for loop: any way to access the whole list?

[Sanjeev Sharma]

within this for loop is there any way to access different pieces of the description and amt?  car-ing and cdr-ing for example?

Or move the identifier definitions into the let*, and pass those to for in some way?

(let*((ratio 9/12))

  (for((description(list "this" "that"))
       (amt(list 4467.61 2428.37)))
    (printf"~a ~a: changed values\n"

           description

           (cat(* amt ratio) -2.))))

[Sanjeev Sharma]

cat is from srfi/54

[Jens Axel Søgaard]

I am not entirely sure this answer is what you are looking for, but the following generator

will produce the pairs of the list instead of the elements. 

(define (in-pairs xs)
  (make-do-sequence
   (λ ()
     (define (pos->element p) p)
     (define (next-position p) (cdr p))
     (define initial-position xs)
     (define (continue-with-pos? p) (pair? p))
     (values pos->element
             next-position
             initial-position
             continue-with-pos?
             #f #f))))

Example:

> (for ([p (in-pairs '(1 2 3 4))])
    (displayln p))
(1 2 3 4)
(2 3 4)
(3 4)
(4)

So you can use 

(define descriptions (list "this" "that"))
(define amounts      (list 4467.61 2428.37))
(for ([d descriptions] [ds (in-pairs descriptions)])
      [a amounts]      [as (in-pairs amounts])
   ...)

And use ds and as to get to other descriptions/amounts than the current (which are in d and a).

/Jens Axel

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Jens Axel Søgaard

[David Storrs]

On Fri, Jun 14, 2019 at 10:45 AM Sanjeev Sharma <thro...@gmail.com> wrote:

within this for loop is there any way to access different pieces of the description and amt?  car-ing and cdr-ing for example?

I'm not entirely clear on what you're looking for, but maybe this helps?

(define lst '(a b c))

(for ([(val idx) (in-indexed lst)])

  (displayln (cons  val idx))

  (when (< idx (sub1 (length lst)))

    (displayln (format "\tnext val: ~a" (list-ref lst (add1 idx))))))

Output:

(a . 0)
        next val: b
(b . 1)
        next val: c
(c . 2)

Or move the identifier definitions into the let*, and pass those to for in some way?

(let*((ratio 9/12))

  (for((description(list "this" "that"))
       (amt(list 4467.61 2428.37)))
    (printf"~a ~a: changed values\n"

           description

           (cat(* amt ratio) -2.))))

--

[Sanjeev Sharma]

thanks, 

the whole idea is to localize the identifiers to the for (preferred) or the let*. 

Reaching back to get the whole list when the for has bound the same identifier is probably asking a bit much.

Within the given strictures I probably need to bind the list in the let* and use a different identifier in the for().

On Friday, June 14, 2019 at 12:46:54 PM UTC-4, David Storrs wrote:

On Fri, Jun 14, 2019 at 10:45 AM Sanjeev Sharma <thro...@gmail.com> wrote:

within this for loop is there any way to access different pieces of the description and amt?  car-ing and cdr-ing for example?

I'm not entirely clear on what you're looking for, but maybe this helps?

(define lst '(a b c))

(for ([(val idx) (in-indexed lst)])

  (displayln (cons  val idx))

  (when (< idx (sub1 (length lst)))

    (displayln (format "\tnext val: ~a" (list-ref lst (add1 idx))))))

Output:

(a . 0)
        next val: b
(b . 1)
        next val: c
(c . 2)

Or move the identifier definitions into the let*, and pass those to for in some way?

(let*((ratio 9/12))

  (for((description(list "this" "that"))
       (amt(list 4467.61 2428.37)))
    (printf"~a ~a: changed values\n"

           description

           (cat(* amt ratio) -2.))))

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