Understanding File I/O and Pointers in Database Design

1. VG101RECITATION 10 By TAs

2. CONTENTS • File I/O • Pointer in database design • About final exam • Q & A

3. FILE I/O #include <iostream> #include <fstream> #include <string> using namespace std; • 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"; • } • } File modes To use .c_str(), you need to include this Check before using

4. 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

5. 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

6. 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 • }

7. 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

8. 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

9. 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.

10. 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

11. 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

12. 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

13. Q & A • Any questions regarding • File I/O • Pointer • Exam