Accumulators

Accumulators calculate aggregated values like sums, maxima, and minima across a variable number of input values. You can use accumulators in these aggregation pipeline stages:

Name	Description
`$accumulator`	Returns the result of a user-defined accumulator function.
`$addToSet`	Returns an array of unique expression values for each group. Order of the array elements is undefined. Changed in version 5.0: Available in the `$setWindowFields` stage.
`$avg`	Returns an average of numerical values. Ignores non-numeric values. Changed in version 5.0: Available in the `$setWindowFields` stage.
`$bottom`	Returns the bottom element within a group according to the specified sort order. New in version 5.2. Available in the `$group` and `$setWindowFields` stages.
`$bottomN`	Returns an aggregation of the bottom `n` fields within a group, according to the specified sort order. New in version 5.2. Available in the `$group` and `$setWindowFields` stages.
`$count`	Returns the number of documents in a group. Distinct from the `$count` pipeline stage. New in version 5.0: Available in the `$group` and `$setWindowFields` stages.
`$first`	Returns the result of an expression for the first document in a group. Changed in version 5.0: Available in the `$setWindowFields` stage.
`$firstN`	Returns an aggregation of the first `n` elements within a group. Only meaningful when documents are in a defined order. Distinct from the `$firstN` array operator. New in version 5.2: Available in the `$group`, expression and `$setWindowFields` stages.
`$last`	Returns the result of an expression for the last document in a group. Changed in version 5.0: Available in the `$setWindowFields` stage.
`$lastN`	Returns an aggregation of the last `n` elements within a group. Only meaningful when documents are in a defined order. Distinct from the `$lastN` array operator. New in version 5.2: Available in the `$group`, expression and `$setWindowFields` stages.
`$max`	Returns the highest expression value for each group. Changed in version 5.0: Available in the `$setWindowFields` stage.
`$maxN`	Returns an aggregation of the `n` maximum valued elements in a group. Distinct from the `$maxN` array operator. New in version 5.2. Available in `$group`, `$setWindowFields` and as an expression.
`$median`	Returns an approximation of the median, the 50th percentile, as a scalar value. New in version 7.0. This operator is available as an accumulator in these stages: `$group` `$setWindowFields` It is also available as an aggregation expression.
`$mergeObjects`	Returns a document created by combining the input documents for each group.
`$min`	Returns the lowest expression value for each group. Changed in version 5.0: Available in the `$setWindowFields` stage.
`$minN`	Returns an aggregation of the `n` minimum valued elements in a group. Distinct from the `$minN` array operator. New in version 5.2. Available in `$group`, `$setWindowFields` and as an expression.
`$percentile`	Returns an array of scalar values that correspond to specified percentile values. New in version 7.0. This operator is available as an accumulator in these stages: `$group` `$setWindowFields` It is also available as an aggregation expression.
`$push`	Returns an array of expression values for documents in each group. Changed in version 5.0: Available in the `$setWindowFields` stage.
`$stdDevPop`	Returns the population standard deviation of the input values. Changed in version 5.0: Available in the `$setWindowFields` stage.
`$stdDevSamp`	Returns the sample standard deviation of the input values. Changed in version 5.0: Available in the `$setWindowFields` stage.
`$sum`	Returns a sum of numerical values. Ignores non-numeric values. Changed in version 5.0: Available in the `$setWindowFields` stage.
`$top`	Returns the top element within a group according to the specified sort order. New in version 5.2. Available in the `$group` and `$setWindowFields` stages.
`$topN`	Returns an aggregation of the top `n` fields within a group, according to the specified sort order. New in version 5.2. Available in the `$group` and `$setWindowFields` stages.

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