Microsoft KB Archive/131993

= PRB: DDX Displays Float/Double in Exponential Format =

Article ID: 131993

Article Last Modified on 11/21/2006

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APPLIES TO

 Microsoft Foundation Class Library 4.2, when used with:  Microsoft Visual C++ 1.5 Professional Edition

 Microsoft Visual C++ 1.51

 Microsoft Visual C++ 1.52 Professional Edition

 Microsoft Visual C++ 5.0 Learning Edition</li></ul>

 Microsoft Visual C++ 2.0 Professional Edition</li></ul>

 Microsoft Visual C++ 2.1</li></ul>

 Microsoft Visual C++ 4.0 Professional Edition</li></ul> </li></ul>

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This article was previously published under Q131993

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1.50 1.51 1.52 | 2.00 2.10 4.00 WINDOWS | WINDOWS NT kbprb

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SYMPTOMS
A floating point or double value may appear unexpectedly in exponential format (scientific notation) in an edit control in an MFC dialog box or formview. This may happen if the following is used to associate a DDX variable of type float or double with the edit control:

DDX_Text(CDataExchange* pDX, int nIDC, float& value)

DDX_Text(CDataExchange* pDX, int nIDC, double& value)

This is true even though the documentation indicates that DDX_Text produces the exponential format only when the decimal point format is not possible.

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CAUSE
In Visual C++ for Windows version 1.5x and Visual C++ 32-bit Edition, version 2.x, the edit control value appears in exponential format because DDX_Text uses gcvt for floats and doubles. The gcvt C Run-time function returns exponential format for all numbers of the format 0.0<x>, where x is any sequence of digits.

In Visual C++ 32-bit Edition, version 4.0, it is less likely that an edit control's value will appear in exponential format, but it is still possible. DDX_Text calls the internal C Run-time function _stprintf with a format specifier of "%.*g" and a precision of either FLT_DIG for floats or DBL_DIG for doubles. As their underlying implementations are the same, _stprintf follows the same rules as does printf. As the Visual C++ 4.0 Books Online point out in the "printf Type Field Characters" topic, the field type "g" yields a

Signed value printed in f or e format, whichever is more compact

for the given value and precision. The e format is used only when

the exponent of the value is less than -4 or greater than or equal

to the precision argument. ..

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RESOLUTION
To work around this behavior, rewrite the DDX_Text function to use fcvt. It's a good idea to create a function that calls fcvt and does all the formatting of the string returned from fcvt. You would call this function from your own DDX_Text.

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STATUS
This behavior is by design.

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MORE INFORMATION
The definitions for the float and the double versions of DDX_Text can be found in the MFC source file DLGFLOAT.CPP in the MFC\SRC subdirectory of the main Visual C++ product directory. We recommend that you familiarize yourself with these functions and those that they call to obtain a solid understanding of their implementations. This will help you rewrite them in the event you decide to do so.

The following provides two sample code backbones for the resolution mentioned above, one for Visual C++ versions 1.5 through 2.2 and one for Visual C++ 4.0. Sample Code I assumes that you have an edit control with ID IDC_EDIT1 on a form view with an associated DDX variable of type float called m_eFloat. The custom DDX_Text function is called DDX_MyFloatText. It uses the function double_to_char, which reads in a double and returns a character string representing the double. Sample Code II makes similar assumptions but supports an edit control with type double as well. The custom functions are called DDX_MyFloatText and DDX_MyDoubleText. They both depend on the internal CRT function _stprintf.

Sample Code I - for Visual C++ 16-bit 1.5x and Visual C++ 32-bit 2.x
// include header files.
 * 1) include <stdio.h>
 * 2) include <stdlib.h>
 * 3) include <string.h>

//............ //............ //............

// The function prototypes void AFXAPI DDX_MyFloatText(CDataExchange* pDX, int nIDC,                           float& value); char *double_to_char (double number); static BOOL PASCAL NEAR _AfxSimpleFloatParse(const char* pszText,                                            double& d);

//............ //............ //............

// Change the DoDataExchange to use DDX_MyFloatText void CDlgfloatView::DoDataExchange(CDataExchange* pDX) {   CFormView::DoDataExchange(pDX); //AFX_DATA_MAP }

//............ //............ //............

// Implementation of DDX_MyFloatText and other helper functions void AFXAPI DDX_MyFloatText(CDataExchange* pDX, int nIDC,                           float& value) {   HWND hWndCtrl = pDX->PrepareEditCtrl(nIDC); char szT[64];

if (pDX->m_bSaveAndValidate) {       ::GetWindowText(hWndCtrl, szT, sizeof(szT)); double d;       if (!_AfxSimpleFloatParse(szT, d)) {           AfxMessageBox(AFX_IDP_PARSE_REAL); pDX->Fail;           // throws exception }       value = (float)d; }   else {       char * pszCvt = double_to_char(value); if (pszCvt) {           int nNewLen = lstrlen(pszCvt); char szOld[64]; // fast check to see if text really changes (reduces           // flash in controls) if (nNewLen > sizeof(szOld) ||               ::GetWindowText(hWndCtrl, szOld, sizeof(szOld)) !=                                nNewLen ||                lstrcmp(szOld, pszCvt) != 0) {               // change it                ::SetWindowText(hWndCtrl, pszCvt); }           delete pszCvt; }       else {           TRACE("DDX_MyFloatText failed to convert float                  value.\n"); pDX->Fail;           // throws exception }   } }


