According to the GHC documentation [1]: rem :: a -> a -> a integer remainder, satisfying (x `quot` y)*y + (x `rem` y) == x
mod :: a -> a -> a integer modulus, satisfying (x `div` y)*y + (x `mod` y) == x div truncates toward negative infinity while quot (which is in Clojure) truncates toward 0. Vincent. [1] http://www.haskell.org/ghc/docs/6.8.3/html/libraries/base/Prelude.html On Feb 10, 8:34 pm, Timothy Pratley <timothyprat...@gmail.com> wrote: > I spoke too soon. Apparently from the Lisp HyperSpec > mod performs the operation floor on number and divisor and returns the > remainder of the floor operation. > Which would indicate that mod1 is not consistent with LISP. > Seeing Java doesn't have a proper mod, it would seem sensible to > follow the LISP/Ruby/Python convention... > > (defn mod2 > "modulus of num and div." > [num div] > (let [m (rem num div)] > (cond > (or (not (integer? num)) (not (integer? div))) > (throw (IllegalArgumentException. > "mod requires two integers")) > (zero? m) 0 > (or (< num 0 div) (< div 0 num)) (+ m div) > :else m))) > > user=> (map #(mod2 % 3) (range -9 9)) > (0 1 2 0 1 2 0 1 2 0 1 2 0 1 2 0 1 2) > user=> (map #(mod2 % -3) (range -9 9)) > (0 -2 -1 0 -2 -1 0 -2 -1 0 -2 -1 0 -2 -1 0 -2 -1) > > >>> map(lambda x: x % -3, range(-9,9) ) > > [0, -2, -1, 0, -2, -1, 0, -2, -1, 0, -2, -1, 0, -2, -1, 0, -2, -1] > > ie: I think the original implementation simply overlooked the case > where rem div num is 0, as per floor. > > Regards, > Tim. > > On Feb 11, 10:46 am, Timothy Pratley <timothyprat...@gmail.com> wrote: > > > Interesting! > > > Based uponhttp://mathforum.org/library/drmath/view/52343.html > > mod might be modified to look like this > > > (defn mod1 > > "modulus of num and div." > > [num div] > > (cond > > (or (not (integer? num)) (not (integer? div))) > > (throw (IllegalArgumentException. > > "mod requires two integers")) > > (< num 0 div) (- (rem num div)) > > (< div 0 num) (rem num (- div)) > > :else (rem num div))) > > > user=> (map #(mod1 % 3) (range -9 9)) > > (0 2 1 0 2 1 0 2 1 0 1 2 0 1 2 0 1 2) > > > > [0, 1, 2, 0, 1, 2, 0, 1, 2, 0, 1, 2, 0, 1, 2, 0, 1, 2] > > > Notably that is still not the same as those other languages! However I > > believe it is 'correct' due to Java and Clojure's definition of div. > > Ruby and Python define integer division of or by a negative number > > differently that C and Java do. Consider the quotient -7/3. Java gives > > -2. Ruby gives -3. > > > Regards, > > Tim. > > --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en -~----------~----~----~----~------~----~------~--~---
