[BLACKBOX] REAL Exponent (Was Another (?) Compiler Trap)

From: Campbell, Robert (SELEX GALILEO, UK) <"Campbell,>
Date: Mon, 3 Mar 2008 12:23:22 -0000

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Hi

Regarding my attempt to extract the exponent of a floating point number
in 'Pure' CP
my 'old' (and slow, but no matter) routine was:

CONST
  ln2 = 0.6931471805599453094172321; (* Correct to 25 digits !!!
*)
BEGIN
  exp := SHORT (ENTIER ((Math.Ln (ABS (x))) / ln2));


This (with some other corrections not relevant to this comment) has
worked for
18 months, then failed today with x = 31.999999999999996; One more or
less
9 and it works. Embarassingly it was a customer who found this!
Fortunately
a strategically placed ASSERT prevented serious consequences.

Odds against, 10^-15 ?, or rather 100%. These details ALWAYS bite
eventually.

The problem, of course, is the Ln rounding.

The line:
 exp := SHORT (ENTIER ((Math.Ln (ABS (x))) / Math.Ln (2.)))

works here, because of the 80 bits left on the stack. But I wouldn't
like to try to *prove* it always works.


Cheers

Robert

SELEX Sensors and Airborne Systems Limited
Registered Office: Sigma House, Christopher Martin Road, Basildon, Essex SS14 3EL
A company registered in England & Wales. Company no. 02426132
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Received on Mon Mar 03 2008 - 13:23:22 UTC