$log (arithmetic expression)

APPLIES TO: MongoDB vCore

The $log operator calculates the logarithm of a number in the specified base.

Syntax

The syntax for the $log operator is as follows:

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

Parameters

Description
<number> Any valid expression that resolves to a positive number.
<base> Any valid expression that resolves to a positive number to be used as the logarithm base.

Example

Let's understand the usage with sample data from the stores dataset to calculate sales metrics using different logarithmic bases.

db.stores.aggregate([
  { $match: { "_id": "40d6f4d7-50cd-4929-9a07-0a7a133c2e74" } },
  {
    $project: {
      name: 1,
      categoryAnalysis: {
        $map: {
          input: "$sales.salesByCategory",
          as: "category",
          in: {
            categoryName: "$$category.categoryName",
            sales: "$$category.totalSales",
            logBase2: { $log: ["$$category.totalSales", 2] },
            logBase10: { $log: ["$$category.totalSales", 10] }
          }
        }
      }
    }
  }
])

This will produce the following output:

{
  "_id": "40d6f4d7-50cd-4929-9a07-0a7a133c2e74",
  "name": "Proseware, Inc. | Home Entertainment Hub - East Linwoodbury",
  "categoryAnalysis": [
    {
      "categoryName": "Sound Bars",
      "sales": 2120,
      "logBase2": 11.051,
      "logBase10": 3.326
    },
    {
      "categoryName": "Home Theater Projectors",
      "sales": 45004,
      "logBase2": 15.458,
      "logBase10": 4.653
    },
    {
      "categoryName": "Game Controllers",
      "sales": 43522,
      "logBase2": 15.410,
      "logBase10": 4.639
    },
    {
      "categoryName": "Remote Controls",
      "sales": 28946,
      "logBase2": 14.822,
      "logBase10": 4.462
    },
    {
      "categoryName": "VR Games",
      "sales": 32272,
      "logBase2": 14.977,
      "logBase10": 4.509
    }
  ]
}

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