$trunc (aggregation)

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Definition

Syntax
Behavior
Example

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 Expression Operators.

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.

`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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