Reading One Item After Another From a File Is Called ____ Access.

Writing Data to Files

The programs you accept written so far require the user to reenter data each time the program runs, because data kept in variables and control properties are stored in RAM and disappears once the plan stops running. If a program is to retain information between the times it runs, it must have a mode of saving it. Data is saved in a file, which is ordinarily stored on a computer'south disk. Once the data is saved in a file, information technology will remain there after the plan stops running. Data that is stored in a file can exist then retrieved and used at a later time. Programmers usually refer to the process of saving data in a file as writing data to the file. When a slice of data is written to a file, it is copied from a variable in RAM to the file. The procedure of retrieving information from a file is known as reading data from the file. When a piece of data is read from a file, it is copied from the file into a variable in RAM.

There's 3 ways to create programs that write data to files and read data from files. When a file is used by a program, three steps must be taken.

1. Open the file — Opening a file creates a connection between the file and the program. Opening an output file usually creates the file on the disk and allows the program to write information to it. Opening an input file allows the program to read data from the file.
2. Process the file — Data is either written to the file (if it is an output file) or read from the file (if it is an input file).
3. Close the file — After the plan is finished using the file, the file must be closed. Closing a file disconnects the file from the program.

File Access Methods

In that location are 2 general ways to access data stored in a file: sequential access and straight admission. When you lot work with a sequential access file, you admission data from the beginning of the file to the cease of the file. If you want to read a slice of information that is stored at the very end of the file, you have to read all of the data that comes before it — you cannot jump directly to the desired data. This is similar to the fashion cassette tape players work. If you want to listen to the last song on a cassette record, you take to either fast-frontwards over all of the songs that come up before it or listen to them. There is no way to spring directly to a specific vocal.

When you piece of work with a random access file (also known as a direct access file), you lot can leap direct to whatsoever slice of data in the file without reading the data that comes earlier it. This is similar to the way a CD player or an MP3 player works. Y'all tin jump direct to whatever vocal that you want to listen to.

In order for a plan to work with a file on the computer's disk, the program must create a file stream object in retentivity. A file stream object is an object that is associated with a specific file and provides a way for the program to work with that file. Information technology is called a "stream" object because a file can be thought of equally a stream of information.

File stream objects piece of work very much like the cin and cout objects. A stream of data may exist sent to cout, which causes values to exist displayed on the screen. A stream of data may be read from the keyboard past cin, and stored in variables. Besides, streams of information may be sent to a file stream object, which writes the data to a file. When data is read from a file, the data flows from the file stream object that is associated with the file, into variables.

Setting Upwards a Plan for File Input/Output

Only as cin and cout require the iostream file to be included in the program, C++ file access requires another header file. The file fstream contains all the declarations necessary for file operations. Information technology is included with the following statement:

#include <fstream>

The fstream header file defines the data types ofstream, ifstream, and fstream. Earlier a C++ program tin can work with a file, it must ascertain an object of one of these data types. The object will be "linked" with an actual file on the computer's disk, and the operations that may be performed on the file depend on which of these three data types you selection for the file stream object. Tabular array beneath lists and describes the file stream data types.

Creating a File Object and Opening a File

Before data can be written to or read from a file, the post-obit things must happen:

• A file stream object must be created
• The file must exist opened and linked to the file stream object.

The following code shows an example of opening a file for input (reading).

          ifstream inputFile;
inputFile.open("Customers.txt");        

The offset argument defines an ifstream object named inputFile. The 2nd statement calls the object's open member function, passing the string "Customers.txt" as an argument. In this statement, the open member function opens the Customers.txt file and links it with the inputFile object. After this code executes, you will be able to apply the inputFile object to read information from the Customers.txt file.

The following code shows an case of opening a file for output (writing).

          ofstream outputFile;
outputFile.open("Employees.txt");        

The first argument defines an ofstream object named outputFile. The 2nd statement calls the object's open member function, passing the cord "Employees.txt" as an statement. In this statement, the open member function creates the Employees.txt file and links information technology with the outputFile object. After this code executes, yous will be able to utilise the outputFile object to write information to the Employees.txt file. It'southward of import to remember that when you call an ofstream object'due south open up member office, the specified file will be created. If the specified file already exists, it will be erased, and a new file with the aforementioned proper name will be created.

