Thanks Erik. Very nice! Although I miss the recursion.<div><div><br></div><div>please help me understand how this works, especially (let loop ([vs '()]). What exactly is going on there?</div><div><br></div><div>Thanks again,</div> <div>-joe<br><br><div class="gmail_quote">On Mon, Feb 13, 2012 at 9:51 PM, Erik Silkensen <span dir="ltr"><<a href="mailto:eriksilkensen@gmail.com">eriksilkensen@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"> <div style="word-wrap:break-word">Instead of calling fib-less-than-n recursively, you could add a loop inside, for example something like,<div><br></div><div><div><div><div>(define (fib-less-than-n n)</div><div> (define fib (mk-fib))</div> <div> (let loop ([vs '()])</div><div> (let ([fib-val (fib)])</div><div class="im"><div> (if (>= fib-val n)</div></div><div> (reverse vs)</div><div> (loop (cons fib-val vs))))))</div></div> </div></div><span class="HOEnZb"><font color="#888888"><div><br></div><div>- Erik</div></font></span><div><div class="h5"><div><br><div><div>On Feb 13, 2012, at 10:41 PM, Joe Gilray wrote:</div><br><blockquote type="cite"> Erik,<div><br></div><div>Intriguing idea and it works "flat", but if I write the following I end up with an infinite loop:</div><div><br></div><div><div>(define (fib-less-than-n n)</div><div> (let ([f1 (mk-fib)])</div> <div> (let ([fib-val (f1)])</div><div> (if (>= fib-val n) '() (cons fib-val (fib-less-than-n n))))))</div><div><br></div>How would I use your idea in this case?</div><div><br></div><div>thanks!</div><div>-joe</div> <div><br><div class="gmail_quote">On Mon, Feb 13, 2012 at 9:18 PM, Erik Silkensen <span dir="ltr"><<a href="mailto:eriksilkensen@gmail.com" target="_blank">eriksilkensen@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin-top:0px;margin-right:0px;margin-bottom:0px;margin-left:0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex"> 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:<br> <br> (define (mk-fib)<br> <div> (let ([n0 -1] [n1 1])<br> (lambda ()<br> (let ([next (+ n0 n1)])<br> (set! n0 n1)<br> (set! n1 next))<br> n1)))<br> <br> </div>- Erik<br> <div><div><br> On Feb 13, 2012, at 10:10 PM, Danny Yoo wrote:<br> <br> > On Mon, Feb 13, 2012 at 11:52 PM, Joe Gilray <<a href="mailto:jgilray@gmail.com" target="_blank">jgilray@gmail.com</a>> wrote:<br> >> Warning: extreme newbie question ahead.<br> >><br> >> I wrote the following fibonacci function:<br> >><br> >> ; function that returns the next fibonacci number each time it is called<br> >> ; invoke as (fib)<br> >> (define fib<br> >> (let ([n0 -1] [n1 1])<br> >> (lambda ()<br> >> (let ([next (+ n0 n1)])<br> >> (set! n0 n1)<br> >> (set! n1 next))<br> >> n1)))<br> ><br> ><br> > One thing you can do is turn fib into a "sequence", and then from a<br> > sequence into a stream that knows how to remember its previous values.<br> ><br> > Here's what it might look like:<br> ><br> > ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;<br> > #lang racket<br> > (require racket/sequence)<br> ><br> > (define fib<br> > (let ([n0 -1] [n1 1])<br> > (lambda ()<br> > (let ([next (+ n0 n1)])<br> > (set! n0 n1)<br> > (set! n1 next))<br> > n1)))<br> ><br> > (define fib-stream<br> > ;; Here's a sequence of the function:<br> > (let ([fib-sequence (in-producer fib 'donttellmecauseithurts)])<br> ><br> > ;; Let's wrap it and turn it into a stream that remembers...<br> > (sequence->stream fib-sequence)))<br> ><br> ><br> > ;; Ok, we've got a stream. Let's look at its first few elements.<br> > (define (peek-fibs n)<br> > (for ([elt fib-stream]<br> > [i (in-range n)])<br> > (displayln elt)))<br> ><br> > (peek-fibs 10)<br> > (printf "-----\n")<br> > (peek-fibs 20)<br> > ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;<br> ><br> </div></div>> ____________________<br> > Racket Users list:<br> > <a href="http://lists.racket-lang.org/users" target="_blank">http://lists.racket-lang.org/users</a><br> <br> </blockquote></div><br></div> </blockquote></div><br></div></div></div></div></blockquote></div><br></div></div>