$sigmoid (aggregation)

Definition

$sigmoid

Performs the sigmoid function, which calculates the percentile of a number in the normal distribution with standard deviation 1.

The $sigmoid expression has the following syntax:

{ $sigmoid: { input: <numeric expression>, onNull: <expression>} }

The arguments can be any valid expression as long as they resolve to all numbers.

The sigmoid function is equivalent to the following algebraic operation:

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Example

This example uses a myScores collection that contains the following documents:

db.myScores.insertMany( [
   { score: 1 },
   { score: 5 },
   {},
   { score: 13 },
   { score: null },
   { score: 21 },
] )

The following aggregation pipeline adds a scaled field to each document and uses $sigmoid to calculate the scaled field value:

db.myScores.aggregate( [
   { $set: {
      scaled: { $sigmoid: "$score" }
   } }
] )

The operation returns the following documents:

{ score: 1, scaled: 0.7310585786 }
{ score: 5, scaled: 0.9933071491 }
{ scaled: null }
{ score: 13, scaled: 0.9999977397 }
{ score: null, scaled: null }
{ score: 19, scaled: 0.9999999992 }