recursion
James Stroud
jstroud at mbi.ucla.edu
Thu Sep 13 13:20:45 EDT 2007
Ian Clark wrote:
> Neil Cerutti wrote:
>> On 2007-09-13, Gigs_ <gigs at hi.t-com.hr> wrote:
>>> Can someone explain me this
>>>
>>>>>> def f(l):
>>> if l == []:
>>> return []
>>> else:
>>> return f(l[1:]) + l[:1] # <= cant figure this, how is all
>>> sum at the end?
>>
>> In plain English, the above program says:
>>
>> The sum of the items in list l is zero if the list is empty.
>> Otherwise, the sum is the value of the first item plus the sum of
>> the rest of the items in the list.
>
> Am I missing something? What does this have to do with summing?
>
> >>> def f(l):
> ... if l == []:
> ... return []
> ... else:
> ... return f(l[1:]) + l[:1]
> ...
> >>> f([1, 2, 3, 4])
> [4, 3, 2, 1]
>
> Ian
Add it up!
Round Sum
0 f([1, 2, 3, 4])
1 f([2, 3, 4]) + [1]
2 f([3, 4]) + [2] + [1]
3 f([4]) + [3] + [2] + [1]
4 f([]) + [4] + [3] + [2] + [1]
Total [] + [4] + [3] + [2] + [1] = [4, 3, 2, 1]
James
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