Random File Access. 1. Random File Access

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1 Random File Access 1. Random File Access In sequential file access, the file is read or written sequentially from the beginning. In random file access, you can skip around to various points in the file to read or write its contents. This can be useful when your file is full of records, and you wish to retrieve a specific record. Rather than reading all of the records until you get to the one you want, you can skip directly to the record you wish to retrieve. Random file access is done by manipulating the file pointer using the seekg() function (for input) and seekp() function (for output). In case you are wondering, the g stands for get and the p for put. The seekg() and seekp() functions take two parameters. The first parameter is an offset that determines how many bytes to move the file pointer. The second parameter is an ios flag that specifies what the offset parameter should be offset from. IOS seek flag beg cur end Meaning The offset is relative to the beginning of the file (default) The offset is relative to the current location of the file pointer The offset is relative to the end of the file A positive offset means move the file pointer towards the end of the file, whereas a negative offset means move the file pointer towards the beginning of the file. For example: inf.seekg(24); // move to 24th byte in file (default is from the beginning) inf.seekg(14, ios::cur); // move forward 14 bytes inf.seekg(-18, ios::cur); // move backwards 18 bytes inf.seekg(22, ios::beg); // move to 22nd byte in file inf.seekg(-28, ios::end); // move to the 28th byte before end of the file inf.seekg(0, ios::beg); // move to beginning of file inf.seekg(0, ios::end); // move to end of file Two other useful functions are tellg() and tellp(), which return the absolute position of the file pointer. This can be used to determine the size of a file: ifstream inf("sample.dat"); inf.seekg(0, ios::end); // move to end of file cout << inf.tellg(); The get() function performs unformatted input operations. The put(c) function writes the character c to the output buffer at the current put position and increases the put pointer to point to the next character. Page 1 of 6 Copyright Enoch Hwang 2009

2 Example 1 #include <iostream> #include <fstream> #include <cstdlib> #include <string> // needed for the getline using namespace std; int main(){ ifstream InFile("Sample.dat"); // If we couldn't open the input file stream for reading if (!InFile){ // Print an error and exit cerr << "Uh oh, Sample.dat could not be opened for reading!" << endl; exit(1); string strdata; InFile.seekg(5); // move to 5th character // Get the rest of the line and print it getline(infile, strdata); cout << strdata << endl; InFile.seekg(8, ios::cur); // move 8 more bytes into file // Get rest of the line and print it getline(infile, strdata); cout << strdata << endl; InFile.seekg(-15, ios::end); // move 15 bytes before end of file // Get rest of the line and print it getline(infile, strdata); cout << strdata << endl; return 0; Page 2 of 6 Copyright Enoch Hwang 2009

3 Example 2: #include <iostream> #include <fstream> #include <cstdlib> using namespace std; int main{ int byten = 10; // open file for binary I/O fstream out("sample.dat", ios::in ios::out ios::binary); if(!out){ cout << "Cannot open file.\n"; return 1; // position write pointer out.seekp(byten, ios::beg); // write a byte out.put('x'); // close the file out.close(); return 0; The fstream class is capable of both reading and writing a file at the same time almost! The big caveat here is that it is not possible to switch between reading and writing arbitrarily. Once a read or write has taken place, the only way to switch between the two is to perform a seek operation. If you don t actually want to move the file pointer, you can always seek to the current position: iofile.seekg(iofile.tellg(), ios::beg); // seek to current file position Page 3 of 6 Copyright Enoch Hwang 2009

4 Example 3: // Program that opens a file, reads it s contents, // and changes all the vowels it finds to a # symbol int main(){ // need to specify both in and out because we're using fstream fstream iofile("sample.dat", ios::in ios::out); // If we couldn't open iofile, print an error if (!iofile){ cerr << "Uh oh, Sample.dat could not be opened!" << endl; exit(1); char chchar; // we're going to do this character by character while (iofile.get(chchar)){ switch (chchar){ // If we find a vowel case 'a': case 'e': case 'i': case 'o': case 'u': case 'A': case 'E': case 'I': case 'O': case 'U': // While there's still data to process // Back up one character iofile.seekp(-1, ios::cur); // Because we did a seek, we can now safely do a write, // so let's write a # over the vowel iofile << '#'; // Now we want to go back to read mode so the next call // to get() will perform correctly. // We'll seekg() to the current location // because we don't want to move the file pointer. iofile.seekg(iofile.tellg(), ios::beg); break; return 0; Page 4 of 6 Copyright Enoch Hwang 2009

5 Example 4 // writing two integers fstream iofile("integers.dat", ios::out ios::binary ios::trunc); int n = 23; iofile.write(reinterpret_cast<const char *>(&n), sizeof(int)); n = 58; iofile.write(reinterpret_cast<const char *>(&n), sizeof(int)); iofile.close(); // reading two integers iofile.open("integers.dat", ios::in ios::binary); iofile.read(reinterpret_cast<char *>(&n), sizeof(int)); iofile.read(reinterpret_cast<char *>(&n), sizeof(int)); iofile.close(); // another way of reading two integers iofile.open("integers.dat", ios::in ios::binary); char bytes[4]; iofile.read((char *)bytes, 4);// an int uses 4 bytes of storage (usually) n = bytes[0] (bytes[1] << 8) (bytes[2] << 16) (bytes[3] << 24); iofile.read((char *)bytes, 4); n = bytes[0] (bytes[1] << 8) (bytes[2] << 16) (bytes[3] << 24); iofile.close(); Page 5 of 6 Copyright Enoch Hwang 2009

6 2. Exercises (Problems with an asterisk are more difficult) 1. Write a program to write a data file with the four lines This is line 1 This is line 2 This is line 3 This is line 4 2. Try Example 1 above reading from the file created in Exercise Try Example Try Example Write a program to store the 10 integers (21, 22, 23, 24, 25, 26, 27, 28, 29, 30) into a random file. 6. Write a program to read in any given integer from the file created in 5 above. The user enters the integer number to be read. For example, the user types in a 4, your program will read in the 4 th number and prints out Same as 5 and 6 above but instead of storing 10 integers, store 10 person records containing the name, address, and phone number. 8. Create a simple address book. The program allows the user to add a person record, find a person record given the person s name, find a person record given the record number, change a person record, and delete a person record. The data is to be stored in a file and is persistent across different runs of the program. Page 6 of 6 Copyright Enoch Hwang 2009

C++ Binary File I/O. C++ file input and output are typically achieved by using an object of one of the following classes:

C++ Binary File I/O. C++ file input and output are typically achieved by using an object of one of the following classes: C++ Binary File I/O C++ file input and output are typically achieved by using an object of one of the following classes: ifstream for reading input only. ofstream for writing output only. fstream for reading