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• Definition
• Behavior
• Example

Definition

$log

New in version 3.2.

Calculates the log of a number in the specified base and returns the result as a double.

$log has the following syntax:

copy

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

The <number> expression can be any valid expression as long as it resolves to a non-negative number.

The <base> expression can be any valid expression as long as it resolves to a positive number greater than 1.

For more information on expressions, see Expressions.

Behavior

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.

Example

A collection examples contains the following documents:

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

The following example uses log₂ in its calculation to determine the number of bits required to represent the value of positiveInt.

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

The operation returns the following results:

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