OpenEuphoria

Re: power overflow

• Posted by jluethje at gmx.de Jun 03, 2002
• 425 views

Hi tone,

nice idea to write this function.
With two changes, it really will be safe, I think.

> Thanks, here is general function I wrote safe_power():

> global constant LOWEST_ATOM = -1e300
> global constant HIGHEST_ATOM = +1e300

> global function log_general (atom b, object x)
> -- general log() function
> -- logarithm for base b and number (or sequence) x

> -- in : b: positive real number, != 1
> --      x: positive real number (or sequence of those numbers)
> -- out: real number
--      (x = 1  -->>  function returns 0 for any base b)

>    return log (x) / log (b)
> end function

> --/*
> -- safe_power [Created on 3. June 2002, 17:59]
> -- The 'safe_power' function does same thing as Euphoria builtin 'power()'
> -- with this difference: if return value of 'power()' is too big
> -- interpreter crashes with error mesage math overflow.
> -- If in this function returned value would be too big
> -- then error 'HIGHEST_ATOM' is returned.
> --
> -- PARAMETERS
> -- 'a'
> --    Same as in built in 'power()'.
> -- 'b'
> --    Same as in built in 'power()'.
> --
> -- RETURN VALUES
> -- The value of power(), or error value 'HIGHEST_ATOM'
> --*/
> global function safe_power (atom a, atom b)
>     atom b_max --// if 'b' is bigger or equal than this then overflow
>     atom a_max --// if 'a' is bigger or equal than this then overflow

-- Additionally:
      if a = 0 and b = 0 then
         return "power(0,0) is not defined"
      end if

>     if a = 0 or b = 0 then --// log (0) crashes
-- Change previous line
-- (see my comments in global function log_general() above):
      if a <= 0 or b <= 0 or a = 1 then

>         return power (a ,b)
>     end if
>     b_max = log_general (a, HIGHEST_ATOM)
>     if b >= b_max then
>         return HIGHEST_ATOM
>     end if
>     a_max = power (HIGHEST_ATOM, 1 / b)
>     if a >= a_max then
>         return HIGHEST_ATOM
>     end if
>     return power (a, b)
> end function


For more information about logarithms, see
http://mathworld.wolfram.com/Logarithm.html

Best regards,
   Juergen