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- $pow (aggregation)
$pow (aggregation)¶
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Definition¶
- $pow¶
3.2 新版功能.
Raises a number to the specified exponent and returns the result. $pow has the following syntax:
{ $pow: [ <number>, <exponent> ] }
The <number> expression can be any valid expression as long as it resolves to a number.
The <exponent> expression can be any valid expression as long as it resolves to a number.
You cannot raise 0 to a negative exponent.
Behavior¶
The result will have the same type as the input except when it cannot be represented accurately in that type. In these cases:
- A 32-bit integer will be converted to a 64-bit integer if the result is representable as a 64-bit integer.
- A 32-bit integer will be converted to a double if the result is not representable as a 64-bit integer.
- A 64-bit integer will be converted to double if the result is not representable as a 64-bit integer.
If either argument resolves to a value of null or refers to a field that is missing, $pow returns null. If either argument resolves to NaN, $pow returns NaN.
| Example | Results |
|---|---|
| { $pow: [ 5, 0 ] } | 1 |
| { $pow: [ 5, 2 ] } | 25 |
| { $pow: [ 5, -2 ] } | 0.04 |
| { $pow: [ -5, 0.5 ] } | NaN |
Example¶
A collection named quizzes contains the following documents:
{
"_id" : 1,
"scores" : [
{
"name" : "dave123",
"score" : 85
},
{
"name" : "dave2",
"score" : 90
},
{
"name" : "ahn",
"score" : 71
}
]
}
{
"_id" : 2,
"scores" : [
{
"name" : "li",
"quiz" : 2,
"score" : 96
},
{
"name" : "annT",
"score" : 77
},
{
"name" : "ty",
"score" : 82
}
]
}
The following example calculates the variance for each quiz:
db.quizzes.aggregate([
{ $project: { variance: { $pow: [ { $stdDevPop: "$scores.score" }, 2 ] } } }
])
The operation returns the following results:
{ "_id" : 1, "variance" : 64.66666666666667 }
{ "_id" : 2, "variance" : 64.66666666666667 }