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if (n:int) < 0 then (-n) > 0 is FALSE
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| Date: | -- (:) |
| 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