如何仅删除非数字字符串的第一个字符？ （MFC，C ++）

Question

I want to remove only the first character in a string that is NOT a digit. The first character can be anything from 'A' to 'Z' or it may be a special character like '&' or '#'. This legacy code is written in MFC. I've looked at the CString class but cannot figure out how to make this work.

I have strings that may look like any of the following: J22008943452GF or 22008943452GF or K33423333333IF or 23000526987IF or #12000895236GF. You get the idea by now.

My dilemma is I need to remove the character in the first position of all the strings, but not the strings that starts with a digit. For the strings that begin with a digit, I need to leave them alone. Also, none of the other characters in the string should not be altered. For example the 'G', 'I' or 'F' in the later part of the string should not be changed. The length of the string will always be 13 or 14 digits.

Here is what I have so far.

CString GAbsMeterCalibration::TrimMeterSNString (CString meterSN)
 {
   meterSN.MakeUpper();
   CString TrimmedMeterSNString = meterSN;
   int strlength = strlen(TrimmedMeterSNString);

    if (strlength == 13)
       {
         // Check the first character anyway, even though it’s
         //  probably okay.  If it is a digit, life’s good.
         //  Return unaltered TrimmedMeterSNString;
       }

    if (strlength == 14)) 
       {
           //Check the first character, it’s probably going
           //  to be wrong and is a character, not a digit.  

           // if I find a char in the first postion of the
           //    string, delete it and shift everything to the 
           //    left.  Make this my new TrimmedMeterSNString
           //  return altered TrimmedMeterSNString;
        }
 }

The string lengths are checked and validated before the calls.

From my investigations, I've found that MFC does not have a regular expression class. Nor does it have the substring methods.

Answer 1

From what I understand, you want to remove the first letter if it is not a digit. So you may make this function simpler:

CString GAbsMeterCalibration::TrimMeterSNString(CString meterSN)
{
    meterSN.MakeUpper();

    int length = meterSN.GetLength();

    // just check the first character is always a digit else remove it
    if (length > 0 && unsigned(meterSN[0] - TCHAR('0')) > unsigned('9'))
    {
        return meterSN.Right(length - 1);
    }

    return meterSN;
}

I am not using function isdigit instead of the conditional trick with unsigned because CString uses TCHAR which can be either char or wchar_t .

Answer 2

The solution is fairly straight forward:

CString GAbsMeterCalibration::TrimMeterSNString(CString meterSN) {
    meterSN.MakeUpper();
    return _istdigit(meterSN.GetAt(0)) ? meterSN :
                                         meterSN.Mid(1);
}

The implementation can be compiled for both ANSI and Unicode project settings by using _istdigit . This is required since you are using CString , which stores either MBCS or Unicode character strings. The desired substring is extracted using CStringT::Mid .

(Note that CString is a typedef for a specific CStringT template instantiation, depending on your project settings.)

Answer 3

How about:

CString GAbsMeterCalibration::TrimMeterSNString (CString meterSN)
 {
   meterSN.MakeUpper();
   CString TrimmedMeterSNString = meterSN;
   int strlength = strlen(TrimmedMeterSNString);

    if (std::isdigit(TrimmedMeterSNString.GetAt(0)) )
       {
         // Check the first character anyway, even though it’s
         //  probably okay.  If it is a digit, life’s good.
         //  Return unaltered TrimmedMeterSNString;
       }
 }

Answer 4

CString test="12355adaddfca";
if((test.GetAt(0)>=48)&&(test.GetAt(0)<=57))
{
//48 and 57 are ascii values of 0&9, hence this is a digit
//do your stuff
 //CString::GetBuffer may help here??
}
else
{
//it is not a digit, do your stuff

}

Compare the ascii value of the first position in the string and you know if it's a digit or not..

Answer 5

I don't know if you've tried this, but, it should work.

CString str = _T("#12000895236GF");
//  check string to see if it starts with digit.
CString result = str.SpanIncluding(_T("0123456789"));
//  if result is empty, string does not start with a number
//  and we can remove the first character.  Otherwise, string
//  remains intact.
if (result.IsEmpty())
    str = str.Mid(1);

Seems a little easier than what's been proposed.