CMPSC 201 - Ch. 8 & Ch. 7.2 Notes

Note that we will not be studying sections 8.3 through 8.6 in this class.

Objective #1: Explain the concept of file stream objects in C++.

• Often a C++ program will use an external data file to receive its input and possibly another file to store its output.
• While it may be convenient to ask the user to enter data via the keyboard, it is more practical to obtain large amounts of input from a previously saved external data file.
• While it may be useful to store output to the screen so that a user can see a program's results, it is also useful to store the output to an external file.
• You must use a file stream object (aka file pointer) for each external file that you plan to work with in your C++ program.
• A file stream itself is simply a path from your program to a physical device such as a floppy disk or the hard disk. Every file stream has a mode which determines which direction the information will flow. Either the information flows from the floppy or hard disk TO the C++ program or vice versa. If the information flows into the C++ program, we call the mode input and the process of getting information from the floppy or hard disk is called "reading". Otherwise, the mode is called output and we call the act of storing information on the floppy or hard disk "writing". By the way, there is no mode that allows information to flow in both directions to the same file stream object.
  
  
• To create a file stream object that would be used to read a value from an external data file, this statement might be used:
  
  ifstream infile;
  
  where ifstream is a C++ keyword and the identifier infile was chosen by the programmer. The programmer could use any valid identifier (i.e. variable name) in place of infile.
  
  
• To create a file stream object that would be used to write data to an external data file, this statement might be used:
  
  ofstream outfile;
  
  where ofstream is a keyword and outfile was chosen by the programmer.
  
  
• Usually these statements are placed at the top of the function (i.e. main) in which the file stream objects and external files are going to be used. These statements are somewhat similar to a variable's declaration statement that should also be located at the top of a program.
  
  
• When using file stream objects, you must use include the header file fstream.h at the top of your program with the compiler directive,
  
  #include <fstream>
  

Objective #2: Use the common file stream methods including open and close.

• After a file stream object is created, you must actually open the named file that exists on the floppy or hard disk. The following statement would be used to actually open a file named "HighScores.txt" and to connect it to the file stream object infile that was instantiated (i.e. declared) with the line above.
  
  infile.open("HighScores.txt");
  
  
• After you are completely finished using an external data file, you should close the file with a statement like,
  
  infile.close( );
  
  
• You can have multiple files opened for both reading and writing at any point during a C++ program however you must have separate file stream objects connected with each one.

Objective #3: Read from and write to sequential files using the necessary file stream methods.

• You can use the extraction operator ( << ) with a file stream object to read from a file that is opened with the input mode, just as you are can use the extraction operator (aka input operator) with the cin object.
  
  
• While the statement,
  
  cin >> age;
  
  would require the user to input his/her age via the keyboard, the statement
  
  infile >> age;
  
  would read the first line of the external text file that is connected to the file stream object infile. The statement,
  
  infile.getline(sentence);
  
  can be used to read the whole first line of the file and store that line in the character array sentence.
  
  
• To write to a file that is opened for output, you use the insertion operator << (aka output operator) with its file stream object.
  
  
• While the statement,
  
  cout << favoriteNum << endl;
  
  would display the value of favorite_num to the screen, the statement
  
  outfile << favoriteNum << endl;
  
  would write it on a single line of the file connected with the file stream object outfile.
  
  
• Use the eof function along with a do while loop to determine whether the end of a sequential-access file has been reached. This function returns a 1 (true) if an attempt has been made to read past the end of the file. For example,
  

do
{
    infile >> x;

    if ( !infile.eof( ) )
    {
            cout << x << endl;
    }

} while ( !infile.eof( ) );

• For one C++ program to access multiple data files, you simply have to create multiple file stream objects (each one pointing to a different data file). For example,
  
  infile.open("inputfile.txt");
  outfile.open("outputfile.txt");
  
  uses two file stream objects, infile (an ifstream file object) and outfile (an ofstream file object). infile points to a external data file named inputfile.txt, which contains data that will be read into the C++ program. outfile points to the external data file named outputfile.txt, which will be written to by the C++ program.
  

Objective #4: Use string variables. (See Ch. 7.2 in your textbook for this discussion of the use of strings. This is the only part of Ch. 7 that we are studying in this class.)

• The simple C++ data type char can only store one character at a time. In order to store a string such as "John Doe", you can use a string variable.
  
  
• You can change one character of a string using subscript notation. For example, after
  
  string studentName = "Jon";
  studentName[0] = 'D';
  
  studentName is now "Don" not "Jon" since the first character of a string is considered to be stored in index position 0. You will see the use of square brackets when we learn about arrays in this course.
  
  
• You must declare a string variable before you use it in a program like any other variable. To declare a string variable, use a statement like,
  
  string name;
  
  
• To declare and initialize a string, use the following:
  
  string name = "John Doe";
  
  
• To obtain the user's name as input you can use the cin object like this,
  
  cout << "Enter your first name: ";
  cin >> firstName;
  cout << "Enter your last name: ";
  cin >> lastName;
  
  where first_name and last_name are two separate character arrays. The user would have to press the Enter key however after each name. Notice the use of the extraction operator (aka input operator) >> after the keyword cin and the use of the insertion operator (aka output operator) << after the keyword cout. Then, you could printout the person's name with
  
  cout << firstName << " " << lastName << endl;
  
  
• You must include the string header file at the top of programs in which you wish to declare or use string variables.
  
  For example,
  
  #include <iostream>
  #include <string>
  using namespace std;
  
  int main()
  {
       string name = "John Doe";
       cout << name << endl;
  
       return 0;
  }
  
  
• However, when opening a file with a user-specified filename as in Program 8.3a on p. 453, you will have to use the following syntax (notably the c_str function)
  
  #include <iostream>
  #include <fstream>
  #include <string>
  using namespace std;

  int main()
  {
       string filename;
       ofstream outfile;

       cin >> filename;
       outfile.open(filename.c_str());

       outfile << "hello" << endl ;

       return 0;
  }

• A string literal is a sequence of characters that is used in a C++ program between double quotes such as in the statement   
  
  cout << "Hello world!";   
  
  where "Hello world!" is the string literal. Note that the string literal, in this case, includes a space and an exclamation point. You do not have to include the string header file in a program when you use string literals.

• A character literal is a single character that is used in a C++ program between single quotes. In the statement,
  
  char initial;
  initial = 'T'; 
  
  the character literal is the uppercase letter T.