Another option is you could turn change your fib function to be 'mk-fib' that
returns a new instance of the fib function each time it's called:
(define (mk-fib)
(let ([n0 -1] [n1 1])
(lambda ()
(let ([next (+ n0 n1)])
(set! n0 n1)
(set! n1 next))
n1)))
- Erik
On Feb 13, 2012, at 10:10 PM, Danny Yoo wrote:
> On Mon, Feb 13, 2012 at 11:52 PM, Joe Gilray <jgil...@gmail.com> wrote:
>> Warning: extreme newbie question ahead.
>>
>> I wrote the following fibonacci function:
>>
>> ; function that returns the next fibonacci number each time it is called
>> ; invoke as (fib)
>> (define fib
>> (let ([n0 -1] [n1 1])
>> (lambda ()
>> (let ([next (+ n0 n1)])
>> (set! n0 n1)
>> (set! n1 next))
>> n1)))
>
>
> One thing you can do is turn fib into a "sequence", and then from a
> sequence into a stream that knows how to remember its previous values.
>
> Here's what it might look like:
>
> ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
> #lang racket
> (require racket/sequence)
>
> (define fib
> (let ([n0 -1] [n1 1])
> (lambda ()
> (let ([next (+ n0 n1)])
> (set! n0 n1)
> (set! n1 next))
> n1)))
>
> (define fib-stream
> ;; Here's a sequence of the function:
> (let ([fib-sequence (in-producer fib 'donttellmecauseithurts)])
>
> ;; Let's wrap it and turn it into a stream that remembers...
> (sequence->stream fib-sequence)))
>
>
> ;; Ok, we've got a stream. Let's look at its first few elements.
> (define (peek-fibs n)
> (for ([elt fib-stream]
> [i (in-range n)])
> (displayln elt)))
>
> (peek-fibs 10)
> (printf "-----\n")
> (peek-fibs 20)
> ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
>
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