Re: [racket-users] for loop: any way to access the whole list?

[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  > 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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>> 
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>

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Re: [racket-users] for loop: any way to access the whole list?

[David Storrs]

On Fri, Jun 14, 2019 at 10:45 AM Sanjeev Sharma  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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> 
> .
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>

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Re: [racket-users] for loop: any way to access the whole list?

[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



Den fre. 14. jun. 2019 kl. 16.45 skrev 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.
>
> --
> You received this message because you are subscribed to the Google Groups
> "Racket Users" group.
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> 
> .
> For more options, visit https://groups.google.com/d/optout.
>


-- 
-- 
Jens Axel Søgaard

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[racket-users] 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.

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