Re: [racket-users] Recursevly processing a list with "wholes"

[Meino . Cramer]

Hi Justin,

thanks for reply and the code! :)

Cheers
Meino


Justin Zamora  [16-11-17 04:02]:
> I think you are looking for something like this:
> 
> #lang racket
> 
> (require test-engine/racket-tests)
> 
> (define (remove-empty-lists lst)
>   (cond
> [(null? lst) '()] ; Check for end of list
> [(null? (first lst)) (remove-empty-lists (rest lst))] ; Skip null
> sublist
> [(not (list? (first lst))) ; Leave non-lists alone
>  (cons (first lst) (remove-empty-lists (rest lst)))]
> [else ; Process the sublists recursively
>  (cons (remove-empty-lists (first lst))
>(remove-empty-lists (rest lst)))]))
> 
> (check-expect (remove-empty-lists '()) '())
> (check-expect (remove-empty-lists '(a)) '(a))
> (check-expect (remove-empty-lists '(a b c)) '(a b c))
> (check-expect (remove-empty-lists '(())) '())
> (check-expect (remove-empty-lists '(() a)) '(a))
> (check-expect (remove-empty-lists '(a ())) '(a))
> (check-expect (remove-empty-lists '(a (b () c) d)) '(a (b c) d))
> (check-expect (remove-empty-lists '((a) (b c) () (e () f (g ())) h (() i
> j)))
>   '((a) (b c) (e f (g)) h (i j)))
> (test)
> 
> Justin
> 
> On Tue, Nov 15, 2016 at 10:52 PM,  wrote:
> 
> > Hi,
> >
> > I have a list of sublists. Some of the sublists are empty.
> > The list should be processed recursevly and the resulting
> > list should no longer contain empty sublists.
> >
> > The code I have so far looks like:
> >
> > (define (step-through-list lst)
> > (if (empty? lst)
> > lst
> > (cons (process-sublist (car lst)) (step-through-list (cdr lst)))
> >
> > but "car" would feed empty sublists to process-sublist as any other
> > sublist. How can I "silently" skip empty sublists and even get out of
> > the recursion without harm (if the last sublist is empty) without
> > switching to iteration?
> > Is there something like "car-not-empty" ? ;)
> > Or do I oversee the obvious here...? ;)))
> >
> > Thank you vary much for any help in advance!
> > Cheers
> > Meino
> >
> >
> > --
> > You received this message because you are subscribed to the Google Groups
> > "Racket Users" group.
> > To unsubscribe from this group and stop receiving emails from it, send an
> > email to racket-users+unsubscr...@googlegroups.com.
> > For more options, visit https://groups.google.com/d/optout.
> >
> 
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Re: [racket-users] Recursevly processing a list with "wholes"

[Justin Zamora]

I think you are looking for something like this:

#lang racket

(require test-engine/racket-tests)

(define (remove-empty-lists lst)
  (cond
[(null? lst) '()] ; Check for end of list
[(null? (first lst)) (remove-empty-lists (rest lst))] ; Skip null
sublist
[(not (list? (first lst))) ; Leave non-lists alone
 (cons (first lst) (remove-empty-lists (rest lst)))]
[else ; Process the sublists recursively
 (cons (remove-empty-lists (first lst))
   (remove-empty-lists (rest lst)))]))

(check-expect (remove-empty-lists '()) '())
(check-expect (remove-empty-lists '(a)) '(a))
(check-expect (remove-empty-lists '(a b c)) '(a b c))
(check-expect (remove-empty-lists '(())) '())
(check-expect (remove-empty-lists '(() a)) '(a))
(check-expect (remove-empty-lists '(a ())) '(a))
(check-expect (remove-empty-lists '(a (b () c) d)) '(a (b c) d))
(check-expect (remove-empty-lists '((a) (b c) () (e () f (g ())) h (() i
j)))
  '((a) (b c) (e f (g)) h (i j)))
(test)

Justin

On Tue, Nov 15, 2016 at 10:52 PM,  wrote:

