# $trunc (aggregation)¶

## 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`

:{ $trunc : [ <number>, <place> ] } FieldTypeDescription`<number>`

numberCan 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:{ $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:

{ _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 }