Module differential_dataflow::operators::count
[−]
[src]
Group records by a key, and apply a reduction function.
The group operators act on data that can be viewed as pairs (key, val). They group records
with the same key, and apply user supplied functions to the key and a list of values, which are
expected to populate a list of output values.
Several variants of group exist which allow more precise control over how grouping is done.
For example, the _by suffixed variants take arbitrary data, but require a key-value selector
to be applied to each record. The _u suffixed variants use unsigned integers as keys, and
will use a dense array rather than a HashMap to store their keys.
The list of values are presented as an iterator which internally merges sorted lists of values. This ordering can be exploited in several cases to avoid computation when only the first few elements are required.
Traits
| CountTotal |
Extension trait for the |
| CountTotalCore |
Extension trait for the |