$sigmoid (aggregation)（聚合）

Definition定义

$sigmoid

Performs the sigmoid function, which calculates the percentile of a number in the normal distribution with standard deviation 1.执行sigmoid函数，该函数计算标准偏差为1的正态分布中数字的百分位数。

The $sigmoid expression has the following syntax:$sigmoid表达式具有以下语法：

{ $sigmoid: { input: <numeric expression>, onNull: <expression>} }

The arguments can be any valid expression as long as they resolve to all numbers.参数可以是任何有效的表达式，只要它们解析为所有数字。

The sigmoid function is equivalent to the following algebraic operation:sigmoid函数等价于以下代数运算：

Example示例

This example uses a myScores collection that contains the following documents:此示例使用包含以下文档的myScores集合：

db.myScores.insertMany( [
   { score: 1 },
   { score: 5 },
   {},
   { score: 13 },
   { score: null },
   { score: 21 },
] )

The following aggregation pipeline adds a scaled field to each document and uses $sigmoid to calculate the scaled field value:以下聚合管道为每个文档添加一个scaled字段，并使用$sigmoid计算scaled字段值：

db.myScores.aggregate( [
   { $set: {
      scaled: { $sigmoid: "$score" }
   } }
] )

The operation returns the following documents:该操作返回以下文档：

{ score: 1, scaled: 0.7310585786 }
{ score: 5, scaled: 0.9933071491 }
{ scaled: null }
{ score: 13, scaled: 0.9999977397 }
{ score: null, scaled: null }
{ score: 19, scaled: 0.9999999992 }