[pro] why :key arguments?

[Svante Carl v. Erichsen]

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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?

Also, passing the key lets you return the complete objects (or
whatever), not just the keys as in the map call.  For example, compare

(quantiles #((a 1) (b 4) (c 2) (d 2) (e 5)) 2 :key #'second)

with

(quantiles (map 'vector #'second
                #((a 1) (b 4) (c 2) (d 2) (e 5)))
           2)

In the first case, you could return (d 2), but not in the second.

Maybe I'm just missing the point.  :)


Best wishes,

Svante


- -- 
Svante Carl v. Erichsen
Wentorfer Str. 96
21029 Hamburg

+49-(0)40-34923721
+49-(0)160-6941474
Svante.v.Erichsen at web.de
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