Mongo shell中的NumberLong算术 [英] NumberLong arithmetic in the Mongo shell

问题描述

如何在Mongo shell中对NumberLong值执行精确算术运算？我的理解是Javascript只有一种数字类型 - 数字 - 通常限于54位浮点精度。

How can I perform precise arithmetic on NumberLong values in the Mongo shell? It's my understanding that Javascript only has one numeric type - number - typically limited to 54-bit floating-point precision.

使用（例如）标准加法的直接算术显示降级强制到低精度类型：

Direct arithmetic using (e.g.) standard addition shows demoting coercion to lower-precision type:

> NumberLong("123456789012345678")+NumberLong("1") 
123456789012345680
> NumberLong("123456789012345678")+NumberLong("2")
123456789012345680

我可以看到如何使用字符串表示提取NumberLong的部分，但这似乎效率低，并且对增量或除法等算术运算没有用。

I can see how to extract portions of a NumberLong using string representations, but this seems inefficient and is not useful for such arithmetic operations as increment or divide.

推荐答案

MongoDB使用 BSON 64位Int类型（ Mongo类型代码18 ）为NumberLong。 db内核可以通过执行更新操作对这些元素（用C ++编写的内核）执行精确算术，但是如果你想在javascript shell中执行此操作，则需要一个像来自Google的这一个。

MongoDB uses the BSON 64 bit Int type (Mongo type code 18) for NumberLong. The db kernel can perform precise arithmetic on these elements (kernel written in C++) through doing update operations but if you want to do this in the javascript shell you'll need a library like this one from Google.

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