$mod (aggregation)

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Definition

Syntax
Behavior
Example

Definition

$mod: Divides one number by another and returns the remainder.

Syntax

The $mod expression has the following syntax:

{ $mod: [ <expression1>, <expression2> ] }

The first argument is the dividend, and the second argument is the divisor. That is, the first argument is divided by the second argument.

Behavior

The arguments can be any valid expression as long as they resolve to numbers. For more information on expressions, see Expression Operators.

Starting in version 7.2, the output data type of the $mod operator is the larger of the two input data types.

Note

Prior to version 7.2, the value and field type of inputs determine the $mod output type if:

The divisor is type double but has an integer value.
The dividend is type int or long.

In this case, MongoDB converts the divisor to the dividend data type before it performs the mod operation. The output data type is the dividend data type.

Negative Dividend

When the dividend is negative, the remainder is also negative. For more details on this behavior, see the official JavaScript documentation.

For an example, see Negative Dividend.

Example

Consider a conferencePlanning collection with the following documents:

db.conferencePlanning.insertMany( [
   { "_id" : 1, "city" : "New York", "hours" : 80, "tasks" : 7 },
   { "_id" : 2, "city" : "Singapore", "hours" : 40, "tasks" : 4 }
] )

The following aggregation uses the $mod expression to return the remainder of the hours field divided by the tasks field:

db.conferencePlanning.aggregate( [
  { $project: { remainder: { $mod: [ "$hours", "$tasks" ] } } }
] )

The operation returns the following results:

[
  { '_id' : 1, 'remainder' : 3 },
  { '_id' : 2, 'remainder' : 0 }
]

Negative Dividend

Consider a modExample collection that contains the following document:

db.modExample.insertOne( [
   { "_id" : 1, "dividend": -13, "divisor": 9 }
] )

This aggregation uses the $mod expression to return the remainder of dividend divided by the divisor field:

db.modExample.aggregate( [
  { $project: { remainder: { $mod: [ "$dividend", "$divisor" ] } } }
] )

The operation returns the following results:

[ { '_id' : 1, 'remainder' : -4 } ]

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