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The primary operation of file processing consists of
opening one. In the strict sense, this means that you are initiating file
processing. This can be done in C by calling one of the fopen-variant
functions. Their syntax are:
FILE *fopen(const char *filename, const char *mode);
FILE *_wfopen(const wchar_t *filename, const wchar_t *mode);
To support Unicode, you should use the _wfopen()
version. In both cases, the first argument specifies the name of the file
you want to work on. If you pass just the name of the file, the compiler
will consider the directory of the current project. Otherwise, you can
provide the complete path of the project. The second argument specifies the
type of operation you want to perform. It is passed as a string.
When this function has been called, if it succeeds in
carrying its operation (we will see what types of operations are available),
it returns a FILE object. If the function fails, it returns NULL.
To create a file, you can pass the second argument of
fopen withe one the following values:
- "w": This specifies that you want to create a new file and write one
or more values to it. If the file exists already, the compiler will
replace it
- "w+": This means that you want to create a new file to read and
write one or more values to it. If the file exists already, its contents
will be replaced
The C language provides various functions you can use to
write values to a file. Some functions deal with strings while others are
supposed to handle any type of value. All functions that a FILE
object as argument. This is the object that holds the file.
To write a character to a FILE object, you can
call one of the following functions:
int fputc(int c, FILE *stream);
wint_t fputwc(wchar_t c, FILE *stream);
The first version of these functions takes a character
as argument. The second takes a wide character. The character would be
written to the file.
To write a string to a file, you can call one of these
functions:
int fputs(const char *str, FILE *stream);
int fputws(const wchar_t *str, FILE *stream);
To write other types of values to a file, you can call
one of the following functions:
size_t fwrite(const void *buffer,
size_t size,
size_t count,
FILE *stream);
To format a value before writting it to the file, you
can call one of these functions:
int fprintf(FILE *stream,
const char *format [, argument ]...);
int _fprintf_l(FILE *stream,
const char *format,
locale_t locale [, argument ]...);
int fwprintf(FILE *stream,
const wchar_t *format [, argument ]...);
int _fwprintf_l(FILE *stream,
const wchar_t *format,
locale_t locale [, argument ]...);
After using a FILE object, you should close it to
free the resources it was using. This is done by calling the fclose()
or _fcloseall() function. Their syntaxes are:
int fclose(FILE *stream);
int _fcloseall(void);
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 Practical
Learning: Writing Values to a Stream
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- On the dialog box, double-click the Save button
- Implement its event as follows:
void CPayrollPreparation1Dlg::OnBnClickedSave()
{
FILE *fstPayroll;
UpdateData(TRUE);
if( m_EmployeeNumber == 0 )
{
AfxMessageBox(L"You must enter an employee number.");
return;
}
if( m_EmployeeName == L"" )
{
AfxMessageBox(L"You must enter an employee name.");
return;
}
if( m_FileName == L"" )
{
AfxMessageBox(L"You must enter a name for the file.");
return;
}
fstPayroll = _wfopen(m_FileName + L".txt", L"w+");
if( fstPayroll == NULL )
AfxMessageBox(L"The file was not created");
else
{
fwprintf(fstPayroll, L"%d\n", m_EmployeeNumber);
fwprintf(fstPayroll, L"%s\n", m_EmployeeName);
fwprintf(fstPayroll, L"%.2f\n", m_GrossPay);
fwprintf(fstPayroll, L"%.2f\n", m_FederalIncomeTax);
fwprintf(fstPayroll, L"%s\n", m_FICATax);
fwprintf(fstPayroll, L"%.2f\n", m_FICARate);
fwprintf(fstPayroll, L"%.2f\n", m_StateTax);
fwprintf(fstPayroll, L"%s\n", m_TotalDeductions);
fwprintf(fstPayroll, L"%s\n", m_NetPay);
fclose(fstPayroll);
}
OnBnClickedReset();
}
- Execute the application to test
- Enter the employee number as 281548
- Enter the employee name as Paul Martin Sutters
- In the Gross Pay, enter 428.79
- In the Federal Income Tax, enter 71.46
- In the State Tax, enter 28.86
- Click Calculate
- Enter the file name as pms
- Click Save
- Close the dialog box and return to your programming environment
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Reading Values From a File
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in the same way, C provides various functions to read
values from a file. Like their writing counterparts, function readers take a
FILE object as argument.
To read a character from a FILE object, you can
call one of the following functions:
int fgetc(FILE *stream);
wint_t fgetwc(FILE *stream);
The first version reads a regular character and the
second reads a wide character. To read a string from a file, you can call
one of these functions:
char *fgets(char *str, int n, FILE *stream);
wchar_t *fgetws(wchar_t *str, int n, FILE *stream);
To read other types of values from a file, you can call
one of the following functions:
size_t fread(void *buffer,
size_t size,
size_t count,
FILE *stream);
To read values that were formatted, you can call one
of these functions:
int fscanf(FILE *stream,
const char *format [, argument ]...);
int _fscanf_l(FILE *stream,
const char *format,
locale_t locale [, argument ]...);
int fwscanf(FILE *stream,
const wchar_t *format [, argument ]...);
int _fwscanf_l(FILE *stream,
const wchar_t *format,
locale_t locale [, argument ]...);
int fscanf_s(FILE *stream,
const char *format [, argument ]...);
int _fscanf_s_l(FILE *stream,
const char *format,
locale_t locale [, argument ]... );
int fwscanf_s(FILE *stream,
const wchar_t *format [,
argument ]...);
int _fwscanf_s_l(FILE *stream,
const wchar_t *format,
locale_t locale [, argument ]...);
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Practical
Learning: Reading Values From a Stream
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- On the dialog box, double-click the Open button
- Implement its event as follows:
void CPayrollPreparation1Dlg::OnBnClickedOpen()
{
FILE *fstPayroll;
wchar_t EmployeeNumber[10];
wchar_t EmployeeName[60];
wchar_t GrossPay[10];
wchar_t FederalIncomeTax[10];
wchar_t FICATax[10];
wchar_t FICARate[10];
wchar_t StateTax[10];
wchar_t TotalDeductions[10];
wchar_t NetPay[10];
UpdateData(TRUE);
if( m_FileName == L"" )
{
AfxMessageBox(L"You must enter a name for the file.");
return;
}
fstPayroll = _wfopen(m_FileName + L".txt", L"r+");
if( fstPayroll == NULL )
AfxMessageBox(L"The file was not opened");
else
{
fgetws(EmployeeNumber, 10, fstPayroll);
fgetws(EmployeeName, 60, fstPayroll);
fgetws(GrossPay, 10, fstPayroll);
fgetws(FederalIncomeTax, 10, fstPayroll);
fgetws(FICATax, 10, fstPayroll);
fgetws(FICARate, 10, fstPayroll);
fgetws(StateTax, 10, fstPayroll);
fgetws(TotalDeductions, 10, fstPayroll);
fgetws(NetPay, 10, fstPayroll);
m_EmployeeNumber = _wtoi(EmployeeNumber);
m_EmployeeName = EmployeeName;
m_GrossPay = _wtof(GrossPay);
m_FederalIncomeTax = _wtof(FederalIncomeTax);
m_FICATax = FICATax;
m_FICARate = _wtof(FICARate);
m_StateTax = _wtof(StateTax);
m_TotalDeductions = TotalDeductions;
m_NetPay = NetPay;
UpdateData(FALSE);
fclose(fstPayroll);
}
}
- Execute the application to test it
- Enter the file name you had previously saved (pms)
- Click Open
- Close the dialog box and return to your programming environment
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