[WORK] groupBy is in! Next: aggregate

[H. S. Teoh via Digitalmars-d]

On Mon, Jan 26, 2015 at 02:50:16PM -0300, Ary Borenszweig via Digitalmars-d wrote:
> On 1/26/15 2:34 PM, Andrei Alexandrescu wrote:
> >>On Mon, Jan 26, 2015 at 11:26:04AM +0000, bearophile via Digitalmars-d
> >>wrote:
> >>>Russel Winder:
> >>>
> >>>>but is it's name "group by" as understood by the rest of the world?
> >>>
> >>>Nope...
[...]
> >My suggestion was to keep the name but change the code of your
> >groupBy implementation to return tuple(key, lazyValues) instead of
> >just lazyValues. That needs to happen only for binary predicates;
> >unary predicates will all have alternating true/false keys.

Huh, what? I think there's some misunderstanding here. The unary version
of the current groupBy translates to a binary predicate:

	groupBy!(a => a.field)

is equivalent to:

	groupBy!((a, b) => a.field == b.field)

I don't see how this has anything to do with alternating keys.


[...]
> That's much more harder to implement than what it does right now. I
> don't know how you'll manage to do the lazyValues thing: you'd need to
> make multiple passes in the range.
> 
> Again, other languages return an associative array in this case.

I think we're talking past each other here. What groupBy currently does
is to group elements by evaluating the predicate on *consecutive runs*
of elements. What some people seem to demand is a function that groups
elements by *global evaluation* of the predicate over all elements.
These two are similar but divergent functions, and conflating them is
not helping this discussion in any way.

If "group by" in other languages refers to the latter function, then
that means "groupBy" is poorly-named and we need to come up with a
better name for it. Changing it to return tuples and what-not seems to
be beating around the bush to me.


T

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Computers are like a jungle: they have monitor lizards, rams, mice, c-moss, binary trees... and bugs.