$log (expression operator)

Definition

$log

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

$log has the following syntax:

{ $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

The default return type is a double. If at least one operand is a decimal, then the return type is a 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.

Example

A collection integers contains the following documents:

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.

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

The operation returns the following results:

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