On Wed, Nov 16, 2016 at 11:10 AM, David Storrs <[email protected]> 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, <[email protected]> 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 <[email protected]> >> [16-11-16 18:26]: >> > >> > > On Nov 15, 2016, at 19:52, [email protected] 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 [email protected]. >> > > 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 [email protected]. >> > 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 [email protected]. >> 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 [email protected]. For more options, visit https://groups.google.com/d/optout.
