$log (aggregation)

Definition定义

$log

Calculates the log of a number in the specified base and returns the result as a double.计算以指定基数表示的数字的对数，并以double形式返回结果。

$log has the following syntax:语法如下：

{ $log: [ <number>, <base> ] }

The <number> expression can be any valid expression as long as it resolves to a non-negative number.<number>表达式可以是任何有效的表达式，只要它解析为非负数。

The <base> expression can be any valid expression as long as it resolves to a positive number greater than 1.<base>表达式可以是任何有效的表达式，只要它解析为大于1的正数。

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 either argument resolves to a value of null or refers to a field that is missing, $log returns null. 如果任一参数解析为null值或引用缺少的字段，$log将返回null。If either argument resolves to NaN, $log returns NaN.如果任一参数解析为NaN，$log将返回NaN。

Example示例

A collection examples contains the following documents:集合examples包含以下文档：

{ _id: 1, positiveInt: 5 }
{ _id: 2, positiveInt: 2 }
{ _id: 3, positiveInt: 23 }
{ _id: 4, positiveInt: 10 }

The following example uses log ₂ in its calculation to determine the number of bits required to represent the value of positiveInt.以下示例在计算中使用log ₂来确定表示positiveInt值所需的位数。

db.examples.aggregate([
   { $project: { bitsNeeded:
      {
         $floor: { $add: [ 1, { $log: [ "$positiveInt", 2 ] } ] } } }
      }
])

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

{ "_id" : 1, "bitsNeeded" : 3 }
{ "_id" : 2, "bitsNeeded" : 2 }
{ "_id" : 3, "bitsNeeded" : 5 }
{ "_id" : 4, "bitsNeeded" : 4 }