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 |