From: dblack@...
Date: 2005-12-13T01:56:29+09:00
Subject: Re: A question about recursive programming

Hi --

On Tue, 13 Dec 2005, sanha lee wrote:

> def all_sum(arr)
>>  return arr if arr.length == 1
>>  temp = arr[-1]
> # sorry about this, it should be return all_sum(arr[0...-1]) +
> all_sum(arr[0...-1]).collect .....
> # not all_sum(arr[1...-1]).collect.
>>  return all_sum(arr[0...-1]) + all_sum(arr[1...-1]).collect {|i| i + temp}
>> end
>
> except that mistake, I can not see any difference between your code and
> mine,
> except that your code doesn't deal with the case where arr.length == 1.
> my result of all_sum([1, 2, 3, 4]) is this [1, 2, 3, 4, 6, 5, 7, 8, 10]
> sorry if i am wrong, no offence.

I'm probably misunderstanding what the original poster wanted.  I
dropped the length == 1 case since in [1,2,3,4], 1 is not the sum of
any two elements.  I was basically doing:

   1 + 2
   1 + 3
   1 + 4
   2 + 3
   2 + 4
   3 + 4


David


>
>
>
>
> On 12/12/05, dblack@wobblini.net <dblack@wobblini.net> wrote:
>>
>> Hi --
>>
>> On Tue, 13 Dec 2005, sanha lee wrote:
>>
>>> I think that it is better solution
>>>
>>> def all_sum(arr)
>>>  return arr if arr.length == 1
>>>  temp = arr[-1]
>>>  return all_sum(arr[0...-1]) + all_sum(arr[1...-1]).collect {|i| i +
>> temp}
>>> end
>>
>> I get a too-deep recursion with that too.  Here's another candidate:
>>
>>   def all_sum(arr)
>>     case arr.size
>>     when 1 then []
>>     else arr[0..-2].map {|i| i + arr[-1]} + all_sum(arr[0..-2])
>>     end
>>   end
>>
>>   p all_sum([1,2,3,4])
>>   => [5, 6, 7, 4, 5, 3]
>>
>>
>> David
>>
>> --
>> David A. Black
>> dblack@wobblini.net
>>
>> "Ruby for Rails", forthcoming from Manning Publications, April 2006!
>>
>>
>

-- 
David A. Black
dblack@wobblini.net

"Ruby for Rails", forthcoming from Manning Publications, April 2006!