C / C++ / MFC

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/// Determines if a string is numeric
/// @param lpszText Text to determine numericy of
/// @param bSigned Determines if the function tests against signed or unsigned
/// numbers
/// @param nBase Numeric base to test against
/// @return True if the string is numeric, false if not
BOOL AfxIsNumeric(LPCTSTR lpszText, BOOL bSigned, int nBase)
{
        CHECK(lpszText != NULL, FALSE);

	// Convert the text to a number using the appropriate sign oriented
	// function
	LPTSTR lpszEndPtr = NULL;
	
	if (bSigned)
        _tcstoi64(lpszText, &lpszEndPtr, nBase);
    else
        _tcstoui64(lpszText, &lpszEndPtr, nBase);        
    
	// If the end pointer points to the null terminator, and it doesn't point to
	// the start of the text, (which would occur on an empty string) it's a
	// valid number
	return ((lpszEndPtr != lpszText) && (*lpszEndPtr == _T('\0')));
}

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#include <ctype.h>
 
bool IsNumeric(const char* p)
{
   int len = strlen(p);
   for (int i = 0; i < len; i++)
   {
      if (!isdigit(p[i]))
         return false;
   }
 
   return true;
}