$pow (aggregation)

Definition

$pow

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

Create a collection called quizzes with the following documents:

db.quizzes.insertMany( [
   {
      _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 }