Re: scheme-question: restricted list inserting

[David Kastrup]

Thomas Morley <address@hidden> writes:

> 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.

Argl.  I messed up one of the more elegant pieces: it's actually just

    (apply list-insert-first-and-thirds (cdr lst) rest)

Less is more.  Sorry for that.

>>                  (cdr lst)))
>>       lst))
>>
>> Something like that?  It's a bit more generic than you asked for, but so
>> what.


-- 
David Kastrup