[racket] do loops in racket, also srfi/25 query

Hi Tom,

thanks for the comments.

As far as reading the data into a vector, that can be accomplished with
"(define vec (read in))"  I was more interested in how to create an srfi/25
array from that vector.

Thanks also for the example of using for*/fold.

-Joe



On Sat, Mar 24, 2012 at 11:26 PM, Tom McNulty <tom.mcnulty at gmail.com> wrote:

> The format of prob11's data is already quite close to what the reader
> expects for a vector, so you can read it directly by including #( ...
>
> (define tbl
>  #(08 02 22 97 38 15 00 40 00 75 04 05 07 78 52 12 50 77 91 08
>        <snip> ...
>    01 70 54 71 83 51 54 69 16 92 33 48 61 43 52 01 89 19 67 48))
>
> A convenience function to read the table is then:
>
> (define (get i j)
>  (vector-ref tbl (+ (* i 20) j)))
>
> Here is my across function, I use a for/fold loop.  The vertical and
> diagonal cases are similar.
>
> (define (across)
>  (for*/fold ([mx 0])
>             ([i (in-range 0 20)]
>              [j (in-range 0 17)])
>             (max mx (* (get i j)
>                        (get i (+ j 1))
>                        (get i (+ j 2))
>                        (get i (+ j 3))))))
>
> Does that help?
>
>
> - Tom
>
> On 2012-03-24, at 11:01 PM, Joe Gilray wrote:
>
> > I was playing around with ProjectEuler #11 and found a lisp solution and
> adapted it to Racket.  Part of the solution had code like the following to
> find products of values in a 20x20 vector.
> >
> > (define (prod-of-vec-lines-of-length-4 v ix iy dx dy)
> >   (do ([p 1 (* p (if (in-vector? x y 20 20) (vector-ref v (+ (* 20 y)
> x)) 0))]
> >        [x ix (+ x dx)]
> >        [y iy (+ y dy)]
> >        [c 0 (add1 c)])
> >     ((= c 4) p)))
> >
> > I realize I've hard-coded this in a very ugly fashion to 20x20 vectors.
> >
> > My questions are:
> >
> > 1) Is there a more Racket-idiomatic way to write the "do" loop above?
> > 2) If I wanted to use srfi/25 to do this in a nicer fashion, how would I
> read in the array? - I liked being able to read in the vector with simply
> (read in) - is there a simply , one-line way to convert a vector to an
> array (make-array (shape 0 20 0 20) vec) puts the entire 400 element vector
> into the first element of the array!
> >
> > Thanks,
> > -joe
> > ____________________
> >  Racket Users list:
> >  http://lists.racket-lang.org/users
>
>
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