如何防止 Ruby 货币浮点数错误

Question

我正在使用 Rails 和货币-rails gem 来处理货币列。

有什么办法可以防止出现浮点数错误吗？（即使是 hack 也行，我只是想确保不会向最终用户显示此类错误）

Rspec 示例：

  it "correctly manipulates simple money calculations" do
    # Money.infinite_precision = false or true i've tried both 
    start_val = Money.new("1000", "EUR")
    expect(start_val / 30 * 30).to eq start_val
  end

结果

Failure/Error: expect(start_val / 30 * 30).to eq start_val

   expected: #<Money fractional:1000.0 currency:EUR>
        got: #<Money fractional:999.99999999999999999 currency:EUR>

   (compared using ==)

   Diff:
   @@ -1,2 +1,2 @@
   -#<Money fractional:1000.0 currency:EUR>
   +#<Money fractional:999.99999999999999999 currency:EUR>

Answer 1

您应该使用小数表示金额。例如，参见 http://ruby-doc.org/stdlib-2.1.1/libdoc/bigdecimal/rdoc/BigDecimal.html。它具有任意精度算法。

编辑：在您的情况下，您可能应该将 Rspec 更改为：

it "correctly manipulates simple money calculations" do
  # Money.infinite_precision = false or true i've tried both 
  start_val = Money.new("1000", "EUR")
  thirty = BigDecimal.new("30")
  expect(start_val / thirty * thirty).to eq start_val
end

EDIT2：在这种情况下，1000/30 不能表示为有限十进制数。您必须使用 Rational class 或进行舍入。示例代码：

it "correctly manipulates simple money calculations" do
  # Money.infinite_precision = false or true i've tried both 
  start_val = Money.new("1000", "EUR")
  expect(start_val.amount.to_r / 30.to_r * 30.to_r).to eq start_val.amount.to_r
end

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