Here's a version of your code that runs in the Intermediate Student
language ("local" isn't part of Beginning Student):
(define (maxval lst)
(local ((define (aux lst max)
(cond [(null? lst) max]
[(> (car lst) max) (aux (cdr lst) (car lst))]
[#t (aux (cdr lst) max)])))
(aux lst 0)))
And here's the full grammar for ISL programs:
http://docs.racket-lang.org/htdp-langs/intermediate.html
Thanks for pointing out that the error message raised by `define` could be
improved.
On Mon, Feb 13, 2017 at 3:48 PM, Matthias Felleisen <[email protected]>
wrote:
>
> You may wish to read up on the design recipe and the necessity of ‘local’
> definitions in HtDP2e:
>
> http://www.ccs.neu.edu/home/matthias/HtDP2e/
>
>
>
>
> > On Feb 13, 2017, at 3:46 PM, Angus Comber <[email protected]> wrote:
> >
> > But it has to have the signature maxval lst - it is a homework problem.
> >
> > So I thought I would need max in the helper function. Basically to keep
> an internal max saved value. Is there another way?
> >
> > On 13 February 2017 at 20:38, Matthias Felleisen <[email protected]>
> wrote:
> >
> > Just lift aux out.
> >
> >
> > > On Feb 13, 2017, at 3:18 PM, Angus Comber <[email protected]>
> wrote:
> > >
> > > If I change the code to:
> > >
> > > (define (maxval lst)
> > > (define (aux lst max)
> > > (cond [(null? lst) max]
> > > [(> (car lst) max) (aux (cdr lst) (car lst))]
> > > [#t (aux (cdr lst) max)]))
> > > (aux lst 0))
> > >
> > > and comment out #lang racket at the top of the file and select
> Beginning Student as the language then I get error:
> > >
> > > define: expected only one expression for the function body, but found
> 1 extra part
> > >
> > > Is this because there is a 2nd define in the function? Or do I need
> some syntax to indicate to run aux at the end of the definition.
> > >
> > > On 12 February 2017 at 21:34, Matthias Felleisen <[email protected]>
> wrote:
> > >
> > > > On Feb 12, 2017, at 3:00 PM, Angus <[email protected]> wrote:
> > > >
> > > > I am a beginner Racket developer by the way.
> > > >
> > > > Here is my maxval function which is supposed to take a list and
> return the largest integer in the list.
> > > >
> > > > (define (maxval lst)
> > > > (define (aux lst max)
> > > > (cond [(null? lst) max]
> > > > [(car lst) > max (aux (cdr lst) (car lst))]
> > > > [#t (aux (cdr lst) max)]))
> > > > (aux lst 0))
> > > >
> > > >
> > > > Using the function always returns the last value in the list
> > > >
> > > >> (maxval (list 1 2 3 5 6 1))
> > > > 1
> > > >
> > > > The code looks correct to me, but I must be missing something. What
> have I done wrong?
> > >
> > >
> > > Your code is 100% correct, conceptually. Sadly, it’s suffering from a
> mistake that I saw many many times in the 90s and before. And that’s
> precisely why we get beginners started on Beginning Student Language in
> DrRacket instead of plain Racket. Try it out for your next few exercises or
> even this one, to see whether the error message would have helped.
> > >
> > > — Matthias
> > >
> > >
> >
> >
>
>
>
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