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- $abs (aggregation)
$abs (aggregation)¶
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Definition¶
- $abs¶
3.2 新版功能.
Returns the absolute value of a number.
$abs has the following syntax:
{ $abs: <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¶
If the argument resolves to a value of null or refers to a field that is missing, $abs returns null. If the argument resolves to NaN, $abs returns NaN.
If the argument resolves to a value of null or refers to a field that is missing, $abs returns null. If the argument resolves to NaN, $abs returns NaN.
| Example | Results |
|---|---|
| { $abs: -1 } | 1 |
| { $abs: 1 } | 1 |
| { $abs: null } | null |
Example¶
A collection ratings contains the following documents:
{ _id: 1, start: 5, end: 8 }
{ _id: 2, start: 4, end: 4 }
{ _id: 3, start: 9, end: 7 }
{ _id: 4, start: 6, end: 7 }
The following example calculates the magnitude of difference between the start and end ratings:
db.ratings.aggregate([
{
$project: { delta: { $abs: { $subtract: [ "$start", "$end" ] } } }
}
])
The operation returns the following results:
{ "_id" : 1, "delta" : 3 }
{ "_id" : 2, "delta" : 0 }
{ "_id" : 3, "delta" : 2 }
{ "_id" : 4, "delta" : 1 }