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$pow (aggregation)

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Definition

$pow

New in version 3.2.

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 }