$abs (aggregation)

Definition定义

$abs

Returns the absolute value of a number.返回数字的绝对值。

$abs has the following syntax:语法如下：

{ $abs: <number> }

The <number> expression can be any valid expression as long as it resolves to a number. <number>表达式可以是任何有效的表达式，只要它解析为数字即可。For more information on expressions, see Expressions.有关表达式的详细信息，请参阅表达式。

Behavior行为

If the argument resolves to a value of null or refers to a field that is missing, $abs returns null. 如果参数解析为null值或引用缺少的字段，$abs返回null。If the argument resolves to NaN, $abs returns NaN.如果参数解析为NaN，$abs返回NaN。

Example示例

A collection ratings contains the following documents:ratings集合包含以下文件：

{ _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:以下示例计算start和end评分值之间的差值：

db.ratings.aggregate([
   {
     $project: { delta: { $abs: { $subtract: [ "$start", "$end" ] } } }
   }
])

The operation returns the following results:该操作返回以下结果：

{ "_id" : 1, "delta" : 3 }
{ "_id" : 2, "delta" : 0 }
{ "_id" : 3, "delta" : 2 }
{ "_id" : 4, "delta" : 1 }