# Accumulators

## Group Accumulators

{% hint style="info" %}
These functions are available for use in the $group stage, accumulators are operators that maintain their state (e.g. totals, maximums, minimums, and related data) as information progress through the pipeline.
{% endhint %}

When used as accumulators in the $group stage, these operators take as input a single expression, evaluating the expression once for each input document, and maintain their stage for the group of documents that share the same group key.

For example:

| Variable    | Access via $   | Brief Description                                                                                     |
| ----------- | -------------- | ----------------------------------------------------------------------------------------------------- |
| accumulator | `$accumulator` | Returns the result of a user-defined accumulator function                                             |
| addToSet    | `$addToSet`    | Returns an array of unique expression values for each group. Order of the array elements is undefined |
| avg         | `$avg`         | Returns an average of numerical values. Ignores non-numeric values                                    |

## Accumulators

{% hint style="info" %}
Some operators that are available as accumulators for the $group stage are also available for use in other. When used in these other stages, these operators do not maintain their state and can take as input either a single argument or multiple arguments.
{% endhint %}

For example:

| Variable     | Access via $    | Brief Description                                                                                           |
| ------------ | --------------- | ----------------------------------------------------------------------------------------------------------- |
| addToSet     | `$addtoSet`     | Returns array of unique pushed values                                                                       |
| avg          | `$avg`          | Calculates the average of a number property or expression in the group.                                     |
| count        | `$count`        | Performs a count of a grouped documents or, if the property is an array, a count of the number of elements. |
| first        | `$first`        | Returns the first value from the group. The result depends on the sort field(s)                             |
| last         | `$last`         | Returns the last value from the group. The result depends on the sort field(s)                              |
| mergeObjects | `$mergeObjects` | Combines multiple documents into a single document                                                          |
| min          | `$min`          | Returns the smallest value from the group.                                                                  |
| max          | `$max`          | Returns the largest value from the group                                                                    |
| push         | `$push`         | Returns the array of pushed values                                                                          |
| sum          | `$sum`          | Calculates the sum of a number property or expression in the group                                          |


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