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$trunc (aggregation)

Definition

$trunc

$trunc truncates a number to a whole integer or to a specified decimal place.

Syntax

The $trunc operator has the following syntax:

{ $trunc : [ <number>, <place> ] }
Field
Type
Description

<number>

number

Can be any valid expression that resolves to a number. Specifically, the expression must resolve to an integer, double, decimal, or long.

$trunc returns an error if the expression resolves to a non-numeric data type.

<place>

integer

Optional Can be any valid expression that resolves to an integer between -20 and 100, exclusive. For example, -20 < place < 100. Defaults to 0 if unspecified.

  • If <place> resolves to a positive integer, $trunc truncates to <place> decimal places.

    For example, $trunc : [1234.5678, 2] truncates to two decimal places and returns 1234.56.

  • If <place> resolves to a negative integer, $trunc replaces <place> digits left of the decimal with 0.

    For example, $trunc : [1234.5678, -2] replaces to two digits left of the decimal with 0 and returns 1200.

  • If the absolute value of <place> exceeds the number of digits to the left of the decimal, $trunc returns 0.

    For example, $trunc : [ 1234.5678, -5] specifies the fifth digit left of the decimal. This exceeds the number of digits left of the decimal and returns 0.

  • If <place> resolves to 0, $trunc truncates all digits to the right of the decimal and returns the whole integer value.

    For example, $trunc : [1234.5678, 0] returns 1234

The <number> expression can be any valid expression as long as it resolves to a number. For more information on expressions, see Expressions.

Behavior

$trunc does not round the truncated data. To round input values to a specified place, use the $round expression.

Returned Data Type

The returned data type matches the data type of the input expression or value.

Truncation Precision with Floating Point Numbers

If you print a double-precision floating point number 4.56 with 20 digits after the decimal separator, you will see 4.55999999999999960920, which is slightly less than 4.56 and is truncated to 4.55.

MongoDB uses the IEEE 754 standard for floating point computations, and the behavior is consistent with that standard.

If you need a floating point number for an application that requires high precison, consider a Decimal128 value. For details, see BSON.Decimal128.

If you need to store a currency value, consider an integer using the lowest currency denomination unit. For example, use an integer with cents or pennies instead of a floating point number.

null, NaN, and +/- Infinity

  • If the argument resolves to a value of null or refers to a field that is missing, $trunc returns null.

  • If the argument resolves to NaN, $trunc returns NaN.

  • If the argument resolves to negative or positive infinity, $trunc returns negative or positive infinity respectively.

Example
Results

{ $trunc: [ NaN, 1] }

NaN

{ $trunc: [ null, 1] }

null

{ $trunc : [ Infinity, 1 ] }

Infinity

{ $trunc : [ -Infinity, 1 ] }

-Infinity

Example

Create a collection named samples with the following documents:

db.samples.insertMany(
   [
      { _id: 1, value: 19.25 },
      { _id: 2, value: 28.73 },
      { _id: 3, value: 34.32 },
      { _id: 4, value: -45.34 }
   ]
)

  • The following aggregation returns value truncated to the first decimal place:

    db.samples.aggregate([
       { $project: { truncatedValue: { $trunc: [ "$value", 1 ] } } }
    ])

    The operation returns the following results:

    { "_id" : 1, "truncatedValue" : 19.2 }
    { "_id" : 2, "truncatedValue" : 28.7 }
    { "_id" : 3, "truncatedValue" : 34.3 }
    { "_id" : 4, "truncatedValue" : -45.3 }

  • The following aggregation returns value truncated to the first place:

    db.samples.aggregate([
       { $project: { truncatedValue: { $trunc: [ "$value", -1 ] } } }
    ])

    The operation returns the following results:

    { "_id" : 1, "truncatedValue" : 10 }
    { "_id" : 2, "truncatedValue" : 20 }
    { "_id" : 3, "truncatedValue" : 30 }
    { "_id" : 4, "truncatedValue" : -40 }

  • The following aggregation returns``value`` truncated to the whole integer:

    db.samples.aggregate([
       { $project: { truncatedValue: { $trunc: [ "$value", 0 ] } } }
    ])

    The operation returns the following results:

    { "_id" : 1, "truncatedValue" : 19 }
    { "_id" : 2, "truncatedValue" : 28 }
    { "_id" : 3, "truncatedValue" : 34 }
    { "_id" : 4, "truncatedValue" : -45 }

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