On Mon, 04 Jul 2011 12:12:33 +0200, Svante Carl v. Erichsen wrote:
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> Am 04.07.2011 11:31, schrieb Tamas Papp:
>> Why do some CL library functions have :key arguments?
>>
>> I am asking because I am working on some statistics functions, and the
>> design choice came up. Specifically, I can write functions like
>>
>> (defun quantiles (sequence quantiles &key (key #'identity))
>> ...)
>>
>> but it is a bit cumbersome. I can make my code simpler by relying on
>> calls like
>>
>> (quantiles (map 'vector key vector) quantiles)
>>
>> but this conses a bit more.
>
> Doesn't quantiles just pass the key to sort? Is there some smarter
> algorithm I am not aware of right now?
Eg
@inproceedings{greenwald2001space,
title={Space-efficient online computation of quantile summaries},
author={Greenwald, M. and Khanna, S.},
booktitle={ACM SIGMOD Record},
volume={30},
number={2},
pages={58--66},
year={2001},
organization={ACM}
}
I am making quantiles a generic function: it should work on objects
returned by the method above, and also on vectors. My problem was that
(quantiles quantile-summary #(0.25 0.5 0.75) :key something)
makes little sense, since the summary is already accumulated. So I will
drop key for the moment, and will probably use compiler macros.
I also thought of a lazy solution that would just save the function and
the object, and traverse once, eg
(quantiles (with-key function object) ...)
but I need to think about that.
> Also, passing the key lets you return the complete objects (or
> whatever), not just the keys as in the map call. For example, compare
That's a good point, I didn't think of that.
Best,
Tamas
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