# Problem with recursion? (was [plt-scheme] Re: Novice needs help writing function )

 From: Yavuz Arkun (yarkun at gmail.com) Date: Fri Jan 4 15:43:54 EST 2008 Previous message: Problem with recursion? (was [plt-scheme] Re: Novice needs help writing function ) Next message: Problem with recursion? (was [plt-scheme] Re: Novice needs help writing function ) Messages sorted by: [date] [thread] [subject] [author]

```Robby Findler wrote:
> On Jan 4, 2008 12:41 PM, Yavuz Arkun <yarkun at gmail.com> wrote:
>
>> Marco Morazan wrote:
>>
>>>> BTW, I first learned about the Fibonacci series in high school, and we
>>>> 2 together to get the next one...", e.g. iteratively. For me the
>>>> recursive definition is contrived, and I saw it for the first time in
>>>> intro programming books. I guess this shows that prior teaching of
>>>> iterative style kills some neurons forever.
>>>>
>>>
>> The iterative model I had in mind was something more like:
>>
>> int fib(N) {
>>     int i, a[2] = {1, 1};
>>     for(i= 3; i <= N; i++) a[i % 2] += a[(i + 1) % 2];
>>     return a[N % 2];
>> }
>>
>
> And yet your English text suggests this definition:
>
>  fib(0)=1
>  fib(1)=1
>  fib(n+2) = fib(n) + fib(n+1)
>
> which, I believe, is even executable code in some programming language
> or other (not that that's particular relevant, tho).
>
>
Well, my English text had the "..." which in my mind was the explicit
loop. The mathematical version you gave makes that an implicit one. I
believe my C version is closer to the English text's spirit because its
formulated as an explicit iteration. What may be confusing in the C
version is the use of % operator to translate the "last 2" into C code.
(BTW, I am no C expert...I am sure it can be written more idiomatically.)

> As far as the mental model of the computer that you seem to work with
> (below), I am amazed at your ability to burrow through (a couple)
> abstractions. I'm remarkably bad at that, myself. But lets not forget
> that the purpose of abstractions to let people build bigger and bigger
> things on top of them and, to that end, it is important to make them
> seamless and then rely on that. Esp. for novices.
>
> Robby
>
>

I didn't mean that I can go down 2 levels in terms of abstraction (e.g.,
Scheme code -> Scheme interpreter -> assembly operations). I mean that I
can follow through only a few recursive calls of Fibonacci before
forgetting where I was. My little brain hurts already.

Yavuz

```

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