VG101RECITATION 10By TAs

CONTENTS

File I/O Pointer in database design About final exam Q & A

FILE I/O void main() { ifstream infile; string fileName; cin >> fileName; infile.open(fileName.c_str(), ios::in); if(infile.fail()) { cout << "File " << fileName << " don't exist\n"; return; } else { cout << "Open file " << fileName << " success\

n"; } }

#include <iostream>#include <fstream>#include <string>using namespace std;

File modes

To use .c_str(), you need to include thisCheck before

using

FILE I/O File modes:

ios::in: opens a file for input

ios::out: opens a file for output

ios::app: appends all output to the end of the file

ios::ate: opens a file for output. If the file already exists, move to the end of the file.

ios::truct: discards the file’s contents if the file already exists. (default action for ios::out)

ios::binary: opens a file for binary input and output

FILE I/O

To use file modes:infile.open(fileName.c_str(), ios::in);

outfile.open(fileName.c_str(), ios::app);

You’ll need ios::app in most cases if you don’t want to overwrite your file data.

For more file modes or useful functions, look up ICP book, chapter 12, or use zhidao.baidu.com

FILE I/O Check before using any file stream object For input:

if(infile.fail()) // handle this exception

For output: ifstream infile; ofstream outfile; infile.open(fileName.c_str()); if( ! infile.fail() ) { cout << "File " << fileName << " already exists\

n"; cout << "Do you want to overwrite it?\n"; // handle user's reply }

FILE I/O

Alternative way for output check, using file modes:

If you want to append the new data to the end of the original file: outfile.open(fileName.c_str(), ios::app); if(outfile.fail()) // handle this exception

If you want to erase the old data in the file and write the new data into it: outfile.open(fileName.c_str()); if(outfile.fail()) // handle this exception

FILE I/O

String and char*: Some functions in C++ are written in C, so

these functions don’t recognize string class.

Use “ .c_str() ” to convert a string to char*infile.open(fileName.c_str(), ios::in);

You need to include <string> if you want to use this function as well as many other string functions

POINTER IN DATABASE DESIGN

Why we use pointer:

1. Pointer is just a variable of an address, it points to an space in memery, so it’s space saving.

2. A teacher may teach several courses, a classroom may be used by several courses. By using pointer, an object can be referenced by N other objects without creating N objects.

POINTER IN DATABASE DESIGN

3. If we change teacher A through pointer in course B, when we look through pointer in course C, teacher A is also changed

4. Using pointer allows us to dynamically add new objects by “new” keyword

5. Pointer form a web of relationship between unique objects, just as the real world

POINTER IN DATABASE DESIGN Where we use pointer and where we don’t:

Notice that we use pointer in class DepartmentT and SchoolT, but we don’t do that in class LocationT and InstructorT

School and department is a relationship between instructors, students, classrooms…while instructor is a real object.

Pointer is a tool to handle relationship or reference

ABOUT FINAL EXAM

All materials will be covered

Focus on skills in programming, not on basic concept

Computer part + paper part

Programming style will be graded

Q & A

Any questions regarding File I/O Pointer Exam