MongoDB $log Operator

MongoDB provides different types of arithmetic expression operators that are used in the aggregation pipeline stages and $log operator is one of them. This operator is used to find the log of a number in the specified base and returns the result as a double.

Syntax:

{ $log: [ <number>, <base> ] }

Here, the number is a valid expression as long as it resolves to a non-negative number and base is a valid expression until it resolves to a positive number greater than 1.

• If the entered value is null, then this operator will return null.
• If the entered value is NaN, then this operator will return NaN.
• If the entered value is a missing field, then this operator will return null.

Examples:

In the following examples, we are working with:

Database: GeeksforGeeks

Collection: example

Document: two documents that contain the details of the shapes in the form of field-value pairs.

Using $log operator:

In this example, we are going to find the log of the value of the side field in the square document. Here, the base value is 10.

db.example.aggregate([{$match:{name: "Square"}},
... {$project: {side:1, logSide: {$log: ["$side", 10]}}}])

Using $log operator in the embedded document: 

In this example, we are going to find the log of the value of the measurement.width field in the rectangle document. Here, the base value is 2.

db.example.aggregate([{$match:{name: "Rectangle"}},
... {$project: {"measurement.width": 1, 
      logWidth: {$log: ["$measurement.width", 2]}}}])

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