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- $log10 (aggregation)
$log10 (aggregation)¶
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Definition¶
- $log10¶
3.2 新版功能.
Calculates the log base 10 of a number and returns the result as a double.
$log10 has the following syntax:
{ $log10: <number> }
The <number> expression can be any valid expression as long as it resolves to a non-negative number. For more information on expressions, see Expressions.
$log10 is equivalent to $log: [ <number>, 10 ] expression.
Behavior¶
If the argument resolves to a value of null or refers to a field that is missing, $log10 returns null. If the argument resolves to NaN, $log10 returns NaN.
| Example | Results |
|---|---|
| { $log10: 1 } | 0 |
| { $log10: 10 } | 1 |
| { $log10: 100 } | 2 |
| { $log10: 1000 } | 3 |
Example¶
A collection samples contains the following documents:
{ _id: 1, H3O: 0.0025 }
{ _id: 2, H3O: 0.001 }
{ _id: 3, H3O: 0.02 }
The following example calculates the pH value of the samples:
db.samples.aggregate( [
{ $project: { pH: { $multiply: [ -1, { $log10: "$H3O" } ] } } }
] )
The operation returns the following results:
{ "_id" : 1, "pH" : 2.6020599913279625 }
{ "_id" : 2, "pH" : 3 }
{ "_id" : 3, "pH" : 1.6989700043360187 }
参见