The problem is that `start' doesn't get a value for a `sqrtcontfrac%' instance until `super-new' in `sqrtcontfrac%' is evaluated. But the expression for the second argument in `super-new' calls `build-repeat', so that `build-repeat' tries to use `start' before it has a value.
I'm not sure of the solution in this case, but it might be to pass the value for `start' into `build-repeat' instead of trying to get the value from the `start' field within `build-repeat'. At Sun, 20 May 2012 02:06:35 -0700, Joe Gilray wrote: > I'm trying to learn to use classes in Racket. I did some successful simple > experiments then moved on to inheritance. When I try to use the code > below, the following code: > > (new sqrtcontfrac% [sqrval i]) leads to the message: sqr: expected > argument of type <number>; given #<undefined> > > Why isn't start being inherited properly? Any help is appreciated! > > Thanks, > -joe > > Here is the class code: > > (module contfrac racket > (require racket/class) > (provide gencontfrac%) > (provide sqrtcontfrac%) > > ; class that represents the continued fraction of a general irrational > number and acts as a base class for continued fractions of other sorts > (define gencontfrac% > (class object% > (super-new) > (init-field start) *; here start is defined* > (init-field repeat) > ; create a n-long list of repeat digits (in reverse order - which > makes creation of a rational easier) > (define/private (expand-repeat n) > (let lp ([c n] [rl '()]) > (if (zero? c) rl (lp (sub1 c) (cons (repeat) rl))))) > ; create a rational representation using the passed number of repeat > digits > (define/public (rational-rep n) > (define rdiglst (expand-repeat n)) > (let lp ([rdigs (rest rdiglst)] [res (first rdiglst)]) > (if (empty? rdigs) (+ start (/ 1 res)) (lp (rest rdigs) (+ (first > rdigs) (/ 1 res)))))) > ; display the continued fraction > (define/public (show) > (printf "[~a; ~a]~n" start repeat)) > )) > > ; define a class that represents the continued fraction of a square root > (define sqrtcontfrac% > (class gencontfrac% > (init sqrval) > (super-new [start (integer-sqrt sqrval)] [repeat (build-repeat > sqrval)]) > (inherit rational-rep show) > (inherit-field start repeat) > ; build the repeating sequence (see > http://en.wikipedia.org/wiki/Continued_fraction) > (define/private (build-repeat n) > (let loop ([adder start] [denom (- n (sqr start))] [result '()]) *; > this is where the error is occurring * > (cond [(zero? denom) '()] ; perfect square > [(= 1 denom) (reverse (cons (+ start adder) result))] ; > found the end of the repeating sequence > [else > (let* ([nextval (quotient (+ adder (integer-sqrt n)) > denom)] [nextadd (- (* nextval denom) adder)]) > (loop nextadd (quotient (- n (sqr nextadd)) denom) (cons > nextval result)))]))) > ; create a n-long list of repeat digits (in reverse order - which > makes creation of a rational easier) > (define/private (expand-repeat n) > (let lp ([c n] [rl '()] [stock repeat]) > (if (zero? c) rl (lp (sub1 c) (cons (first stock) rl) (if (empty? > (rest stock)) repeat (rest stock)))))) > )) > ) > ____________________ > Racket Users list: > http://lists.racket-lang.org/users ____________________ Racket Users list: http://lists.racket-lang.org/users
