C / C++ / MFC

2-Feb-09 23:42

Dear All,

I have an issue with floating point precision with Visual Studio 2005 (2008 as well)
See the sample code.

const float f = 0.2f * 3; 
CString str; 
str.Format(_T( "%f"), f );

When you check the watch window, the variable will be having value 0.60000002(I expected 0.6). To confirm this again, I tried running with WinDBG, it has given a value 0.6000000238 in it's watch window

I doubt this may affect my calculation which should be very precise. I tried different floating point models using /fp compiler option. But always it has the same output. But the formatted string below gives 0.600000 as the result.

What could be the reason for this issue? Any solutions?

-Sarath.
"Great hopes make everything great possible" - Benjamin Franklin

My blog - Sharing My Thoughts

Re: floating point precision issue [modified]

Cedric Moonen2-Feb-09 23:53

Cedric Moonen

2-Feb-09 23:53

I don't understand where is your problem exactly: only with the precision or when formating the string ?
For the precision problem, you can use a double instead of a float, you'll gain much more precision.
For the formating, you can supply the number of decimal points you want to show in the format string (just add a number before the 'f').

Cédric Moonen
Software developer

Charting control [v1.5]
OpenGL game tutorial in C++

Sarath C3-Feb-09 0:38

3-Feb-09 0:38

OK

try this code. then things will be more clear.

float f = 0.2f 0.6f;
 f -= 0.2f;
 f -= 0.2f;
 f -= 0.2f;

 if( f == 0.0f ) // this condition will be failed
 {
     // f == 0;
 }

In this case, after subtracting 0.2 three times, the value should reach to 0.0f. But it's fails. because of the trailing additional digits in the floating point representation. The same case exists for double type as well.

-Sarath.
"Great hopes make everything great possible" - Benjamin Franklin

My blog - Sharing My Thoughts

modified on Tuesday, February 3, 2009 6:59 AM

Cedric Moonen3-Feb-09 0:48

Cedric Moonen

3-Feb-09 0:48

When working with doubles or floats, you should never compare numbers directly. This won't work. Instead, you have to use precision range when comparing them (for instance if the value is between -0.000001 and 0.000001, then you can say it is equal to 0. That's the only way to work with floating point values, because they will never be 100% precise and you have to keep that in mind.

Cédric Moonen
Software developer

Charting control [v1.5]
OpenGL game tutorial in C++

Sarath C3-Feb-09 0:55

3-Feb-09 0:55

Cedric Moonen wrote:
for instance if the value is between -0.000001 and 0.000001, then you can say it is equal to 0. That's the only way to work with floating point values, because they will never be 100% precise and you have to keep that in mind

Yeap the sign associated with it was the real issue. it may remain negative (even I expect 0.0f)

-Sarath.
"Great hopes make everything great possible" - Benjamin Franklin

My blog - Sharing My Thoughts

Stuart Dootson3-Feb-09 1:06

3-Feb-09 1:06

Sarath. wrote:
the sign associated with it was the real issue

Shouldn't be - the IEEE standard requires that positive or negative zeros compare equal.

CPallini3-Feb-09 0:00

CPallini

3-Feb-09 0:00

With double data type you'll get a better approximation (you know you can't get exact values)
Smile | :)

Stuart Dootson3-Feb-09 0:38

3-Feb-09 0:38

It's because standard floats and doubles cannot exactly represent 0.6.

The representation of 0.6 as a float can be determined by working out what sum of binary fractions (1/2ⁿ) best approximates it - you can see it's going to be something like 0.5 + 0.0625 + 0.03125 + .... - SpeedCrunch[^] (a nice calculator) gives it as this:

0b0.1001100110011001100110011001100110011001100110011001100110011001101

Infinitely recurring binary time...

Read this document[^] for the full story.

Compile time error for 'string.h'

David Crow3-Feb-09 4:47

David Crow

3-Feb-09 4:47

See here and here. In short, not all floating-point numbers can be stored in binary without some rounding.

"Old age is like a bank account. You withdraw later in life what you have deposited along the way." - Unknown

"The brick walls are there for a reason...to stop the people who don't want it badly enough." - Randy Pausch

Supriya Tonape2-Feb-09 23:33

2-Feb-09 23:33

Hello,
I am compling one of the sample projects which I got after installing WMSDK.

I am using VS2009 and I compiled the project from "C:\WMSDK\WMFSDK11\samples\Metadataedit" but getting error which tells that it's not getting declarations of some symbols/variables used in 'string.h'. I am confused thinking what setting am I missing ? can anybody please guide why I am getting those error ?

