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$toDouble (expression operator)

Definition

$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.

$toDouble has the following syntax:

{
   $toDouble: <expression>
}

The $toDouble takes any valid expression.

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

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

Behavior

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 base ₁₀ 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-base ₁₀ 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.

Tip

To convert strings in bases other than 10, use $convert with the base option.

The following table lists some conversion to double examples:

Example	Results
`$toDouble: true`	1
`$toDouble: false`	0
`$toDouble: 2.5`	2.5
`$toDouble: Int32(5)`	5
`$toDouble: Long(10000)`	10000
`$toDouble: "-5.5"`	-5.5
`$toDouble: ISODate("2018-03-27T05:04:47.890Z")`	1522127087890

Subnormal Numbers

Starting in MongoDB 8.3, the server is able to parse the full range of all representable double precision floating point numbers. This includes subnormal numbers where the most significant digit has leading zeroes and the exponent has the least possible value. For details, see Subnormal Number.

In earlier versions of MongoDB, the server returns an error when you try to parse these numbers. The following example raises an error in versions earlier than MongoDB 8.3:

db.t.insertOne( { v: "7.08263e-317" } )
db.t.aggregate([
  {
    $project: {
      converted: {
        $convert: { input: "$v", to: "double" },
      }
    }
   }
])

This example fails with an error similar to the following:

MongoServerError[ConversionFailure]: Executor error during aggregate command on namespace: test.t ::
caused by :: Failed to parse number '7.08263e-317' in $convert with no onError value: Out of range

Example

Create a collection weather with the following documents:

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:

// 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.

Back

$toDecimal

$toHashedIndexKey