File I/O Christian Schumacher, Info1 D-MAVT 2013

Transcription

1 File I/O Christian Schumacher, Info1 D-MAVT 2013 Input and Output in C++ Stream objects Formatted output Writing and reading files References

2 General Remarks I/O operations are essential for computer programs. Input operations let us communicate with the program Output operations let the program communicate with us Typical input/output devices: Keyboard, mouse, screen and files C++ provides set of streams Classes that make I/O communication easy and OS independent 2

3 Stream Objects and Classes A stream is a transfer of data elements between a program and I/O devices. Every stream has a direction (I/O) and a device connected to it: E.g. - Input (direction) stream from file (device) E.g. - Output (direction) stream to console (device) Previously seen C++ classes and stream objects are: istream cin: input variable hooked up to the keyboard. ostream cout: output variable hooked up to the screen. Stream objects operating on files are instances of ifstream and ofstream classes.

4 C++ Standard I/O #include <iostream> #include <string> using namespace std; int main() cout << What s your name: << endl; string fname; cin >> fname; cout << Hello World << fname <<! << endl; return 0; C++ library includes the header file <iostream> definining stdin and stdout stream objects. stdin of a program is the keyboard, accessed through cin. stdout of a program is the screen, accessed through cout.

5 Formatted Output Additional Resources: (1) (2) C++ Primer, Ch. 17 Manipulate output data to make it easier to read or post-process more readable than 1.299e-3 C++ header <iomanip> provides manipulators to format output: #include <iomanip> Decimal precision cout.precision(3);... << setprecision(3) <<... Fill character cout.fill( * );... << setfill( * ) <<... double pi = ; cout << pi; Output: cout << setprecision(3) << pi; Output: 3.14 cout.precision(4); cout << pi; Output:

6 Streams and Files <fstream> Streaming data from/to files (i.e. read/write) is almost identical to console I/O. Except file streams have to be instantiated and opened by the user-program. // Instantiate ofstream object fout ofstream fout; fout.open( myfile.dat ); fout << Hello ; // writing to file cout << Hello ; // writing to screen

7 fstream Important I/O functions Open/close Files void open(const char* filename); void close(); Check for success after opening bool is_open(); End of file bool eof(); Extract formatted data: >> Insert data with format: << 7

8 Step-by-Step Streaming to Files 1. Include header: #include <fstream> 2. Create instance of fstream object: ofstream fout; 3. Open file: fout.open("mydat"); 4. Transfer data from/to file: fout << Hello World!\n ; 5. Close file: fout.close(); 8

9 Example: Simple write and read #include <iostream> #include <fstream> using namespace std; int main() ofstream fout; #include <iostream> #include <fstream> using namespace std; int main() ifstream fin; fout.open("mydat"); if(!fout.is_open()) cerr << IO error\n"; return 1; int a = 12345; fout << a; //write to file fout.close(); fin.open("mydat"); if(!fin.is_open()) cerr << IO error\n"; return 1; int a; fin >> a; // read from file fin.close(); return 0; 9 return 0;

10 Read/write character by character Useful functions for scanning files char-by-char: ostream& put(char c); int get(); ofstream fout; fout.open("file.txt"); fout.put('t'); fout.put('e'); fout.put('s'); fout.put('t'); fout.close(); ifstream fin; fin.open("file.txt"); char in[4]; for (int i = 0; i < 4; i++) in[i] = fin.get(); fin.close(); <fstream> offers many other functions 10

11 Recap: References Arguments of functions can be declared as a reference Add & in front of variable name void test(int zahl) void test(int &zahl) Instead of copying the value of a variable that is passed, the reference is directly linked it As with pointers, every change to the reference changes the passed variable References are like static pointers without the need of dereferencing 11

12 References References can also be declared within functions Syntax: type &refname = varname; int a = 4; int &b = a; // b is a reference to a b = 5; // a is now 5 Referenced variable has to be specified when defining the variable, cannot be changed Original variable and its reference are in every way identical Give same address when using & operator 12

13 Function Overloading Multiple functions can have the same name if they can be distinguished Number of arguments and/or type of arguments have to be different float add(float a, float b) return a + b; float add(float a, float b, float c) return a + b + c; int add(int a, int b) return a + b; Compiler chooses correct function from function call add(1.0f, 2.0f) -> float add(float a, float b) add(1.0f, 2.0f, 3.0f) -> float add(float a, float b, float c) add(1, 2) -> int add(int a, int b) 13

14 Übung 9 Aufgabe 2: Vektor normalisieren struct Vector float x; float y; float z; ; Vector v; v.x = 1.0f; v.y = 2.5f; v.z = 1.2f; 14