,6,7]
	p format(a)