> Hi,
>
> I have a list of sublists. Some of the sublists are empty.
> The list should be processed recursevly and the resulting
> list should no longer contain empty sublists.
>
> The code I have so far looks like:
>
> (define (step-through-list lst)
> (if (empty? lst)
> lst
> (cons (process-sublist (car lst)) (step-through-list (cdr lst)))
>
> but "car" would feed empty sublists to process-sublist as any other
> sublist. How can I "silently" skip empty sublists and even get out of
> the recursion without harm (if the last sublist is empty) without
> switching to iteration?
> Is there something like "car-not-empty" ? ;)
> Or do I oversee the obvious here...? ;)))
>
> Thank you vary much for any help in advance!
> Cheers
> Meino
>
>
> --
> You received this message because you are subscribed to the Google Groups
> "Racket Users" group.
> To unsubscribe from this group and stop receiving emails from it, send an
> email to racket-users+unsubscr...@googlegroups.com.
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Re: [racket-users] Recursevly processing a list with "wholes"

[Meino . Cramer]

Hi David,

thanks for the reply and the link! :)

I have a list of sublist, which I want to process recursively.
Some sublists are empty, some are filled.
The typical pattern is (as far as I know)

(define (process-list lst)
(cons (do-somthing-with-sublist (car lst)) (process-lst (car lst

But this has the drawback, that empty sublists (via 'car lst') are
processed also. So I need a "kind of car" which returns not the next
sublist but the next not-empty sublist and skipping any empty sublist
without breaking the recursion.

Others on the list already posted some solutions for what I try to 
acchieve and this is only to clearify things.
Is it now understandable, what I try to acchieve?

Cheers
Meino





David Storrs  [16-11-16 20:16]:
> Hi Meino,
> 
> As a suggestions, you might want to read through this page on Stack
> Overflow about how to ask technical questions:
> http://stackoverflow.com/help/how-to-ask   The summary is:  tell us what
> you're trying to achieve, what you've tried, and what *specifically* is not
> working.
> 
> The question you're asking here (about walk-lists) is hard to help with,
> because it's not clear what the context is.  I'll take my best shot, though:
> 
> On Wed, Nov 16, 2016 at 10:04 AM,  wrote:
> 
> > Hi John,
> >
> > thank you for your reply ! :)
> >
> > ...no, not a homework...
> > I want to teach myself some racket and hope 'to get it' finally...
> > Since I am no native speaker I have the deficulty to search for
> > things, which names I dont know...
> >
> > One example of my code is:
> >
> > (define (trim-list lst)
> >   (if (empty? lst)
> > lst
> > (cons (string-trim (car lst)) (trim-list (cdr lst)
> >
> >
> > (define (walk-sublists lst)
> >   (if (empty? lst)
> > lst
> > (cons (trim-list (car lst)) (walk-sublists (cdr lst)
> >
> > I want to avoid to call trim-list, if 'car lst' returns an empty list.
> > Instead 'car lst' sghould return the next non-empty list...
> >
> > I will take a look in the reference manual for 'filter' and 'compose'.
> >
> > Cheers
> > Meino
> >
> >
> >
> You might try something like this:
> 
> (define (trim-list lst)
>   (if (empty? lst)
> lst
> (cons (string-trim (car lst))
> (trim-list (cdr lst)
> >
> >
> >
> >
> > 'John Clements' via Racket Users 
> > [16-11-16 18:26]:
> > >
> > > > On Nov 15, 2016, at 19:52, meino.cra...@gmx.de wrote:
> > > >
> > > > Hi,
> > > >
> > > > I have a list of sublists. Some of the sublists are empty.
> > > > The list should be processed recursevly and the resulting
> > > > list should no longer contain empty sublists.
> > >
> > > Is this homework?
> > >
> > > if so: can you show your test cases?
> > >
> > > if not: (filter (compose not null?) l).
> > >
> > >
> > > John Clements
> > >
> > > >
> > > > The code I have so far looks like:
> > > >
> > > > (define (step-through-list lst)
> > > >(if (empty? lst)
> > > >lst
> > > >(cons (process-sublist (car lst)) (step-through-list (cdr lst)))
> > > >
> > > > but "car" would feed empty sublists to process-sublist as any other
> > > > sublist. How can I "silently" skip empty sublists and even get out of
> > > > the recursion without harm (if the last sublist is empty) without
> > > > switching to iteration?
> > > > Is there something like "car-not-empty" ? ;)
> > > > Or do I oversee the obvious here...? ;)))
> > > >
> > > > Thank you vary much for any help in advance!
> > > > Cheers
> > > > Meino
> > > >
> > > >
> > > > --
> > > > You received this message because you are subscribed to the Google
> > Groups "Racket Users" group.
> > > > To unsubscribe from this group and stop receiving emails from it, send
> > an email to racket-users+unsubscr...@googlegroups.com.
> > > > For more options, visit https://groups.google.com/d/optout.
> > >
> > >
> > >
> > > --
> > > You received this message because you are subscribed to the Google
> > Groups "Racket Users" group.
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> > >
> >
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Re: [racket-users] Recursevly processing a list with "wholes"

