Rounding Question

[Michael8]

Hello:

I have a simple program that consists of the following 3 lines of code:

double precision :: temp
temp = 0.0625write(6, '(F10.3)') temp

When I compile (using compiler version 10.1.014) the code in Debug mode and run it,it writes out the number 0.062. But when I use Release mode I get a result of 0.063.

Why is there a difference?

Thanks.
Michael

[abhimodak]

Hi

I would just like to add that we have come across this issue when carefully monitoring changes due to the compiler upgrades and visual studio. I can reproduce this error for 0.0625 when using the IVF9.1 integration with VS2003 and switching the debug/optimize modes. However, this doesn't seem to happen when using VS 2005.

--------
Program Test_Rounding
! Purpose: To test rounding in the print-out.
Implicit None
!
Integer, Parameter :: DOUBLE = kind(1.0d0)
Real(DOUBLE) :: a
!
a = 0.0625
Write(*,"(A,F7.3)") "a=", a
!
End Program Test_Rounding
--------

This gives:

(1) VS2003, ifort /O2 => 0.062

(2) VS2003, ifort /debug:full => 0.063

(3) VS2005, ifort /O2 => 0.063

(4) VS2005, ifort /debug:full => 0.063

Sincerely
Abhi

[Steven_L_Intel1]

Interesting. My test was using VS2005 which may be why I did not see the same results. When I get information from the developers as to what is causing the difference I'll let you know.

[jimdempseyatthecove]

Abhi,

It might also be of interest to print out the Hex format of a as well as the SIZEOF(0.0625).

This would indicate if the anomaly were attributed to

a) The value of the literal 0.0625
b)The type of the literal as beingreal(4) or real(8)
c) If the anomaly is attributed to the write statement or "=" statement

Jim Dempsey

[Steven_L_Intel1]

.0625 is exactly representable even in single precision. The difference, whatever it is, is internal to the run-time library ans not the compiled code.

[Michael8]

Abhi,

Thanks for pointing out that there is a difference between VS2003 and VS2005. As Steve suggested, that may have been why he was unable to replicate my results, as I was using VS2003. Even though he was eventually able to reproduce this type of inconsistency with some linker setting, it bothered me that he couldn't reproduce it exactly as-is based on the code I posted and him using the exact same compiler version.

Thanks.
Michael

[Steven_L_Intel1]

When using VS2003, I can reproduce the difference by selecting the non-debug libraries and not changing any other options. So it really is a library difference. I will be VERY interested to learn why.

[Michael8]

Quoting - Steve Lionel (Intel)

When using VS2003, I can reproduce the difference by selecting the non-debug libraries and not changing any other options. So it really is a library difference. I will be VERY interested to learn why.

Hello:

Sorry to bring back this old thread, but we're running into a very similar issue to this one again. Were you ever able to find out from the developers why there is a difference in rounding?

Thanks.
Michael

[Steven_L_Intel1]

The issue is still open and I do not have a response from the engineers. I will ping them.

[Alexandra_K_]

Hi,

we have a similar problem with rounding in the F-Format.
It doesn't round correctly.

We have tried various compiler-settings, but we got always the same (wrong) result.
We have written a simple code to demonstrate this:

program roundTest

real*4 rtemp

rtemp=6.25
write(*,'(''rtemp: '', f5.1, f10.5)') rtemp, rtemp
rtemp=6.75
write(*,'(''rtemp: '', f5.1, f10.5)') rtemp, rtemp

end program roundTest

Output:

rtemp: 6.2 6.25000
rtemp: 6.8 6.75000

Do you have any explanations or solutions?
What should we do?

Thanks a lot!
Sincerely, Alexandra and Zuzana

[Steven_L_Intel1]

Alexandra and Zuzana,

The results you are seeing here are correct. The Fortran standard does not specify the method of rounding for F format, and our implementation follows the IEEE floating point "round to even" rule which says that if a value to be rounded is exactly halfway between two representable values, round such that the last digit is even.

If you explicitly want "round away from zero", add RC as a format edit descriptor or open the file with ROUND='COMPATIBLE'. Other choices are 'UP', 'DOWN', 'ZERO' and 'NEAREST'.

[Alexandra_K_]

Hi Steve,

thanks a lot for your quick answer! You were right with your proposal.
It works now properly.

However, we would like to point it out that the Intel-manual is not exhaustive.
We were looking forwards and backwards through
the documentation, but we were not able to find those assignments.

We have found it on the website of IBM.
http://publib.boulder.ibm.com/infocenter/compbgpl/v9v111/index.jsp
under: Fortran...Control edit descriptors

Thanks again,
Alexandra and Zuzana

[Steven_L_Intel1]

Yes - we have not yet documented all the Fortran 2003 features we support. We'll address that in the next major release.