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## Accumulators 

Accumulators, available only for the $group stage, compute values by combining documents that share the same group key. Accumulators take as input a single expression, evaluating the expression once for each input document, and maintain their state for the group of documents. 

<img src='content_image/736738.jpg'>

## 7.4.2 Aggregation Commands Comparison 

The following table provides a brief overview of the features of the MongoDB aggregation commands. 

