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if (n:int) < 0 then (-n) > 0 is FALSE
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Date: | 2006-12-07 (18:34) |
From: | Pal-Kristian Engstad <pal_engstad@n...> |
Subject: | if (n:int) < 0 then (-n) > 0 is FALSE |
Using OCaml 3.08.3 on the following function: let pow x n = let rec aux x n acc = if n == 0 then acc else if n == 1 then acc *. x else if n land 1 == 0 then aux (x*.x) (n/2) acc else aux (x*.x) ((n-1)/2) (x*.acc) in if n >= 0 then aux x n 1.0 else 1.0 /. (aux x (-n) 1.0) ;; I tested this function with # pow 1.0 (1024 * 1024 * 1024) To find that it loops forever. The reason is that 1024*1024*1024=2^30 cannot be represented as a positive number on 32-bit platforms, hence it silently converts it to -1073741824, or -2^30. The reason this loops again is that -n = -(-20^30) = -20^30......, still negative! This is obviously a bug - has it since been fixed? But more alarmingly - why is there no warning? Thanks, PKE. -- Pål-Kristian Engstad (engstad@naughtydog.com), Lead Programmer, ICE team, Naughty Dog, Inc., 1601 Cloverfield Blvd, 6000 North, Santa Monica, CA 90404, USA. Ph.: (310) 633-9112. "Most of us would do well to remember that there is a reason Carmack is Carmack, and we are not Carmack.", Jonathan Blow, 2/1/2006, GD Algo Mailing List