[racket] Looping with look-behind and look-forward

The line (displayln (/ (+ prior current next))) needs to be changed to
 (displayln (/ (+ prior current next) 3))

Thanks,

On Sat, May 26, 2012 at 9:20 PM, Matthias Felleisen <matthias at ccs.neu.edu>wrote:

>
> Do you mean that if you operated on a list it would look like this:
>
> #lang racket
>
> (define (running-average-of-3 l2)
>  (define-values (_1 _2)
>    (for/fold ((current (second l2)) (prior (first l2))) ((next (rest (rest
> l2))))
>      (displayln (/ (+ prior current next)))
>      (values next current)))
>  (void))
>
> (running-average-of-3 '(1 2 3 4 5 6 7 8 9 0))
>
>
>
>
> On May 26, 2012, at 12:56 AM, Harry Spier wrote:
>
> > I can use for/fold to loop through a sequence while having the current
> > element and  the previous element of the sequence still available by
> > doing something like this.
> >
> > (for/fold ([previous-element-in-sequence '()][list-being-created '()])
> >  ( [current-element-in-sequence sequence])
> >  (do some stuff)
> >  ....
> >  (values current-element-in-sequence
> >          (cons something list-being-created)))
> >
> > But what I want is to be able to loop through a sequence while having
> > the prior, the current and the next element available.
> >
> > I'm sequencing through a representation of lines of a black and white
> > page of text showing where the black pixels are and from that
> > producing a graph showing the connectivity of the black pixel strips
> > on each line to the ones on the lines above and below it.
> >
> > Using for/fold I'm able to code the graph of connectedness of the
> > black strips on each line to the prior line in a very straightforward
> > way, but I'd like to graph in both directions both to the prior line
> > and to the next line at the same time.
> >
> > Is there a simple way to loop through a sequence and have the prior,
> > current and next element available at the same time.
> >
> > Thanks,
> > Harry
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>
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