Thanks Erik. Very nice! Although I miss the recursion. please help me understand how this works, especially (let loop ([vs '()]). What exactly is going on there?
Thanks again, -joe On Mon, Feb 13, 2012 at 9:51 PM, Erik Silkensen <eriksilken...@gmail.com>wrote: > Instead of calling fib-less-than-n recursively, you could add a loop > inside, for example something like, > > (define (fib-less-than-n n) > (define fib (mk-fib)) > (let loop ([vs '()]) > (let ([fib-val (fib)]) > (if (>= fib-val n) > (reverse vs) > (loop (cons fib-val vs)))))) > > - Erik > > On Feb 13, 2012, at 10:41 PM, Joe Gilray wrote: > > Erik, > > Intriguing idea and it works "flat", but if I write the following I end up > with an infinite loop: > > (define (fib-less-than-n n) > (let ([f1 (mk-fib)]) > (let ([fib-val (f1)]) > (if (>= fib-val n) '() (cons fib-val (fib-less-than-n n)))))) > > How would I use your idea in this case? > > thanks! > -joe > > On Mon, Feb 13, 2012 at 9:18 PM, Erik Silkensen > <eriksilken...@gmail.com>wrote: > >> 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) >> > >> ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; >> > >> > ____________________ >> > Racket Users list: >> > http://lists.racket-lang.org/users >> >> > >
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