$exp (expression operator)（表达式运算符）

Definition定义

$exp

Raises Euler's number (i.e. e ) to the specified exponent and returns the result.将欧拉数（即e）幂乘到指定的指数并返回结果。

$exp has the following syntax:具有以下语法：

{ $exp: <exponent> }

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

Behavior行为

The default return type is a double. If at least one operand is a decimal, then the return type is a decimal.默认返回类型是double。如果至少有一个操作数是decimal，则返回类型是decimal。

If the argument resolves to a value of null or refers to a field that is missing, $exp returns null. If the argument resolves to NaN, $exp returns NaN.如果参数解析为null值或引用缺少的字段，$exp将返回null。如果参数解析为NaN，$exp将返回NaN。

Example示例

A collection named accounts contains the following documents:名为accounts的集合包含以下文档：

db.accounts.insertMany( [
   { _id: 1, interestRate: .08, presentValue: 10000 },
   { _id: 2, interestRate: .0825, presentValue: 250000 },
   { _id: 3, interestRate: .0425, presentValue: 1000 }
] )

The following example calculates the effective interest rate for continuous compounding:以下示例计算连续复利的实际利率：

db.accounts.aggregate( [ { $project: { effectiveRate: { $subtract: [ { $exp: "$interestRate"}, 1 ] } } } ] )

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

{ "_id" : 1, "effectiveRate" : 0.08328706767495864 }
{ "_id" : 2, "effectiveRate" : 0.08599867343905654 }
{ "_id" : 3, "effectiveRate" : 0.04341605637367807 }