$log (aggregation)
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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 than1
.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 | Results |
---|---|
{ $log: [ 100, 10 ] } | 2 |
{ $log: [ 100, Math.E ] } where Math.E is a JavaScript
representation 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 log 2 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 }