[David Storrs]

On Wed, Nov 16, 2016 at 11:10 AM, David Storrs 
wrote:

> Hi Meino,
>
> As a suggestions, you might want to read through this page on Stack
> Overflow about how to ask technical questions:
> http://stackoverflow.com/help/how-to-ask   The summary is:  tell us what
> you're trying to achieve, what you've tried, and what *specifically* is not
> working.
>
> The question you're asking here (about walk-lists) is hard to help with,
> because it's not clear what the context is.  I'll take my best shot, though:
>
> On Wed, Nov 16, 2016 at 10:04 AM,  wrote:
>
>> Hi John,
>>
>> thank you for your reply ! :)
>>
>> ...no, not a homework...
>> I want to teach myself some racket and hope 'to get it' finally...
>> Since I am no native speaker I have the deficulty to search for
>> things, which names I dont know...
>>
>> One example of my code is:
>>
>> (define (trim-list lst)
>>   (if (empty? lst)
>> lst
>> (cons (string-trim (car lst)) (trim-list (cdr lst)
>>
>>
>> (define (walk-sublists lst)
>>   (if (empty? lst)
>> lst
>> (cons (trim-list (car lst)) (walk-sublists (cdr lst)
>>
>> I want to avoid to call trim-list, if 'car lst' returns an empty list.
>> Instead 'car lst' sghould return the next non-empty list...
>>
>> I will take a look in the reference manual for 'filter' and 'compose'.
>>
>> Cheers
>> Meino
>>
>>
>>
> You might try something like this:
>
> (define (trim-list lst)
>   (if (empty? lst)
> lst
> (cons (string-trim (car lst))
> (trim-list (cdr lst)
>


Dangit, GMail, hitting 'tab, space' does not necessarily mean I want to
send, it might just mean I want to indent some code.  Sorry for that, all.


(define (trim-list lst) ...)

;;  process a list, but skip any element of the list that is null
(define (walk-sublists lst)
   (cond[(null? lst) null]
[(null? (car lst)) (walk-lists (cdr lst))]
[else (cons (trim-list (car lst))
  (walk-sublists (cdr lst]))

Code not tested, but let me know if it makes sense.


Dave


>>
>>
>> 'John Clements' via Racket Users 
>> [16-11-16 18:26]:
>> >
>> > > On Nov 15, 2016, at 19:52, meino.cra...@gmx.de wrote:
>> > >
>> > > Hi,
>> > >
>> > > I have a list of sublists. Some of the sublists are empty.
>> > > The list should be processed recursevly and the resulting
>> > > list should no longer contain empty sublists.
>> >
>> > Is this homework?
>> >
>> > if so: can you show your test cases?
>> >
>> > if not: (filter (compose not null?) l).
>> >
>> >
>> > John Clements
>> >
>> > >
>> > > The code I have so far looks like:
>> > >
>> > > (define (step-through-list lst)
>> > >(if (empty? lst)
>> > >lst
>> > >(cons (process-sublist (car lst)) (step-through-list (cdr lst)))
>> > >
>> > > but "car" would feed empty sublists to process-sublist as any
>> other
>> > > sublist. How can I "silently" skip empty sublists and even get out of
>> > > the recursion without harm (if the last sublist is empty) without
>> > > switching to iteration?
>> > > Is there something like "car-not-empty" ? ;)
>> > > Or do I oversee the obvious here...? ;)))
>> > >
>> > > Thank you vary much for any help in advance!
>> > > Cheers
>> > > Meino
>> > >
>> > >
>> > > --
>> > > You received this message because you are subscribed to the Google
>> Groups "Racket Users" group.
>> > > To unsubscribe from this group and stop receiving emails from it,
>> send an email to racket-users+unsubscr...@googlegroups.com.
>> > > For more options, visit https://groups.google.com/d/optout.
>> >
>> >
>> >
>> > --
>> > You received this message because you are subscribed to the Google
>> Groups "Racket Users" group.
>> > To unsubscribe from this group and stop receiving emails from it, send
>> an email to racket-users+unsubscr...@googlegroups.com.
>> > For more options, visit https://groups.google.com/d/optout.
>> >
>>
>> --
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>> email to racket-users+unsubscr...@googlegroups.com.
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>>
>
>

