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comp.lang.lisp

Re: Another code review perhaps?

William James

8/3/2015 3:45:00 PM

Arthur Lemmens wrote:

> > Define iterative and recursive versions of a function that takes an
> > object x and a vector v, and returns a list of all the objects that
> > immediately precede x in v.
>
> Graham doesn't like LOOP, but I do. So here's a LOOP-version:
>
> (defun precedes (object vector)
> (loop for x across vector
> and i from 0
> when (and (equal x object) (> i 0))
> collect (elt vector (1-i))))

He obviously didn't test it even once.

Gauche Scheme:

(use srfi-42 :only (list-ec))
(define (precedes x vec)
(list-ec (:vector y (index i) vec)
(and (equal? x y) (> i 0))
(~ vec (- i 1))))

(precedes 4 #(4 2 3 4 5 4))
===>
(3 5)


>
> > Define iterative and recursive versions of a function that takes an
> > object and a list, and returns a new list in which the object appears
> > between each pair of elements in the original list:
>
> This one is also pretty easy with LOOP:
>
> (defun intersperse (x list)
> (loop for (a . rest) on list
> collect a
> when rest
> collect x))

gosh> (intersperse '+ '(1 2 3))
(1 + 2 + 3)

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2 Answers

William James

11/29/2015 12:51:00 AM

0

WJ wrote:

> Arthur Lemmens wrote:
>
> > > Define iterative and recursive versions of a function that takes an
> > > object x and a vector v, and returns a list of all the objects that
> > > immediately precede x in v.
> >
> > Graham doesn't like LOOP, but I do. So here's a LOOP-version:
> >
> > (defun precedes (object vector)
> > (loop for x across vector
> > and i from 0
> > when (and (equal x object) (> i 0))
> > collect (elt vector (1-i))))


Look at that:

(1-i)

Don't you think that that should be:

(1- i)

or

(- i 1)

?

Consider the incredible arrogance and stupidity of one who
would post code like that without testing it.

Ocaml:

let array_filter_segments size func array =
let result = ref [] in
for i = (Array.length array) - size downto 0 do
match func (Array.sub array i size) with
Some x -> result := x::!result
| None -> ()
done;
!result ;;

let precedes item vector =
array_filter_segments 2
(fun [|a;b|] -> if item=b then Some a else None)
vector ;;

precedes 4 [|4; 2; 3; 4; 5; 4|];;

[3; 5]

William James

3/31/2016 3:12:00 PM

0

WJ wrote:

> WJ wrote:
>
> > Arthur Lemmens wrote:
> >
> > > > Define iterative and recursive versions of a function that takes an
> > > > object x and a vector v, and returns a list of all the objects that
> > > > immediately precede x in v.
> > >
> > > Graham doesn't like LOOP, but I do. So here's a LOOP-version:
> > >
> > > (defun precedes (object vector)
> > > (loop for x across vector
> > > and i from 0
> > > when (and (equal x object) (> i 0))
> > > collect (elt vector (1-i))))
>
>
> Look at that:
>
> (1-i)
>
> Don't you think that that should be:
>
> (1- i)
>
> or
>
> (- i 1)
>
> ?
>
> Consider the incredible arrogance and stupidity of one who
> would post code like that without testing it.
>
> Ocaml:
>
> let array_filter_segments size func array =
> let result = ref [] in
> for i = (Array.length array) - size downto 0 do
> match func (Array.sub array i size) with
> Some x -> result := x::!result
> | None -> ()
> done;
> !result ;;
>
> let precedes item vector =
> array_filter_segments 2
> (fun [|a;b|] -> if item=b then Some a else None)
> vector ;;
>
> precedes 4 [|4; 2; 3; 4; 5; 4|];;
>
> [3; 5]

Another way:

let precedes item vector =
let result = ref [] in
Array.iteri
(fun i x ->
if i > 0 && x = item then result := vector.(i-1) :: !result)
vector;
List.rev !result ;;

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highest court, which is comprised entirely of Jews and Catholics. The three
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www.jta.org/2016/03/16/news-opinion/united-states/obama-to-name-jewish-federal-judge-to-supreme-court