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- $log (aggregation)
$log (aggregation)¶
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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:
{ $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 | 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 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 }
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