# [racket] Using future and touch

 From: Robby Findler (robby at eecs.northwestern.edu) Date: Wed Jul 24 22:47:00 EDT 2013 Previous message: [racket] Using future and touch Next message: [racket] Using future and touch Messages sorted by: [date] [thread] [subject] [author]

```When I run this, the future visualizer shows that gcd and square-number?
block the futures. square-number? is implemented in TR and if you take it,
you find that integer-sqrt also blocks the futures. I'm not sure if those
functions can be made to run safely in futures or not.

Robby

On Wed, Jul 24, 2013 at 7:26 PM, Joe Gilray <jgilray at gmail.com> wrote:

> I have a ProjectEuler problem that I wanted to speed up so I thought I
> would try to speed it up with future / touch.
>
> I tried the following:
>
> ; function that searches for progressive numbers for a given range of b
> values
> (define (find-progressive-num b-start b-end b-incr lim)
>   (for/sum ([b (in-range b-start b-end b-incr)])
>     (let loopa ([a (add1 b)] [suma 0])
>       (cond
>         [(> (gcd a b) 1) (loopa (add1 a) suma)]
>         [(>= (* a a a b) lim) suma]
>         [else
>          (let loopc ([c 1] [sumc 0])
>            (define n (+ (* a a a c c b) (* c b b)))
>            (cond
>              [(>= n lim) (loopa (add1 a) (+ suma sumc))]
>              [(square-number? n) (loopc (add1 c) (+ sumc n))]
>              [else (loopc (add1 c) sumc)]))]))))
>
> ; ProjectEuler problem #141
> ; n = q * d + r
> ; q/d = d/r = a/b (where a and b are relatively prime)
> ; so d = a*r/b and q = a^2 * r / b^2
> ; since a and b are coprime r must be divisible by b^2 or r = c*b^2
> ; substituting: d = a*c*b and q = a^2*c
> ; n = a^3 * c^2 * b + c * b^2
> (define (euler141)
>   (define lim 10000000000)
>   (let ([f1 (future (λ () (find-progressive-num 1 1000 2 lim)))])
>     (+ (find-progressive-num 2 1000 2 lim) (touch f1))))
>
> Unfortunately this runs no faster than the sequential version.  I tried
> using the future-visualizer but I couldn't understand what it was telling
> me (I guess some operation is blocking).  I also tried finer grained
> threads (one for each value of b), but that did no better.
>
> Can anyone give me some pointers to successfully using future / touch?
>
> Thanks,
> -Joe
>
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>   http://lists.racket-lang.org/users
>
>
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