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Re: [racket-users] Recursevly processing a list with "wholes"

[David Storrs]

Hi Meino,

As a suggestions, you might want to read through this page on Stack
Overflow about how to ask technical questions:
http://stackoverflow.com/help/how-to-ask   The summary is:  tell us what
you're trying to achieve, what you've tried, and what *specifically* is not
working.

The question you're asking here (about walk-lists) is hard to help with,
because it's not clear what the context is.  I'll take my best shot, though:

On Wed, Nov 16, 2016 at 10:04 AM,  wrote:

> Hi John,
>
> thank you for your reply ! :)
>
> ...no, not a homework...
> I want to teach myself some racket and hope 'to get it' finally...
> Since I am no native speaker I have the deficulty to search for
> things, which names I dont know...
>
> One example of my code is:
>
> (define (trim-list lst)
>   (if (empty? lst)
> lst
> (cons (string-trim (car lst)) (trim-list (cdr lst)
>
>
> (define (walk-sublists lst)
>   (if (empty? lst)
> lst
> (cons (trim-list (car lst)) (walk-sublists (cdr lst)
>
> I want to avoid to call trim-list, if 'car lst' returns an empty list.
> Instead 'car lst' sghould return the next non-empty list...
>
> I will take a look in the reference manual for 'filter' and 'compose'.
>
> Cheers
> Meino
>
>
>
You might try something like this:

(define (trim-list lst)
  (if (empty? lst)
lst
(cons (string-trim (car lst))
(trim-list (cdr lst)
>
>
>
>
> 'John Clements' via Racket Users 
> [16-11-16 18:26]:
> >
> > > On Nov 15, 2016, at 19:52, meino.cra...@gmx.de wrote:
> > >
> > > Hi,
> > >
> > > I have a list of sublists. Some of the sublists are empty.
> > > The list should be processed recursevly and the resulting
> > > list should no longer contain empty sublists.
> >
> > Is this homework?
> >
> > if so: can you show your test cases?
> >
> > if not: (filter (compose not null?) l).
> >
> >
> > John Clements
> >
> > >
> > > The code I have so far looks like:
> > >
> > > (define (step-through-list lst)
> > >(if (empty? lst)
> > >lst
> > >(cons (process-sublist (car lst)) (step-through-list (cdr lst)))
> > >
> > > but "car" would feed empty sublists to process-sublist as any other
> > > sublist. How can I "silently" skip empty sublists and even get out of
> > > the recursion without harm (if the last sublist is empty) without
> > > switching to iteration?
> > > Is there something like "car-not-empty" ? ;)
> > > Or do I oversee the obvious here...? ;)))
> > >
> > > Thank you vary much for any help in advance!
> > > Cheers
> > > Meino
> > >
> > >
> > > --
> > > You received this message because you are subscribed to the Google
> Groups "Racket Users" group.
> > > To unsubscribe from this group and stop receiving emails from it, send
> an email to racket-users+unsubscr...@googlegroups.com.
> > > For more options, visit https://groups.google.com/d/optout.
> >
> >
> >
> > --
> > You received this message because you are subscribed to the Google
> Groups "Racket Users" group.
> > To unsubscribe from this group and stop receiving emails from it, send
> an email to racket-users+unsubscr...@googlegroups.com.
> > For more options, visit https://groups.google.com/d/optout.
> >
>
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Re: [racket-users] Recursevly processing a list with "wholes"

[Matthias Felleisen]

> On Nov 16, 2016, at 1:04 PM, meino.cra...@gmx.de wrote:
> 
> ...no, not a homework...
> I want to teach myself some racket and hope 'to get it' finally...
> Since I am no native speaker I have the deficulty to search for
> things, which names I dont know...


