$log

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

Syntax

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

Parameters

<number>stringrequired

Any valid expression that resolves to a positive number.

<base>stringrequired

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

Examples

Sample Data

{
  "_id": "0fcc0bf0-ed18-4ab8-b558-9848e18058f4",
  "sales": {
    "salesByCategory": [
      { "categoryName": "Wine Accessories", "totalSales": 34440 }
    ]
  }
}

Calculate the log of total sales

Calculate the log of sales volumes per category in base 2 and 10.

Query:

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] }
          }
        }
      }
    }
  }
])

Output:

[
  {
    "_id": "40d6f4d7-50cd-4929-9a07-0a7a133c2e74",
    "name": "Proseware, Inc. | Home Entertainment Hub",
    "categoryAnalysis": [
      { "categoryName": "Sound Bars", "sales": 2120, "logBase2": 11.051, "logBase10": 3.326 }
    ]
  }
]