Re: Hints on recursion

matteo d'addio 81 wrote:
>Pascal Bourguignon ha scritto:
>>
>> For (twice 'e '(e . rest)) = (once 'e . rest)
>> and (once 'e '(e . rest)) = (none 'e . rest)
>
> Hi, I'm a CL beginner too.
>
>> (once 'e . rest)
> ^
> What this point stand for?

Prolog [Head | Tail], [A, B | Tail]
Lisp (car . cdr), (first second . rest)

A list, e.g. '(1 2 3), is essentially shorthand for the dotted
equivalent,
in this case: '(1 . (2 . (3 . nil)))

So if we substitute a list, for example '(g h i), for rest into
Pascal's

(twice 'e '(e . rest)) = (once 'e . rest)

we get

(twice 'e '(e g h i)) = (once 'e '(g h i))

See http://www.lispworks.com/documentation/HyperSpec/Body/02_da.htm

--
Geoff

.

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