Thanks in Advance.

regards,
Supriya

Cedric Moonen2-Feb-09 23:37

Cedric Moonen

2-Feb-09 23:37

Are you talking about std::string ? If yes, then you have to #include <string></string> and not #include <string.h></string.h>.

EDIT: BTW, it is in general usefull to provide the exact error message, and not your interpretation of the message.

Cédric Moonen
Software developer

Charting control [v1.5]
OpenGL game tutorial in C++

Supriya Tonape2-Feb-09 23:44

2-Feb-09 23:44

sorry here are some of the errors..

1>c:\program files\microsoft visual studio 9.0\vc\include\string.h(48) : error C2065: '_MaxCount' : undeclared identifier
1>c:\program files\microsoft visual studio 9.0\vc\include\string.h(48) : error C3861: '_Out_opt_bytecap_': identifier not found
1>c:\program files\microsoft visual studio 9.0\vc\include\string.h(48) : error C2144: syntax error : 'void' should be preceded by ')'
1>c:\program files\microsoft visual studio 9.0\vc\include\string.h(48) : warning C4229: anachronism used : modifiers on data are ignored
1>c:\program files\microsoft visual studio 9.0\vc\include\string.h(48) : error C2059: syntax error : ')'

Regards,
Supriya

prasad_som3-Feb-09 3:27

prasad_som

3-Feb-09 3:27

Check if your problem is similar to one described here[^].

David Crow3-Feb-09 4:50

David Crow

3-Feb-09 4:50

Supriya Tonape wrote:
can anybody please guide why I am getting those error ?

Have you tried including string (instead of string.h) instead?

Supriya Tonape12-Feb-09 0:33

12-Feb-09 0:33

Yes I had tried that.
(sorry for late reply, I cudnt check mails.)
Regards,
Supriya.

Unicode Strings

user30342-Feb-09 23:31

user3034

2-Feb-09 23:31

Hi,

My C++ program (VS2005) has two parts. one part writes into a file and the other part reads from it.
The folder name is japanese. eg: C:\Documents and Settings\管理者
To read the file in this location,I am supposed to use an API (third party library) which takes the file location as a char*. I have the file location(C:\Documents and Settings\管理者) in a CString.

How do I convert this into a char* ?

I tried using the T2A macros, but the japanese string is not getting extracted. the char* pointer shows some junk charaters.

Any inputs on this ?

Thanks in advance.

Re: Unicode Strings

Sarath C2-Feb-09 23:50

2-Feb-09 23:50

It's better to seek the help of 3rd party library to provide Unicode interfaces. Other workarounds like renaming to ASCII and restoring it back after use will be painful.

-Sarath.
"Great hopes make everything great possible" - Benjamin Franklin

My blog - Sharing My Thoughts

Re: Unicode Strings

Supriya Tonape3-Feb-09 1:00

How to append text in word document caption?

3-Feb-09 1:00

I m not sure but check if this helps you,

CString buf = _T("salut"); //the first string
char * Result; //the result
long len = wcslen (buf); //the length of "salut"
wcstombs (Result,buf,len); //conversion to char *
Result[len] = '\0';

SNI2-Feb-09 21:48

SNI

2-Feb-09 21:48

Hi,
I would like to add some text in the cation of document. e.g. If I have a Test.doc then the caption for this document is Test.doc but I want to add some more text to it like "Mydoc" so the caption will become "Test.doc Mydoc". I know we need to change some registery entries but I want to change it for all office documents. Actually I m changing the extension of these documents as Test.gdoc so for all office docs with extension it should attach this caption.
Pls let me know that how could we achieve this.

Thanks
SNI

SNI

Re: How to append text in word document caption?

Stuart Dootson2-Feb-09 22:14

Re: How to append text in word document caption?

2-Feb-09 22:14

Using MFC? Override CFrameWnd::OnUpdateFrameTitle.

Here's the implementation I used to add a 'document is modified' marker:

void CMainFrame::OnUpdateFrameTitle(BOOL bAddToTitle)
{
   if ((GetStyle() & FWS_ADDTOTITLE) == 0)
      return;     // leave it alone!

   CDocument* pDocument = GetActiveDocument();
   if (bAddToTitle && pDocument != NULL)
      UpdateFrameTitleForDocument(pDocument->GetTitle() + (pDocument->IsModified()?"*":""));
   else
      UpdateFrameTitleForDocument(NULL);
}

SNI2-Feb-09 22:34

SNI

2-Feb-09 22:34

THanks for your reply but i need this for Winword document. Once we open any document (Test.doc), it will show its name at top left but I want to append my string to the cation of it "Test.doc (MyDoc)".
Do you have any idea how to do it?

SNI

SNI

Re: How to append text in word document caption?

Stuart Dootson2-Feb-09 23:16