Introduction to MongoDB $ln Operator

The $ln operator is a mathematical operator in MongoDB used to calculate the natural logarithm (base e) of a number. In MongoDB, the $ln operator can be used with other operators like $project and $addFields in the aggregation pipeline.

Syntax

The syntax of the $ln operator is as follows:

{ $ln: <numberExpression> }

Here, <numberExpression> represents a numeric expression that can be a numeric field, a numeric variable, or a numeric constant.

Use Cases

The $ln operator can be used in the aggregation pipeline to calculate the natural logarithm of a number and can be used in various statistical analyses. For example, calculating the logarithm of sales revenue can be used to analyze sales trends and market size.

Examples

Here are two examples of using the $ln operator.

Example 1: Calculating the logarithm of order amounts

Suppose we have a collection of orders that contains the total amount for each order, and we need to calculate the natural logarithm of the order amount to analyze the distribution of order amounts.

The data in the orders collection is as follows:

{ "_id" : 1, "totalAmount" : 100 }
{ "_id" : 2, "totalAmount" : 50 }
{ "_id" : 3, "totalAmount" : 200 }
{ "_id" : 4, "totalAmount" : 80 }

We can use the following aggregation pipeline to calculate the natural logarithm of the order amount:

db.orders.aggregate([{ $project: { logAmount: { $ln: "$totalAmount" } } }])

After running the above aggregation operation, the output is as follows:

{ "_id" : 1, "logAmount" : 4.605170185988092 }
{ "_id" : 2, "logAmount" : 3.912023005428146 }
{ "_id" : 3, "logAmount" : 5.298317366548036 }
{ "_id" : 4, "logAmount" : 4.382026634673881 }

In the output result, the logAmount field represents the natural logarithm of the order amount, which can be used to analyze the distribution of order amounts.

Example 2

Suppose we have a students collection that contains the score information for each student. We want to calculate the natural logarithm of the math score for each student. Here’s an example of the data:

{ name: "Alice", math_score: 80 }
{ name: "Bob", math_score: 90 }
{ name: "Charlie", math_score: 75 }

We can use the following aggregation pipeline to calculate the natural logarithm of the math score for each student:

db.students.aggregate([
  {
    $project: {
      name: 1,
      log_math_score: { $ln: ["$math_score"] }
    }
  }
])

After running the above aggregation pipeline, we get the following result:

{ "name": "Alice", "log_math_score": 4.382026634673881 }
{ "name": "Bob", "log_math_score": 4.499809670330265 }
{ "name": "Charlie", "log_math_score": 4.31748811353631 }

In the above result, the natural logarithm of the math score for each student is calculated and represented using the new log_math_score field.

Example 3: Calculate Natural Logarithm of a Field Value in a Nested Document

Suppose we have the following document:

{
  "_id": 1,
  "name": "Alice",
  "scores": {
    "math": 80,
    "english": 90,
    "science": 75
  }
}

We want to calculate the natural logarithm of the scores.math field value. We can use the following aggregation pipeline:

db.students.aggregate([
  {
    $project: {
      name: 1,
      ln_math: { $ln: "$scores.math" }
    }
  }
])

In the above aggregation pipeline, we first use the $project operator to select the fields to output. Here, we select the name field and a new field ln_math, whose value is the natural logarithm of the $scores.math field value. Then, we use the $ln operator to calculate the natural logarithm.

Running the above aggregation pipeline, we get the following output:

{ "_id": 1, "name": "Alice", "ln_math": 4.382026634673881 }

This result tells us that the natural logarithm of the scores.math field value is 4.382026634673881.

Conclusion

In this article, we introduced the syntax and use cases of the $ln operator. We can use the $ln operator to calculate the natural logarithm of numerical field values. Using the $ln operator allows us to more easily calculate mathematical functions in aggregation pipelines.