$log (aggregation)
Definition
New in version 3.2.
Calculates the log of a number in the specified base and returns theresult 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 positivenumber 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 ismissing, $log
returns null
. If either argument resolves toNaN
, $log
returns NaN
.
Example | Results |
---|---|
{ $log: [ 100, 10 ] } | 2 |
{ $log: [ 100, Math.E ] } where Math.E is a JavaScriptrepresentation for e. | 4.605170185988092 |
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 todetermine the number of bits required to represent the value ofpositiveInt
.
- 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 }
See also