Ranges/algorithms for aggregation

[H. S. Teoh]

On Fri, Mar 21, 2014 at 04:10:12PM +0000, Justin Whear wrote:
[...]
> This pull request[1] for groupBy has been hanging around for a year
> now, driving me to copy-and-paste the implementation into a couple of
> my projects.  Using it, you could do this:
> 
> auto tuples = ... // get your list of (B, foo), (B, bar), etc.
> auto output = tuples.sort!`a[0] < b[0]`
>                     .groupBy!`a[0] == b[0]`;
> // output is a range of:
> //    [
> //     [(A, begga)],
> //     [(B, foo), (B, bar), (B, big)],
> //     [(C, ble)]
> //    ]
> 
> The advantage being that output isn't an array at all but a lazy range
> of lazy ranges.
> 
> 1 https://github.com/D-Programming-Language/phobos/pull/1186

Be aware, though, that groupBy only compares *adjacent* elements for
equivalence; it does not sort the input. So if your input has equivalent
elements interspersed with non-equivalent elements, you will have the
equivalent elements split into multiple runs in the output.

Example:
	auto data = [
		tuple(1, "a")
		tuple(1, "b")
		tuple(2, "c")
		tuple(1, "d")
	];
	writeln(data.groupBy!((a,b) => a[0] == b[0]));

Will output:
	[[tuple(1, "a"), tuple(1, "b")], [tuple(2, "c")], [tuple(1, "d")]]

Which may not be what the OP wants.


T

-- 
Unix was not designed to stop people from doing stupid things, because that would also stop them from doing clever things. -- Doug Gwyn