Some refactors to reduce the API surface area, make operations dryer and more maintainable, better naming

calculator_refactor_1.rb

class Calculator

  ORDERED_OPERATIONS = %W(* / + -) 

  def calculate(input)
    input = input.split(" ")

    while input.length > 1 do
      ORDERED_OPERATIONS.each do | op |
        i = input.find_index(op)
        if i 
          prev = input[i-1].to_f
          nex =  input[i+1].to_f
          result = operate(prev, op, nex)
          input = update_input_with_result(input, result, i)
        end
      end
    end
    input.first
  end

  private

  def operate(prev, op, nex)
    prev.send(op, nex)
  end

  def update_input_with_result(input, result, i)
    input.slice!((i-1)..(i+1)) #extract evaluated items
    input[i-1] = result #insert result
    input
  end
end


input = '7 - 2 + 3 / 4 * 5'
expected = (7 - (2 + (3 / (4 * 5.to_f))))
actual = Calculator.new.calculate(input)
puts actual
puts (expected == actual)