$ln (expression operator)

Definition

$ln

Calculates the natural logarithm ln (i.e log _e) of a number and returns the result as a double.

$ln has the following syntax:

{ $ln: <number> }

The <number> expression can be any valid expression as long as it resolves to a non-negative number. For more information on expressions, see Expressions.

$ln is equivalent to $log: [ <number>, Math.E ] expression, where Math.E is a JavaScript representation for Euler's number e.

Behavior

The default return type is a double. If at least one operand is a decimal, then the return type is a decimal.

If the argument resolves to a value of null or refers to a field that is missing, $ln returns null. If the argument resolves to NaN, $ln returns NaN.

Example

A collection sales contains the following documents:

db.sales.insertMany( [
   { _id: 1, year: "2000", sales: 8700000 },
   { _id: 2, year: "2005", sales: 5000000 },
   { _id: 3, year: "2010", sales: 6250000 }
] )

The following example transforms the sales data:

db.sales.aggregate( [ { $project: { x: "$year", y: { $ln: "$sales"  } } } ] )

The operation returns the following results:

{ "_id" : 1, "x" : "2000", "y" : 15.978833583624812 }
{ "_id" : 2, "x" : "2005", "y" : 15.424948470398375 }
{ "_id" : 3, "x" : "2010", "y" : 15.648092021712584 }