Programming II with C++ (CSNB244) Lab 10. Topics: Files and Stream

Transcription

1 Topics: Files and Stream In this lab session, you will learn very basic and most common I/O operations required for C++ programming. The second part of this tutorial will teach you how to read and write from a file. Files and Stream This requires another standard C++ library called fstream, which defines three new data types: Data type / Class Description Default Mode Parameter ofstream Represents the output file stream ios::out Use to create files Use to write information to files ifstream Represents the input file stream ios::in Use to read information from files fstream Represents the file stream generally - ofstream and ifstream Use to create files Use to write information to files Use to read information from files ios::in ios::out To perform file processing in C++, must include the following header files; #include <iostream> Now you can create some variables that can use these data types. Now you can declare some variables by using data types available in fstream library. Example; fstream afile; Flow to perform file processing; 1. Open file 2. Read/write operation from/to file 3. Close file Opening file A file must be opened before you can read from it or write to it. Either the ofstream or fstream object may be used to open a file for writing and ifstream object is used to open a file for reading purpose only. First, let s create a file, file1.txt at the same location as your C++ project folder. Right click New Text Document Rename it to file1.txt Following is the standard syntax for open() function THIS IS A PRE-DEFINED FUNCTION, which is a member of fstream, ifstream, and ofstream objects. void open(const char *filename, ios::openmode mode); First argument: const char *filename Usage: Specifies the name and location of the file to be opened Page 1 of 5

2 Second argument: ios::openmode mode Usage: defines the mode in which the file should be opened Mode Flag available; Mode Flag Description ios::app Append mode. All output to that file to be appended to the end. ios::ate Open a file for output and move the read/write control to the end of the file. ios::in Open a file for reading. ios::out Open a file for writing. ios::trunc If the file already exists, its contents will be truncated before opening the file. Can combine these flags by using OR. Example: wfile.open("../file1.txt", ios::out ios::trunc ); You can also exclude the second argument. rfile.open("../file1.txt "); Full codes example wfile.open("../file1.txt", ios::out ios::trunc); rfile.open("../file1.txt "); Note that: 1. file1.txt is a plain data file. The file name is file1. The extension of this file is.txt. the file name can be any name ( legal characters for file name). Checking the status of the opened file You can also check either your file is successfully opened by using the following statement; Objectofthefile.is_open() Example if(wfile.is_open()) cout << "File opened" << endl; Closing File When a C++ program terminates it automatically closes flushes all the streams, release all the allocated memory and close all the opened files. But it is always a good practice that a programmer should close all the opened files before program termination. Following is the standard syntax for close() function, which is a member of fstream, ifstream, and ofstream objects. Example: Page 2 of 5

3 wfile.close(); rfile.close(); Full codes example Programming II with C++ (CSNB244) Lab 10 wfile.open("../file1.txt", ios::out ios::trunc ); wfile.close(); rfile.open("../file1.txt"); rfile.close(); Write data to a file While doing C++ programming, you write information to a file from your program using the stream insertion operator (<<) just as you use that operator to output information to the screen. The only difference is that you use an ofstream or fstream object instead of the cout object. We have declared a variable named wfile previously. By using this variable, we have opened file1.txt. Now we will use the following sentences into file1.txt. Hello world void main() ofstream myfile; myfile.open("file1.txt"); if (myfile.is_open()) cout << "Successful open the file\n"; myfile << "Hello World.\n"; myfile.close(); Exercise 1 Write data to file Write a program to print the following sentences into a text file. This is my first exercise on file stream. Hoorey! I managed to write this text into a file. The program should check whether the opening file is exists or not. It should notify the current status of file opening, writing and closing to the user. Page 3 of 5

4 Read the content of a file Note: You must ensure the file that you want to read MUST exists. getline function is a function to read line by line until the end of file. getline(objectfile,varibletostorestring)) // reading a text file #include <iostream> string word; ifstream myfile("read.txt"); if (myfile.is_open()) while (getline(myfile, word)) cout << word << '\n'; myfile.close(); cout << "Unable to open file"; Read and write in one program fstream rwfile; string line; rwfile.open("../myfile2.txt"); //writing to file if (rwfile.is_open()) cout << "Status File Opened" << endl; cout << "Writing to file...\n"; rwfile << "Of course i am happy! \nthis my first program.\n"; rwfile.close(); rwfile.open("../myfile2.txt"); //read from file if (rwfile.is_open()) Page 4 of 5

5 cout << "Status File Opened" << endl; while (getline(rwfile, line)) cout << line << endl; rwfile.close(); Page 5 of 5