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Re: scheme-question: restricted list inserting
From: |
Thomas Morley |
Subject: |
Re: scheme-question: restricted list inserting |
Date: |
Sun, 17 Sep 2017 00:40:49 +0200 |
2017-09-17 0:29 GMT+02:00 David Kastrup <address@hidden>:
> 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.
Yep, works. I will use it this way.
For my current use-case the even more generic 'list-alternate' would
mean the need to build l2 first, i.e one unneeded step more.
Thanks again,
Harm