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$trunc (aggregation)

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Definition

$trunc

Changed in version 4.2..

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

MongoDB 4.2 adds the following syntax for $trunc:

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{ $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. e.g. -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

Prior to MongoDB 4.2, $trunc truncated the input value to the whole integer. MongoDB 4.2 continues supporting the pre-4.2 syntax and behavior:

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{ $trunc: <number> }

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

If truncating to a specific decimal place, the data type returned by $trunc matches the data type of the input expression or value.

If truncating to a whole integer value, $trunc returns an integer.

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

A collection named samples contains the following documents:

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{ _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:

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

    The operation returns the following results:

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    { "_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:

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

    The operation returns the following results:

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    { "_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:

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

    The operation returns the following results:

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

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