Docs Home → Develop Applications → MongoDB Manual

# $log (aggregation)

On this page

## Definition

`$log`

*New in version 3.2*.Calculates the log of a number in the specified base and returns the result as a double.

`$log`

has the following syntax:{ $log: [ <number>, <base> ] } The

`<number>`

expression can be any valid expression as long as it resolves to a non-negative number.The

`<base>`

expression can be any valid expression as long as it resolves to a positive number greater than`1`

.For more information on expressions, see Expressions.

## Behavior

If either argument resolves to a value of `null`

or refers to a field that is
missing, `$log`

returns `null`

. If either argument resolves to
`NaN`

, `$log`

returns `NaN`

.

Example | Results |
---|---|

`{ $log: [ 100, 10 ] }` | `2` |

`{ $log: [ 100, Math.E ] }` where `Math.E` is a JavaScript
representation for e. | `4.605170185988092` |

## Example

A collection `examples`

contains the following documents:

{ _id: 1, positiveInt: 5 } { _id: 2, positiveInt: 2 } { _id: 3, positiveInt: 23 } { _id: 4, positiveInt: 10 }

The following example uses log _{2} in its calculation to
determine the number of bits required to represent the value of
`positiveInt`

.

db.examples.aggregate([ { $project: { bitsNeeded: { $floor: { $add: [ 1, { $log: [ "$positiveInt", 2 ] } ] } } } } ])

The operation returns the following results:

{ "_id" : 1, "bitsNeeded" : 3 } { "_id" : 2, "bitsNeeded" : 2 } { "_id" : 3, "bitsNeeded" : 5 } { "_id" : 4, "bitsNeeded" : 4 }