[WORK] groupBy is in! Next: aggregate

[via Digitalmars-d]

On Friday, 23 January 2015 at 20:19:31 UTC, Ary Borenszweig wrote:
> On 1/23/15 3:08 PM, Andrei Alexandrescu wrote:
>> So H.S. Teoh awesomely took
>> https://github.com/D-Programming-Language/phobos/pull/2878 to
>> completion. We now have a working and fast relational "group 
>> by" facility.
>>
>> See it at work!
>>
>> ----
>> #!/usr/bin/rdmd
>>
>> void main()
>> {
>>     import std.algorithm, std.stdio;
>>     [293, 453, 600, 929, 339, 812, 222, 680, 529, 768]
>>         .groupBy!(a => a & 1)
>>         .writeln;
>> }
>> ----
>>
>> [[293, 453], [600], [929, 339], [812, 222, 680], [529], [768]]
>>
>> The next step is to define an aggregate() function, which is a 
>> lot
>> similar to reduce() but works on ranges of ranges and 
>> aggregates a
>> function over each group. Continuing the previous example:
>>
>>     [293, 453, 600, 929, 339, 812, 222, 680, 529, 768]
>>         .groupBy!(a => a & 1)
>>         .aggregate!max
>>         .writeln;
>>
>> should print:
>>
>> [453, 600, 929, 812, 529, 768]
>>
>> The aggregate function should support aggregating several 
>> functions at
>> once, e.g. aggregate!(min, max) etc.
>>
>> Takers?
>>
>>
>> Andrei
>
> In most languages group by yields a tuple of {group key, group 
> values}.
>
> For example (Ruby or Crystal):
>
> a = [1, 4, 2, 4, 5, 2, 3, 7, 9]
> groups = a.group_by { |x| x % 3 }
> puts groups #=> {1 => [1, 4, 4, 7], 2 => [2, 5, 2], 0 => [3, 9]}
>
> In C# it's also called group by: 
> http://www.dotnetperls.com/groupby
>
> Java: 
> http://docs.oracle.com/javase/8/docs/api/java/util/stream/Collectors.html#groupingBy-java.util.function.Function-
>
> SQL: http://www.w3schools.com/sql/sql_groupby.asp
>
> So I'm not sure "groupBy" is a good name for this.

You are talking about two different functions here. group by and 
partition by. The function that has been implemented is often 
called partition by.

The best example I know of:

https://clojuredocs.org/clojure.core/group-by
https://clojuredocs.org/clojure.core/partition-by