It is possible to ascertain a file stream object and open a file in one statement. Hither is an example:

ifstream inputFile("Customers.txt");

This statement defines an ifstream object named inputFile and opens the Customer.txt file. Here is an instance that defines an ofstream object named outputFile and opens the Employees.txt file:

ofstream outputFile("Employees.txt");

Closing a File

The opposite of opening a file is closing it. Although a program'southward files are automatically closed when the program shuts downwardly, it is a proficient programming practice to write statements that close them. Hither are two reasons a program should close files when it is finished using them:

• Well-nigh operating systems temporarily shop data in a file buffer before it is written to a file. A file buffer is a small "property section" of retentiveness that file-leap data is first written to. When the buffer is filled, all the data stored there is written to the file. This technique improves the arrangement'south performance. Closing a file causes whatsoever unsaved data that may still exist held in a buffer to exist saved to its file. This means the information will be in the file if you need to read information technology later on in the same programme.
• Some operating systems limit the number of files that may exist open at one time. When a program closes files that are no longer being used, it will not deplete more of the operating system's resources than necessary.

Calling the file stream object's close member function closes a file. Hither is an instance:

inputFile.close();

Writing Information to a File

The plan demonstrates an case that reads strings equally input from the keyboard so writes those strings to a file. The program asks the user to enter the showtime names of three friends, and then it writes those names to a file named Friends.txt. After this code has executed, we can open the Friends.txt file using a text editor and look at its contents.

          // This programme writes user input to a file.
#include <iostream>
#include <fstream>
#include <string>
using namespace std;          int main()
{
            ofstream outputFile;
            cord name1, name2, name3;          // Open an outputFile;
            outputFile.open("Friends.txt");          // Get the names of three friends.
            cout << "Enter the names of three friends.\n";
            cout << "Frind #i: ";
            cin >> name1;
            cout << "Frind #two: ";
            cin >> name2;
            cout << "Frind #3: ";
            cin >> name3;          // Where the names to the file.
            outputFile << name1 << endl;
            outputFile << name2 << endl;
            outputFile << name3 << endl;
            cout << "The names were saved to a file.\northward";          // Close the file
            outputFile.shut();
            render 0;
}        

Reading Data from a File

The >> operator not only reads user input from the cin object, but also information from a file. Assuming inputFile is an ifstream object, the following statement shows the >> operator reading information from the file into the variable name:

          inputFile >> name;        

Let's look at an example. The program below assumes the file Friends.txt exists, opens the file, reads the names and displays them on the screen, and so closes the file.

          // This plan reads information from a file.
#include <iostream>
#include <fstream>
#include <string>
using namespace std;          int main()
{
            ifstream inputFile;
            string name;          inputFile.open("Friends.txt");
            cout << "Reading data from the file.\n";          inputFile >> name;          // Read name i from the line
            cout << name << endl;       // Display name 1          inputFile >> proper name;          // Read name 2 from the line
            cout << proper name << endl;       // Display proper noun ii          inputFile >> name;          // Read name three from the line
            cout << name << endl;       // Display name three          inputFile.close();          // Close the file
            return 0;
}        

Using Loops to Process Files

Although some programs utilize files to store simply minor amounts of information, files are typically used to hold large collections of data. When a program uses a file to write or read a large amount of information, a loop is typically involved.

Quite often a program must read the contents of a file without knowing the number of items that are stored in the file. For example, suppose you need to write a plan that displays all of the items in a file, but you exercise not know how many items the file contains. You can open up the file and then utilise a loop to repeatedly read an item from the file and display it. Even so, an mistake will occur if the program attempts to read beyond the end of the file. The program needs some mode of knowing when the end of the file has been reached so it will not endeavour to read beyond it.

Fortunately, the >> operator not only reads data from a file, only also returns a true or simulated value indicating whether the data was successfully read or non. If the operator returns truthful, so a value was successfully read. If the operator returns false, it means that no value was read from the file.

There is a way to make up one's mind whether the open fellow member part successfully opened the file. After you telephone call the open up member function, you tin examination the file stream object every bit if information technology were a Boolean expression.

In each of the previous examples, the proper name of the file that is opened is hard-coded every bit a 11 string literal into the programme. In many cases, you lot will desire the user to specify the name of a file for the program to open. In C++ eleven, you can pass a string object as an argument to a file stream object's open up fellow member function.

The programme below shows an example. This version prompts the user to enter the name of the file. In line 15, the proper noun that the user enters is stored in a string object named filename. In line 18, the filename object is passed as an argument to the open function.

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Source: https://medium.com/@ctchalland/files-b75bb27606e6

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