Meino, have you considered reading 'Die Macht der Abstraktion?’ (This is a 
guess based on your high-level domain.) — Matthias

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Re: [racket-users] Recursevly processing a list with "wholes"

[Meino . Cramer]

Hi John,

thank you for your reply ! :)

...no, not a homework...
I want to teach myself some racket and hope 'to get it' finally...
Since I am no native speaker I have the deficulty to search for
things, which names I dont know...

One example of my code is:

(define (trim-list lst)
  (if (empty? lst)
lst
(cons (string-trim (car lst)) (trim-list (cdr lst)


(define (walk-sublists lst)
  (if (empty? lst)
lst
(cons (trim-list (car lst)) (walk-sublists (cdr lst)

I want to avoid to call trim-list, if 'car lst' returns an empty list.
Instead 'car lst' sghould return the next non-empty list...

I will take a look in the reference manual for 'filter' and 'compose'.

Cheers
Meino





'John Clements' via Racket Users  [16-11-16 
18:26]:
> 
> > On Nov 15, 2016, at 19:52, meino.cra...@gmx.de wrote:
> > 
> > Hi,
> > 
> > I have a list of sublists. Some of the sublists are empty.
> > The list should be processed recursevly and the resulting
> > list should no longer contain empty sublists.
> 
> Is this homework? 
> 
> if so: can you show your test cases?
> 
> if not: (filter (compose not null?) l).
> 
> 
> John Clements
> 
> > 
> > The code I have so far looks like:
> > 
> > (define (step-through-list lst)
> >(if (empty? lst)
> >lst
> >(cons (process-sublist (car lst)) (step-through-list (cdr lst)))
> > 
> > but "car" would feed empty sublists to process-sublist as any other
> > sublist. How can I "silently" skip empty sublists and even get out of
> > the recursion without harm (if the last sublist is empty) without
> > switching to iteration?
> > Is there something like "car-not-empty" ? ;)
> > Or do I oversee the obvious here...? ;)))
> > 
> > Thank you vary much for any help in advance!
> > Cheers
> > Meino
> > 
> > 
> > -- 
> > You received this message because you are subscribed to the Google Groups 
> > "Racket Users" group.
> > To unsubscribe from this group and stop receiving emails from it, send an 
> > email to racket-users+unsubscr...@googlegroups.com.
> > For more options, visit https://groups.google.com/d/optout.
> 
> 
> 
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Re: [racket-users] Recursevly processing a list with "wholes"

['John Clements' via Racket Users]

> On Nov 15, 2016, at 19:52, meino.cra...@gmx.de wrote:
> 
> Hi,
> 
> I have a list of sublists. Some of the sublists are empty.
> The list should be processed recursevly and the resulting
> list should no longer contain empty sublists.

Is this homework? 

if so: can you show your test cases?

if not: (filter (compose not null?) l).


John Clements

> 
> The code I have so far looks like:
> 
> (define (step-through-list lst)
>(if (empty? lst)
>lst
>(cons (process-sublist (car lst)) (step-through-list (cdr lst)))
> 
> but "car" would feed empty sublists to process-sublist as any other
> sublist. How can I "silently" skip empty sublists and even get out of
> the recursion without harm (if the last sublist is empty) without
> switching to iteration?
> Is there something like "car-not-empty" ? ;)
> Or do I oversee the obvious here...? ;)))
> 
> Thank you vary much for any help in advance!
> Cheers
> Meino
> 
> 
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[racket-users] Recursevly processing a list with "wholes"

[Meino . Cramer]

Hi,

I have a list of sublists. Some of the sublists are empty.
The list should be processed recursevly and the resulting
list should no longer contain empty sublists.

The code I have so far looks like:

(define (step-through-list lst)
(if (empty? lst)
lst
(cons (process-sublist (car lst)) (step-through-list (cdr lst)))

but "car" would feed empty sublists to process-sublist as any other
sublist. How can I "silently" skip empty sublists and even get out of
the recursion without harm (if the last sublist is empty) without
switching to iteration?
Is there something like "car-not-empty" ? ;)
Or do I oversee the obvious here...? ;)))

Thank you vary much for any help in advance!
Cheers
Meino


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