CSc Introduc/on to Compu/ng. Lecture 19 Edgardo Molina Fall 2011 City College of New York

Transcription

1 CSc Introduc/on to Compu/ng Lecture 19 Edgardo Molina Fall 2011 City College of New York 18

2 Standard Device Files Logical file object: Stream that connects a file of logically related data to a program Physical file object: Stream that connects to hardware device such as keyboard, screen, or printer Actual physical device assigned to your program for data entry is formally called standard input file cin method calls are routed to this standard input file cout method calls are written to a device that has been assigned as standard output file C++ for Engineers and Scientists, Third Edition 19

3 Other Devices Keyboard, display, error, and log streams are connected automatically to stream objects cin, cout, cerr, and clog when iostream header file is included in program C++ for Engineers and Scientists, Third Edition 20

4 Using FileStreams File streams are similar to standard input streams >> for reading from ifstream << for writing to ofstream Example: ofstream fout( myfile.txt ); fout >> text that will be saved in the file >> endl; fout.close() C++ for Engineers and Scientists, Third Edition 21

5 Random File Access File access: Refers to process of retrieving data from a file Two types of file access Sequential file access Random file access File organization: Refers to the way data is stored in a file The files you have used and will continue to use have a sequential organization, meaning characters in file are stored in a sequential manner C++ for Engineers and Scientists, Third Edition 22

6 Random File Access (continued) Each open file has been read in a sequential manner, meaning characters are accessed one after another, which is called sequential access Although characters are stored sequentially, they don t have to be accessed in same way In random access, any character in opened file can be read without having to read all characters stored ahead of it first To provide random access, each ifstream object creates a file position marker automatically This markers is a long integer representing an offset from the beginning of file C++ for Engineers and Scientists, Third Edition 23

7 Random File Access (continued) Table 8.4 File Position Marker Methods C++ for Engineers and Scientists, Third Edition 24

8 Random File Access (continued) seek() method allows programmer to move to any position in file Character s position is referred to as its offset from the start of file C++ for Engineers and Scientists, Third Edition 25

9 #include <iostream> #include <fstream> #include <string> #include <cstdlib> using namespace std; Program 8.7 int main() { string filename = "test.dat"; char ch; long offset, last; ifstream infile(filename.c_str()); if (infile.fail()) // check for successful open { cout << "\nthe file was not successfully opened" << "\n Please check that the file currently exists" << endl; exit(1); } infile.seekg(0l,ios::end); // move to the end of the file last = infile.tellg(); // save the offset of the last character for(offset = 1L; offset <= last; offset++) { infile.seekg(-offset, ios::end); ch = infile.get(); cout << ch << " : "; } infile.close(); cout << endl; } return C++ for 0; Engineers and Scientists, Third Edition 26

10 ios::beg ios::curr ios::end Random Access Modes C++ for Engineers and Scientists, Third Edition 27

11 Appending #include <iostream> #include <fstream> #include <string> #include <cstdlib> using namespace std; int main() { string filename = "test.dat"; //ofstream outfile(filename.c_str()); ofstream outfile(filename.c_str(), ios_base::app); cout << "Pointer at " << outfile.tellp() << endl; outfile << "Testing \n"; outfile.close(); } return 0; C++ for Engineers and Scientists, Third Edition 28

12 File Streams as Function Arguments A file steam object can be used as a function argument The function s formal parameter must be a reference to the appropriate stream, either ifstream& or ofstream& Examples: inout(), getopen() C++ for Engineers and Scientists, Third Edition 29

13 #include <iostream> #include <fstream> #include <cstdlib> #include <string> using namespace std; Program 8.8 void inout(ofstream&); // function prototype int main() { string fname = "list.dat"; // here is the file we are working with ofstream outfile; outfile.open(fname.c_str()); } if (outfile.fail()) // check for a successful open { cout << "\nthe output file " << fname } << " was not successfully opened" << endl; exit(1); inout(outfile); // call the function return 0; void inout(ofstream& fileout) { const int NUMLINES = 5; // number of lines of text string line; int count; cout << "Please enter five lines of text:" << endl; for (count = 0; count < NUMLINES; count++) { getline(cin,line); fileout << line << endl; } cout << "\nthe file has been successfully written. << endl; C++ for Engineers and Scientists, Third Edition return; 30 }

14 Common Programming Errors Forgetting to open file before attempting to read from it or write to it Using file s external name in place of internal file stream name when accessing a file Opening file for output without first checking that file with the same name already exists Opening existing file for output overwrites that file Not understanding that end of file is detected only after EOF marker has been read and passed over C++ for Engineers and Scientists, Third Edition 31

15 Common Programming Errors (continued) Attempting to detect end of file by using character variables for EOF marker Any variable used to accept EOF marker must be declared an integer variable Using integer argument with seekg() and seekp () functions This offset must be long integer constant or variable C++ for Engineers and Scientists, Third Edition 32

16 Chapter Summary Data file is any collection of data stored together in an external storage medium under a common name Data file is connected to file stream by using fstream open method File can be opened in input and output mode All file streams must be declared as objects of ifstream or ofstream class In addition to any files opened in a function, standard stream objects cin, cout, and cerr are declared and opened automatically when a program runs C++ for Engineers and Scientists, Third Edition 33