The most literal translation is
#lang racket
(define select-room (lambda (number rooms)
(let loop ((l (mcar rooms)))
(if (not (null? l))
(if (= (mcar(mcar l)) number)
(mcar l)
(loop (mcdr l)))
(void))))) ;<-- explicit "else" branch
You can replace (void) with whatever you want to signal that nothing
is found, like 'not-found, but the more idiomatic versions is to
return #f.
(define select-room (lambda (number rooms)
(let loop ((l (mcar rooms)))
(if (not (null? l))
(if (= (mcar(mcar l)) number)
(mcar l)
(loop (mcdr l)))
#f)))) ;<-- explicit "else" branch
It's not possible to return "nothing", you must return something, like
(void) or #f or 'not-found. When you are going to ignore the result,
it's better to return (void), but if you want to signal that something
is not found it's better to return #f, or raise an error. (Actually,
you can return "nothing", but let's hide that under the carpet for few
months, because it doesn't play nice with most racket code.)
But now set-symbol has to check that traps is not #f:
(define set-symbol!
(lambda (symbol number_of_room)
(let ((traps (mcdr(select-room number_of_room environment))))
(if traps
(set-mcdr! traps (mcons symbol (mcdr traps)))
(void)))) ;<-- explicit "else" branch
Here, it's better to return (void) because you are going to ignore the
result, and set-mcdr! returns also (void). The default printer ignore
voids results and prints "nothing", so (void) will be better here.
(I renamed set-symbol as set-symbol!, because the name of most
functions that mutate their argument have bang. It's not a rule, only
a style recommendation.)
A more idiomatic version use when, unless, and and or, but I'm triying
to make the minimal changes to your code.
Gustavo
On Sun, Apr 10, 2016 at 11:30 AM, Joan Martin <joanmar1...@gmail.com> wrote:
> Thank for your answer,
> but how should I change the procedure and ged rid of Racket warning?
> I cant figure it out.
>
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