From: Marcin Borkowski <mbork@mbork.pl>
To: help-gnu-emacs@gnu.org
Subject: Re: How to mapcar or across a list?
Date: Wed, 15 Jul 2015 23:02:01 +0200 [thread overview]
Message-ID: <87d1ztm8xi.fsf@mbork.pl> (raw)
In-Reply-To: <871tg9m9os.fsf@kuiper.lan.informatimago.com>
On 2015-07-15, at 22:45, Pascal J. Bourguignon <pjb@informatimago.com> wrote:
> Marcin Borkowski <mbork@mbork.pl> writes:
>
>> Hi there,
>>
>> so here's my problem: I have a list of Boolean values, and I want to
>> `mapcar' an `or' across it (IOW, I want to test whether at least one of
>> them is true). Of course, (apply #'or my-list) does not work. Of
>> course, I can (cl-reduce (lambda (x y) (or x y)) my-list) -- but is
>> there a better method?
>>
>> BTW, my-list doesn't really exist: it is a result of `mapcar'ing
>> a function taking some value and yielding a Boolean value, so bonus
>> points if the method does not process the whole list.
>
> It's too late, lisp is not a lazy language.
Well, *after* I mapcar then it's too late, but your own answer (below)
shows that mapcar itself is not necessary (as I suspected).
>> (Note: I just noticed that my-list is in fact sorted so that all true
>> values (if any) will occur at the beginning anyway, so I can just test
>> the first one. This means that my problem is purely academic, though
>> still interesting, I guess.)
>
> First you want to reduce a list of booleans to a single value, so what
> makes you thing that mapcar is the right too for that? There's the word
> "map" in "mapcar", didn't you notice?
My bad, you're right, of course.
> map reduce
>
> x1 --> r1 x1 -\
> x2 --> r2 x2 --+-> r
> x3 --> r3 x3 -/
Please, do not shame a mathematician. I hope notany (pun intended;-))
of my students will read this...
> Also, mapcar allocates a new list of same length as the original list
> for the result, so it is costly. At least, you could consider mapc.
True.
> With mapc (or mapcar) you could perform your task with some cl magic:
>
> (require 'cl)
> (setf lexical-binding t)
>
> (defun* or-all (list)
> (mapc (lambda (element)
> (when element
> (return-from or-all t)))
> list)
> nil)
>
> (defun* and-all (list)
> (mapc (lambda (element)
> (unless element
> (return-from and-all nil)))
> list)
> t)
>
> (or-all '(nil nil nil)) --> nil
> (or-all '(nil t nil)) --> t
>
> (and-all '(t t t)) --> t
> (and-all '(t nil t)) --> nil
>
>
> But I fail to see how it's better than:
>
> (defun* or-all (list)
> (reduce (lambda (a b) (or a b)) list))
>
> (defun* and-all (list)
> (reduce (lambda (a b) (and a b)) list))
Of course, but - as I expected - someone already felt that there should
be a more elegant way, and hence the abstraction called `some', `every' etc.
> or than just:
>
> (some 'identity '(nil nil nil)) --> nil
> (some 'identity '(nil t nil)) --> t
> (some 'identity '(t t t )) --> t
>
> (every 'identity '(nil nil nil)) --> nil
> (every 'identity '(t nil t )) --> nil
> (every 'identity '(t t t )) --> t
>
> There are also the negations:
>
> (notevery 'identity '(nil nil nil)) --> t
> (notevery 'identity '(nil t nil)) --> t
> (notevery 'identity '(t t t )) --> nil
>
> (notany 'identity '(nil nil nil)) --> t
> (notany 'identity '(t nil t )) --> nil
> (notany 'identity '(t t t )) --> nil
>
> And since your booleans are computed by a function with mapcar, you
> could avoid computing this function for all the elements of the original
> list, and you could avoid allocating the temporary list by having some
> call this function:
>
>
> So instead of:
>
> (reduce (lambda (a b) (or a b))
> (mapcar 'complex-and-lengthy-predicate list))
>
> use:
>
> (some 'complex-and-lengthy-predicate list)
How cool. Again: I felt that something like that should exist, so I'm
not surprised that it actually does.
> For example:
> (some (lambda (x) (print x) (evenp x)) '(1 2 3 5 7 9 11))
> prints:
> 1
>
> 2
> --> t
>
>
> Learn more about Common Lisp:
> http://www.lispworks.com/documentation/HyperSpec/Front/
> http://cliki.net/
Well, maybe I should. I have a bunch of students who want to study CL,
I should be able to help them;-). (I read parts of the Cl spec, most of
"Practical Common Lisp" and small parts of "On Lisp", but that was a few
years ago...)
Thanks again,
--
Marcin Borkowski
http://octd.wmi.amu.edu.pl/en/Marcin_Borkowski
Faculty of Mathematics and Computer Science
Adam Mickiewicz University
next prev parent reply other threads:[~2015-07-15 21:02 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <mailman.6968.1436991281.904.help-gnu-emacs@gnu.org>
2015-07-15 20:45 ` How to mapcar or across a list? Pascal J. Bourguignon
2015-07-15 21:02 ` Marcin Borkowski [this message]
2015-07-15 23:56 ` Barry Margolin
2015-07-15 20:14 Marcin Borkowski
2015-07-15 20:42 ` Rasmus
2015-07-15 20:55 ` Marcin Borkowski
2015-07-15 22:21 ` Emanuel Berg
2015-07-15 22:21 ` John Mastro
[not found] ` <mailman.6977.1436998965.904.help-gnu-emacs@gnu.org>
2015-07-15 22:28 ` Pascal J. Bourguignon
2015-07-15 22:36 ` Emanuel Berg
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