$toDouble(aggregation)¶

~~On this page~~本页内容

~~Definition~~定义
~~Behavior~~行为
~~Example~~示例

Definition定义¶

$toDouble¶

~~New in version 4.0.~~版本4.0中的新功能。

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.

$toDouble ~~has the following syntax:~~语法如下所示：

{
   $toDouble: <expression>
}

The $toDouble takes any valid expression.

The $toDouble is a shorthand for the following $convert expression:

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

Behavior行为¶

The following table lists the input types that can be converted to a double:

Input Type	Behavior
Boolean	Returns `0` for `false`. Returns `1` for `true`.
Double	No-op. Returns the double.
Decimal	Returns the decimal value as a double. The decimal value must fall within the minimum and maximum value for a double. You cannot convert a decimal value whose value is less than the minimum double value or is greater than the maximum double value.
Integer	Returns the int value as a double.
Long	Returns the long value as a double.
String	Returns the numerical value of the string as a double. The string value must be of a base₁₀ numeric value (e.g. `"-5.5"`, `"123456"`) and fall within the minimum and maximum value for a double. You cannot convert a string value of a non-base₁₀ number (e.g. `"0x6400"`) or a value that falls outside the minimum and maximum value for a double.
Date	Returns the number of milliseconds since the epoch that corresponds to the date value.

The following table lists some conversion to double examples:

~~Example~~示例	~~Results~~结果
`$toDouble: true`	1
`$toDouble: false`	0
`$toDouble: 2.5`	2.5
`$toDouble: NumberInt(5)`	5
`$toDouble: NumberLong(10000)`	10000
`$toDouble: "-5.5"`	-5.5
`$toDouble: ISODate("2018-03-27T05:04:47.890Z")`	1522127087890

Example示例¶

Create a collection weather with the following documents:

db.weather.insert( [
   { _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:

// 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:

{ "_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 }

Note

If the conversion operation encounters an error, the aggregation operation stops and throws an error. To override this behavior, use $convert instead.