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Aggregation Operators

$round (aggregation)

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  • Definition
  • Behavior
  • Example

Definition

$round

New in version 4.2.

$round rounds a number to a whole integer or to a specified decimal place.

$round has the following syntax:

{ $round : [ <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.

$round 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, $round rounds to <place> decimal places.

    For example, $round : [1234.5678, 2] rounds to two decimal places and returns 1234.57.

  • If <place> resolves to a negative integer, $round rounds using the digit <place> to the left of the decimal.

    For example, $round : [1234.5678, -2] uses the 2nd digit to the left of the decimal (3) and returns 1200.

    If the absolute value of <place> equals or exceeds the number of digits to the left of the decimal, $round returns 0.

    For example, $round : [ 1234.5678, -4] specifies the fourth digit to the left of the decimal. This equals the number of digits left of the decimal and returns 0.

  • If <place> resolves to 0, $round rounds using the first digit to the right of the decimal and returns rounded integer value.

    For example, $round : [1234.5678, 0] returns 1235.

Behavior

Rounding to Even Values

When rounding on a value of 5, $round rounds to the nearest even value. For example, consider the following sample documents:

{_id : 1, "value" : 10.5},
{_id : 2, "value" : 11.5},
{_id : 3, "value" : 12.5},
{_id : 4, "value" : 13.5}

$round : [ "$value", 0] returns the following:

{_id : 1, "value" : 10},
{_id : 2, "value" : 12},
{_id : 3, "value" : 12},
{_id : 4, "value" : 14}

The value 10.5 is closest to the even value 10, while the values 11.5 and 12.5 are closest to the even value 12. Rounding to the nearest even value supports more even distribution of rounded data than always rounding up or down.

Returned Data Type

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

If rounding to a whole integer value, $round returns the value as an integer.

null, NaN, and +/- Infinity

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

  • If the first argument resolves to NaN, $round returns NaN.

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

Example
Results
{ $round: [ NaN, 1] }
NaN
{ $round: [ null, 1] }
null
{ $round : [ Infinity, 1 ] }
Infinity
{ $round : [ -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: -45.39 }
   ]
)

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

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

    The operation returns the following results:

    { "_id" : 1, "roundedValue" : 19.2 }
    { "_id" : 2, "roundedValue" : 28.7 }
    { "_id" : 3, "roundedValue" : 34.3 }
    { "_id" : 4, "roundedValue" : -45.4 }

  • The following aggregation returns value rounded using the first digit to the left of the decimal:

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

    The operation returns the following results:

    { "_id" : 1, "roundedValue" : 10 }
    { "_id" : 2, "roundedValue" : 20 }
    { "_id" : 3, "roundedValue" : 30 }
    { "_id" : 4, "roundedValue" : -50 }

  • The following aggregation returns value rounded to the whole integer:

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

    The operation returns the following results:

    { "_id" : 1, "roundedValue" : 19 }
    { "_id" : 2, "roundedValue" : 29 }
    { "_id" : 3, "roundedValue" : 34 }
    { "_id" : 4, "roundedValue" : -45 }

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$reverseArray (aggregation)