The body of a for loop is not in tail position wrt to the for loop (you can see this in Check Syntax-- there is a gap in the tail arrows between the let that binds 'next-z' and the call to 'next-z'.
Robby On Tue, Mar 6, 2012 at 5:03 AM, Pierpaolo Bernardi <olopie...@gmail.com> wrote: > Hello, > > I was expecting the procedure 'fa' below to run in constant memory, > as, in my understanding, It doesn't use any non-tail recursive loops, > it does not build any data structure, and only performs arithmetic > operations on small integers. > > But some of my assumptions must be wrong, as the calls to next-z > accumulate and eventually fill the available memory. > > Can someone explain me where's my mistake? > > (A secondary note: the debugger shows the value of the squares vector > as a 100 element vector, with no hint that the value is abbreviated. > Pretty confusing, IMHO). > > Thanks. > > Pierpaolo > > ================ > #lang racket > > (define (perfect-square? n) > (= n (sqr (integer-sqrt n)))) > > (define (fa) > (let ((squares (for/vector ((i (in-range 1 10000))) > (* i i)))) > (let/ec return > (let next-sum ((sum 3)) > (let ((limit-z (quotient sum 3))) > (let next-z ((z 1)) > (if (= z limit-z) > (next-sum (add1 sum)) > (for ((y+z (in-vector squares))) > (let ((y (- y+z z))) > (if (> (+ y z) sum) > (next-sum (add1 sum)) > (let ((x (- sum z y))) > (if (and (perfect-square? (+ x y)) > (perfect-square? (- x y)) > (perfect-square? (+ x z)) > (perfect-square? (- x z))) > (return (list x y z sum)) > (next-z (add1 z)))))))))))))) > > ================ > ____________________ > Racket Users list: > http://lists.racket-lang.org/users ____________________ Racket Users list: http://lists.racket-lang.org/users
