$ln (aggregation)

Definition定义

$ln

Calculates the natural logarithm ln (i.e log _e) of a number and returns the result as a double.计算一个数字的自然对数ln（即log _e），并以双精度返回结果。

$ln has the following syntax:具有以下语法：

{ $ln: <number> }

The <number> expression can be any valid expression as long as it resolves to a non-negative number. <number>表达式可以是任何有效的表达式，只要它解析为非负数即可。For more information on expressions, see Expressions.有关表达式的详细信息，请参阅表达式。

$ln is equivalent to $log: [ <number>, Math.E ] expression, where Math.E is a JavaScript representation for Euler's number e.相当于$log: [ <number>, Math.E ]表达式，其中Math.E是欧拉数e的JavaScript表示。

Behavior行为

If the argument resolves to a value of null or refers to a field that is missing, $ln returns null. 如果参数解析为null值或引用了缺失的字段，$ln将返回null。If the argument resolves to NaN, $ln returns NaN.如果参数解析为NaN，$ln将返回NaN。

Example实例

A collection sales contains the following documents:sales集合包含以下文档：

{ _id: 1, year: "2000", sales: 8700000 }
{ _id: 2, year: "2005", sales: 5000000 }
{ _id: 3, year: "2010", sales: 6250000 }

The following example transforms the sales data:以下示例转换sales数据：

db.sales.aggregate( [ { $project: { x: "$year", y: { $ln: "$sales"  } } } ] )

The operation returns the following results:该操作返回以下结果：

{ "_id" : 1, "x" : "2000", "y" : 15.978833583624812 }
{ "_id" : 2, "x" : "2005", "y" : 15.424948470398375 }
{ "_id" : 3, "x" : "2010", "y" : 15.648092021712584 }