Python and Lisp : car and cdr

[Ethan Furman]

Lie Ryan wrote:
> On 06/18/11 00:45, Franck Ditter wrote:
>> Hi, I'm just wondering about the complexity of some Python operations 
>> to mimic Lisp car and cdr in Python...
>>
>> def length(L) :
>>   if not L : return 0
>>   return 1 + length(L[1:])
>>
>> Should I think of the slice L[1:] as (cdr L) ? I mean, is the slice
>> a copy of a segment of L, or do I actually get a pointer to something
>> inside L ? Is the above function length O(n) or probably O(n^2) ? 
>> Where are such implementation things (well) said ?
>>
>> Thanks,
>>
>>      franck
> 
> Your function does not mimic Lisp's car/cdr. This one does:
> 
> 
> def car(L):
>     return L[0]
> def cdr(L):
>     return L[1]

IANAL (I am not a Lisper), but shouldn't that be 'return L[1:]' ?


> def length(L):
>     if not L: return 0
>     return 1 + length(cdr(L))

How is this different from regular ol' 'len' ?

~Ethan~