Re: TAxis bug

Valery Fine (fine@bnl.gov)
Fri, 10 Apr 1998 11:03:37 -0500


Dear Fred
On 9 Apr 98 at 23:22, you wrote:

> There is a Visual C++ runtime library function called _controlfp()
> which sets the IEEE floating-point status word. The syntax is
> _controlfp(value, mask); the mask indicates which bits of the status
> word you want to change. There are lots of constants defined for
> various parts of this word. In order to set round-to-nearest mode,
> the syntax would be:
>
> _controlfp(_RC_NEAR, _MCW_RC);
>
> The other rounding mode choices are _RC_CHOP, _RC_UP, and _RC_DOWN.

Thank you very much for your clarification.

I am aware about this function but I doubt ROOT should change the
default setting.

Another question whether ROOT should supply some "common" method to
give the user an opportunity to change this option on his / her own
via TSystem class.

Since ROOT is multi-platform package it should be understood how
this method will be implemented for the rest supported platforms too.

For the time being I would advice those finding it is essential
to control the rounding "by hand" call this function himself
/herself from his/her method.

From another hand I'd like to call your attention that the C / C++
docs says:

Floating to Integral
====================

When an object of floating type is converted to an integral type,
the fractional part is truncated. No rounding takes place in the
conversion process. Truncation means that a number like 1.3 is
converted to 1, and -1.3 is converted to -1.

THAT'S TRUE for ANY CURRENT MathCPU bits setting (it is a LAW for
each platform / compiler implementation and NO hardware/ CPU flag /
mask are "allowed" to change it)

And just another LAW:

Integral to Floating
====================
When an object of integral type is converted to a floating type and
the original value cannot be represented exactly, the result is
either the next higher or the next lower representable value.

Valery

=================================================================
Dr. Valeri Faine (Fine)
------------------- Phone: +1 516 344 7806
Brookhaven National Laboratory FAX : +1 516 344 4206
Bldg. 510A /STAR mailto:fine@bnl.gov
Upton, New York, 11973-5000 http://nicewww.cern.ch/~fine
USA

Dr. Valery Fine Telex : 911621 dubna su
-----------
LCTA/Joint Inst.for Nuclear Res. Phone : +7 09621 6 40 80
141980 Dubna, Moscow region Fax : +7 09621 6 51 45
Russia mailto:fine@main1.jinr.dubna.su