 * 1) define PRECISION 5

char *double_to_char (double number) {  char *buffer,*temp ;

int decimal_spot, sign, count, current_location = 0, zeropos;

temp = _fcvt (number, PRECISION, &decimal_spot, &sign) ;

if (strlen (temp) > PRECISION) buffer = new char[(strlen (temp) + 3)]; else buffer = new char[(PRECISION + 3)];

if (buffer == NULL) {     OutputDebugString("Memory allocating attempt has failed in"                        "'double_to_char'\n") ; return (NULL) ; }

/* Add negative sign if required. */

if (sign) buffer [current_location++] = '-' ;

/* Place decimal point in the correct location. */

if (decimal_spot > 0) {     strncpy (&buffer [current_location], temp, decimal_spot) ; buffer [decimal_spot + current_location] = '.' ; strcpy (&buffer [decimal_spot + current_location + 1],                     &temp [decimal_spot]) ;

}  else {     buffer [current_location++] = '0'; buffer [current_location] = '.' ; for(count = current_location-(1+sign);         count<abs(decimal_spot); count++) buffer [count + (current_location+1)] = '0' ; strcpy (&buffer [count + (current_location+1)], temp) ; }

zeropos = strlen(buffer)-3; if (buffer[zeropos+2] == '0') {

while (buffer[zeropos--] == '0') buffer[zeropos+2] = '\0';

if (buffer[zeropos+1] != '.') buffer[zeropos+2] = '\0';

}  return (buffer) ; }

static BOOL PASCAL NEAR _AfxSimpleFloatParse(const char* pszText,                                            double& d) { ASSERT(pszText != NULL); while (*pszText == ' ' || *pszText == '\t') pszText++;

ASSERT(!::IsDBCSLeadByte(*pszText)); char chFirst = pszText[0]; d = strtod(pszText, (char**)&pszText); if (d == 0.0 && chFirst != '0') return FALSE;  // could not convert while (*pszText == ' ' || *pszText == '\t') pszText++; ASSERT(!::IsDBCSLeadByte(*pszText));

if (*pszText != '\0') return FALSE;  // not terminated properly

return TRUE; }

Sample Code II - for Visual C++ 32-bit 4.0 --

//............ //............ //............

// The function prototypes void AFXAPI DDX_MyFloatText(CDataExchange* pDX, int nIDC, float& value); void AFXAPI DDX_MyDoubleText(CDataExchange* pDX, int nIDC, double& value); void AFXAPI _MyAfxTextFloatFormat(CDataExchange* pDX, int nIDC,                                 void* pData, double value, int nSizeGcvt,                                  int nSizeType);

//............ //............ //............

// Change the DoDataExchange to use DDX_MyFloatText or DDX_MyDoubleText void CDlgfloatView::DoDataExchange(CDataExchange* pDX) {   CFormView::DoDataExchange(pDX); //AFX_DATA_MAP }

//............ //............ //............

// Implementation of DDX_MyFloatText, DDX_MyDoubleText and //  _MyAfxTextFloatFormat.


 * 1) include <float.h>
 * 2) define PRECISION 8

void AFXAPI DDX_MyFloatText(CDataExchange* pDX, int nIDC, float& value) {   _MyAfxTextFloatFormat(pDX, nIDC, &value, value, PRECISION, FLT_DIG); }

void AFXAPI DDX_MyDoubleText(CDataExchange* pDX, int nIDC, double& value) {   _MyAfxTextFloatFormat(pDX, nIDC, &value, value, PRECISION, DBL_DIG); }

void AFXAPI _MyAfxTextFloatFormat(CDataExchange* pDX, int nIDC,                                 void* pData, double value, int nSizeGcvt,                                  int nSizeType) {   ASSERT(pData != NULL);

HWND hWndCtrl = pDX->PrepareEditCtrl(nIDC);

// Make sure your buffer is big enough. Strings returned by       // _stprintf using the "f" specifier tend to be longer // than those returned using the "g" specifier. TCHAR szBuffer[64];

if (pDX->m_bSaveAndValidate) {       ::GetWindowText(hWndCtrl, szBuffer, _countof(szBuffer)); double d;       if (!AfxSimpleFloatParse(szBuffer, d)) {           AfxMessageBox(AFX_IDP_PARSE_REAL); pDX->Fail;           // throws exception }       if (nSizeType == FLT_DIG) *((float*)pData) = (float)d; else *((double*)pData) = d;   } else {       _stprintf(szBuffer, _T("%.*f"), nSizeGcvt, value); AfxSetWindowText(hWndCtrl, szBuffer); } }

Additional query words: 1.50 2.00 2.10 4.00

Keywords: kbcode kbdlg kbprb KB131993

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