On Apr 25, 2013, at 2:31 PM, "Ford, Joe" <jf...@hollandhall.org> wrote:
> I have a group of high school students with a question... can someone please > explain to beginner Racket users the differences between these three boolean > functions: eq? equal? eqv? Welcome to Racket v5.3.4.5. > (require math) > (eq? (factorial 100) (factorial 100)) ;; small numbers are the same pointers > in the system #t > (eq? (factorial 1000) (factorial 1000)) ;; big number are distinct pointers #f > (eqv? (factorial 1000) (factorial 1000)) ;; ... but they point to observably > equal numbers, so eqv? uses = to compare them #t > (eqv? (cons 1 2) (cons 1 2)) ;; distinct pointers point to allocated > structures, on the other hand, and eqv? compares pointers with pointer > equality #f > (equal? (cons 1 2) (cons 1 2)) ;; so we need structural equality to find out > whether they contain the same pieces down to the leaves #t > We have read the help menu verbage visible from DrRacket, but simply don't > understand what it is saying. See HtDP's section on extensional (equal?) vs intensional equality (eq?). As much as I like our help system, I don't think it's suitably for truly novice programmers. > Maybe that's lack of context or vocabulary... but we're struggling a bit. To > test simple variations of application, we wrote some simple code (shown > below) but don't understand why the results are what they are: > > (define FOUR "four") > (define A (make-posn 4 5)) > (define B (make-posn (+ 3 1) (- 6 1))) > "-------------" > (equal? FOUR "four") > (equal? 4 (+ 1 3)) > (equal? 4 (posn-x (make-posn 4 3))) > (equal? A B) > "-------------" > (eq? FOUR "four") > (eq? 4 (+ 1 3)) > (eq? 4 (posn-x (make-posn 4 3))) > (eq? A B) > "---------------" > (eqv? FOUR "four") > (eqv? 4 (+ 1 3)) > (eqv? 4 (posn-x (make-posn 4 3))) > (eqv? A B) > > > Why in the world would the above-defined A and B be considered "equal?" but > not "eq?" or "eqv?"? See beginning of message. ____________________ Racket Users list: http://lists.racket-lang.org/users
