$log (expression operator)（表达式运算符）

Definition定义

$log

Calculates the log of a number in the specified base and returns the result as a double.计算指定基数内数字的对数，并将结果作为双精度数返回。

$log has the following syntax:具有以下语法：

{ $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

The default return type is a double. If at least one operand is a decimal, then the return type is a decimal.默认返回类型是double。如果至少有一个操作数是decimal，则返回类型是decimal。

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

Example示例

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

db.integers.insertMany( [
   { _id: 1, int: 5 },
   { _id: 2, int: 2 },
   { _id: 3, int: 23 },
   { _id: 4, int: 10 }
] )

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

db.integers.aggregate([
   { $project: { bitsNeeded:
      {
         $floor: { $add: [ 1, { $log: [ "$int", 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 }