From Newsgroup: comp.lang.lisp

Pascal Costanza wrote:

On 19/05/2014 22:33, Kay Hamacher wrote:

Hi there,

I want to map/reduce a vector:

(reduce #'+
(map 'vector
#'(lambda(x) BODY )
)
)

However, BODY has alternating signs for the
terms to be added up (first term is positive,
second is negative, third is positive,....).
One could definitely do this with some global/state
variable in the lambda.

But I guess there's a much more elegant
alternative in LISP. Any hints?

(defun example (vector)
(loop for index below (length vector)
sum (* (svref vector index)
(- 1 (* (rem index 2) 2)))))

Gauche Scheme

"!" is similar to "do".

(define (example vec)
(! (sum + (* x sign)
sign 1 (- sign)
x :across vec)
(not x)))

(example #(400 2 500 3))

===>
895

Given:

(define-syntax !-aux
(syntax-rules (<> @ + - cons cdr :in :across :if ! )
[(_ (:if bool z ...) ((v i u) seen ...) stuff ...)
(!-aux (z ...)
((v i (if bool u v)) seen ...) stuff ...) ]
[(_ (x :in lst z ...) seen (lets ...) stuff ...)
(!-aux (x (and (pair? xs)(pop! xs)) <> z ...)
seen
(lets ... (xs lst)) stuff ...) ]
[(_ (x :across vec z ...) seen (lets ...) stuff ...)
(!-aux (x (and (< i (vector-length v))
(begin0 (vector-ref v i) (inc! i))) <>
z ...)
seen (lets ... (v vec) (i 0)) stuff ...) ]
[(_ (a b <> z ...) (seen ...) stuff ...)
(!-aux (z ...) (seen ... (a b b)) stuff ...) ]
[(_ (a b + z ...) (seen ...) stuff ...)
(!-aux (z ...) (seen ... (a b (+ 1 a))) stuff ...) ]
[(_ (a + n z ...) (seen ...) stuff ...)
(!-aux (z ...) (seen ... (a 0 (+ n a))) stuff ...) ]
[(_ (a b - z ...) (seen ...) stuff ...)
(!-aux (z ...) (seen ... (a b (- a 1))) stuff ...) ]
[(_ (a cons b z ...) (seen ...) stuff ...)
(!-aux (z ...) (seen ... (a '() (cons b a))) stuff ...) ]
[(_ (a b cdr z ...) (seen ...) stuff ...)
(!-aux (z ...) (seen ... (a b (cdr a))) stuff ...) ]
[(_ (a b c z ...) (seen ...) stuff ...)
(!-aux (z ...) (seen ... (a b c)) stuff ...) ]
[(_ (a b) (seen ...) stuff ...)
(!-aux () (seen ... (a b)) stuff ...) ]
[(_ (a) (seen ...) stuff ...)
(!-aux () (seen ... (a '())) stuff ...) ]
;;
[(_ () seen lets a b c ! action ...)
(!-aux () seen lets (a b c) #t (action ...)) ]
[(_ () seen lets a b ! action ...)
(!-aux () seen lets (a b) #t (action ...)) ]
[(_ () seen lets a ! action ...)
(!-aux () seen lets a #t (action ...)) ]
;;
[(_ () ((a b c) z ...) lets bool)
(!-aux () ((a b c) z ...) lets bool a) ]
[(_ () ((a b c) z ...) lets bool @)
(!-aux () ((a b c) z ...) lets bool (reverse a)) ]
[(_ () seen lets bool @ result stuff ...)
(!-aux () seen lets bool (reverse result) stuff ...) ]
[(_ () seen lets bool (what @ x z ...) stuff ...)
(!-aux () seen lets bool (what (reverse x) z ...) stuff ...) ]
[(_ () seen lets bool (what x @ y z ...) stuff ...)
(!-aux () seen lets bool (what x (reverse y) z ...) stuff ...) ]
[(_ () ((a b c) z ...) lets 0 stuff ...)
(!-aux () ((a b c) z ...) lets (= 0 a) stuff ...) ]
[(_ () seen lets bool result stuff ...)
(let lets (do seen (bool result) stuff ...)) ]
))
(define-syntax !
(syntax-rules ()
[(_ specs bool stuff ...)
(!-aux specs () () bool stuff ...) ]
))
--
[T]he problem is that lispniks are as cultish as any other devout group and basically fall down frothing at the mouth if they see [heterodoxy].
--- Kenny Tilton
The good news is, it's not Lisp that sucks, but Common Lisp. --- Paul Graham --- Synchronet 3.21a-Linux NewsLink 1.2

From Newsgroup: comp.lang.lisp

"B. Pym" <Nobody447095@here-nor-there.org> writes:

Pascal Costanza wrote:

On 19/05/2014 22:33, Kay Hamacher wrote:

Hi there,

I want to map/reduce a vector:

(reduce #'+
(map 'vector
#'(lambda(x) BODY )
)
)

However, BODY has alternating signs for the
terms to be added up (first term is positive,
second is negative, third is positive,....).
One could definitely do this with some global/state
variable in the lambda.

But I guess there's a much more elegant
alternative in LISP. Any hints?

(defun example (vector)
(loop for index below (length vector)
sum (* (svref vector index)
(- 1 (* (rem index 2) 2)))))

Gauche Scheme

"!" is similar to "do".

(define (example vec)
(! (sum + (* x sign)
sign 1 (- sign)
x :across vec)
(not x)))

(example #(400 2 500 3))

Common LisprCOs rCylooprCO macro also supports iterating over
vectors, much as the rCydolistrCO macro does for lists.

"The for-as-across subclause": http://www.ai.mit.edu/projects/iiip/doc/CommonLISP/HyperSpec/Body/sec_6-1-2-1-5.html

This eliminates the need for accessing sequence items
through indexing and eliminates the possibility of an
off-by-one error.

The for-as-equals subclause allows a variable to be set to
an initial value and, optionally, then be set to a different
value on subsequent iterations.

"The for-as-equals-then subclause": http://www.ai.mit.edu/projects/iiip/doc/CommonLISP/HyperSpec/Body/sec_6-1-2-1-4.html

Using these constructs, the above solution can be written
either as:

(defun alternating-signs (vector)
(loop
for item across vector and sign = 1 then (- sign)
sum (* item sign)))

or,

(defun alternating-signs (vector)
(loop
for item across vector
for sign = 1 then (- sign)
sum (* item sign)))

(alternating-signs #(4 -9 11 -19))

43

--
The lyf so short, the craft so long to lerne.
- Geoffrey Chaucer, The Parliament of Birds.
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