$toDouble (expression operator)（表达式运算符）

Definition定义

$toDouble

Converts a value to a double. If the value cannot be converted to an double, $toDouble errors. If the value is null or missing, $toDouble returns null.将值转换为双精度。如果该值不能转换为double，则$toDouble错误。如果该值为null或缺失，$toDouble将返回null。

$toDouble has the following syntax:具有以下语法：

{
   $toDouble: <expression>
}

The $toDouble takes any valid expression.$toDouble接受任何有效的表达式。

The $toDouble is a shorthand for the following $convert expression:$toDouble是以下$convert表达式的简写：

{ $convert: { input: <expression>, to: "double" } }

Behavior行为

The following table lists the input types that can be converted to a double:下表列出了可以转换为double的输入类型：

The following table lists some conversion to double examples:下表列出了一些双重转换示例：

Example示例

Create a collection weather with the following documents:使用以下文档创建集合weather：

db.weather.insertMany( [
   { _id: 1, date: new Date("2018-06-01"), temp: "26.1C" },
   { _id: 2,  date: new Date("2018-06-02"), temp: "25.1C" },
   { _id: 3,  date: new Date("2018-06-03"), temp: "25.4C" },
] )

The following aggregation operation on the weather collection parses the temp value and converts to a double:以下对weather集合的聚合操作解析temp值并将其转换为double值：

// Define stage to add degrees field with converted value

tempConversionStage = {
   $addFields: {
      degrees: { $toDouble: { $substrBytes: [ "$temp", 0, 4 ] } }
   }
};


db.weather.aggregate( [
   tempConversionStage,
] )

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

{ "_id" : 1, "date" : ISODate("2018-06-01T00:00:00Z"), "temp" : "26.1C", "degrees" : 26.1 }
{ "_id" : 2, "date" : ISODate("2018-06-02T00:00:00Z"), "temp" : "25.1C", "degrees" : 25.1 }
{ "_id" : 3, "date" : ISODate("2018-06-03T00:00:00Z"), "temp" : "25.4C", "degrees" : 25.4 }