$log10 (expression operator)

Definition

$log10

Calculates the log base 10 of a number and returns the result as a double.

$log10 has the following syntax:

{ $log10: <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.

$log10 is equivalent to $log: [ <number>, 10 ] expression.

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, $log10 returns null. If the argument resolves to NaN, $log10 returns NaN.

Example

Create a collection named samples with the following documents:

db.samples.insertMany(
   [
      { _id: 1, H3O: 0.0025 },
      { _id: 2, H3O: 0.001 },
      { _id: 3, H3O: 0.02 }
   ]
)

The following example calculates the pH value of the samples:

db.samples.aggregate( [
   { $project: { pH: { $multiply: [ -1, { $log10: "$H3O" } ] } } }
] )

The operation returns the following results:

{ "_id" : 1, "pH" : 2.6020599913279625 }
{ "_id" : 2, "pH" : 3 }
{ "_id" : 3, "pH" : 1.6989700043360187 }