2017-09-16 22:20 GMT+02:00 David Kastrup <d...@gnu.org>:
> Thomas Morley <thomasmorle...@gmail.com> 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
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