$sigmoid (expression operator)

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:

click to enlarge

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 }