Map, Filter, and Reduce in Lisp

[S. Robert James]

I'm familiar with map/filter/reduce from other languages. But I can't
find a simple doc on how to do these in Lisp! What I find perplexing
is that I thought that Lisp invented this paradigm?

What is the simplest way of map / filter / reducing a collection
(perhaps a list) in Lisp?

[Pedro Kroger]

On Feb 24, 11:14 pm, "S. Robert James" <srobertja...@gmail.com> wrote:
> What is the simplest way of map / filter / reducing a collection
> (perhaps a list) in Lisp?

It depends on what you want to do. Have you looked for mapcar, mapcan,
reduce, and many other mapping functions? [1] A few examples:

(reduce #'+ '(1 2 3 4)) => 10

(mapcar #'sqrt '(1 2 3 4)) => (1.0 1.4142135 1.7320508 2.0)

(mapcan #'(lambda (x) (if (oddp x) (list x))) '(1 2 3 4 5))

You should also take look at Practical Common Lisp [2].

Pedro Kroger

[1] http://www.lisp.org/HyperSpec/Body/fun_mapccm_ma_istcm_mapcon.html
[2] http://www.gigamonkeys.com/book/

[Lars Rune Nøstdal]

uhm .. have you checked the hyperspec?

http://www.lispworks.com/documentation/HyperSpec/Front/index.htm
http://www.lispworks.com/documentation/HyperSpec/Body/f_map.htm
http://www.lispworks.com/documentation/HyperSpec/Body/f_reduce.htm

`remove-if-not' is probably what you're after when you mention `filter':

http://www.lispworks.com/documentation/HyperSpec/Body/f_rm_rm.htm

you can download the hyperspec for offline reading here:

http://www.lispworks.com/documentation/common-lisp.html)

--
Lars Rune Nøstdal
http://nostdal.org/

[Vassil Nikolov]

I don't have a reference ready, but note that filtering in Common
Lisp can be done with REMOVE, REMOVE-IF, and REMOVE-IF-NOT; here is a
trivial example (one of many possible):

(reduce #'+ (map 'list #'round (remove-if #'symbolp '(foo 1.2 bar 3.4 baz 5.6))))
=> 10

(see also MAPCAR).

---Vassil.

--
Our programs do not have bugs; it is just that the users' expectations
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