Re: [Camllist] Runtime overflow and what to do
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Date:   (:) 
From:  Pierre Weis <pierre.weis@i...> 
Subject:  Re: [Camllist] Runtime overflow and what to do 
Hi again, > On Sun, Oct 13, 2002 at 12:29:58PM +0200, Pierre Weis wrote: > > > let ( * ) n m = > > if num_bits_int n + num_bits_int m < length_of_int > > then n * m else raise (Overflow ("*", string_of_int n, string_of_int m)) > > Does it work with negative numbers? And with multiplication between a > positive and a negative number? Yes it does. > > Just to let people play with the writting of the module :), > > num_bits_int is left as an exercise to the interested readers (3 lines > > of Caml code using trivial bit shuffling). > > I am interested in knowing it, because I already searched that. > Without a loop or a predefined array of powers of two, I don't > know how to do that. Nor did I. I think there exists processors that do have an instruction to implement this function but this is not the general case: you need to write a loop or use a predefined array of powers of two or write a dichotomic algorithm or whatever... For instance: let num_bits_int n = let rec num_bits n = if n = 0 then 0 else succ (num_bits (n lsr 1)) in num_bits (abs n);; (No dichotomy, here. Experiments with a dichotomic algorithm is also interesting, but deciding which is faster is difficult since benchmarking is almost impossible: the results are too much dependant of the value of integers manipulated in the program.) Pierre Weis INRIA, Projet Cristal, Pierre.Weis@inria.fr, http://pauillac.inria.fr/~weis/  To unsubscribe, mail camllistrequest@inria.fr Archives: http://caml.inria.fr Bug reports: http://caml.inria.fr/bin/camlbugs FAQ: http://caml.inria.fr/FAQ/ Beginner's list: http://groups.yahoo.com/group/ocaml_beginners