Re: scheme-question: restricted list inserting

[Thomas Morley]

2017-09-16 22:20 GMT+02:00 David Kastrup <address@hidden>:
> Thomas Morley <address@hidden> writes:
>
>> Hi all,
>>
>> what's the best (less expensive) method to insert elements only at the
>> head of a list and between first and second element of said list.
>> But don't insert an element at list-end if the list is of length 1.
>>
>> I do have:
>>
>> (define (list-insert-first-and-third lst arg1 arg2)
>>   (if (pair? lst)
>>       (append
>>         (list arg1)
>>         (list (car lst))
>>         (if (pair? (cdr lst))
>>             (list arg2)
>>             '())
>>         (cdr lst))
>>       lst))
>>
>> (display (list-insert-first-and-third '(1 2 3 4 5) "a" "b"))
>>
>> --> (a 1 b 2 3 4 5)
>>
>> This looks clumsy, though.
>>
>> Any hint for a better code?
>
> (define (list-insert-first-and-third lst arg1 . rest)
>   (if (pair? lst)
>       (cons* arg1 (car lst)
>              (if (pair? rest)

As far as I can tell the line below misses one argument
>                  (apply list-insert-first-and-third (cdr lst) (cdr rest))
            (apply list-insert-first-and-thirds (cdr lst) (car rest) (cdr rest))
works for me.
>                  (cdr lst)))
>       lst))
>
> Something like that?  It's a bit more generic than you asked for, but so
> what.

No problem with a more generic code. More, with this hint I did:
(define (list-alternate l1 l2)
  (if (and (pair? l1) (pair? l2))
      (cons* (car l2) (car l1)
             (cond ((null? (cdr l1)) '())
                   ((null? (cdr l2)) (cdr l1))
                   (else (list-alternate (cdr l1) (cdr l2)))))
      l1))
Ok, other arguments, but let me quote you "but so what" :)

> At any rate, it would appear that cons* would be a good building-block
> even in your non-recursive approach.

I missed cons*, I should really implement it into my vocabulary.

Thanks a lot,
  Harm