Program Input and the Software Design Process ROBERT REAVES

File Input and Output  File is a named area in a secondary storage that holds a collection of information.  Why would we want to read from a file instead of a keyboard?  Little mistakes.  Easier to enter in large amounts of data multiple times.  Why would we want to write to a file?  Allows us to look at the output over and over without a rerun.  Output of one program can be used as input to another.

Using Files  If we want file I/O in program we have to do:  Request the preprocessor to include the header file fstream.  #include  Use declaration statements to declare the file streams we will use.  Prepare each file for reaching and writing by using a function named open.  Specify the name of the file stream in each input or output statement.

Using Files  Through the header file fstream, the C++ standard library defines two data types, ifstream and ostream.  ifstream represents a stream of characters coming from an input file.  ostream represents a stream of characters going to an output file.  ALL the operators we have learned about cout and cin are valid with these data types.  ifstream uses (>>) operator, get function, ignore function.  ostream uses (<<) operator, endl, setw, setprecision, etc..

Declaring File Streams  Just like we declared int, char, float, etc.. We declare file streams:  int x;  float y;  ifstream myIn;  ostream myOut;  NOTE: Cannot read and write from the same file using the ifstream and the ostream.

Opening Files  Opening a file causes the computer’s operating system to perform certain actions that allow us to proceed with file I/O.  Example of opening a file for reading and writing.  myIn.open(“input.txt”); //Name of the file is irrelevant.  myOut.open(“output.txt”); //Name of the file is irrelevant.  Each of these are a function call to the open function. Both are different open functions, one is associated with the ifstream while the other the ostream.

Open function  Associates a stream variable used in your program with a physical file on disk.  Open, with an input file, sets the file’s reading marker to the first piece of data in the file.  Open, with an output file, checks whether the file already exists.  If it does:  Erases all the old contents of the file. Sets the writing marker to the beginning.  If it doesn’t:  Creates a new, empty file for you. Sets the writing marker to the beginning.  NOTE: Want to open files before any kind of I/O attempts are made.

Close Function  Closing a file causes the operating system to perform certain wrap- up activities on it and to break the connection between the stream variable and the file.  Close function associated with both the ifstream and ostream.  Do we always been to close files?

Run-Time Input of File Names  Open function associated with the ifstream data type requires an argument that specifies the name of the actual data file.  Program will only work with this particular file, we don’t we want! We can to be able to run with any file. =)  Command technique is to prompt the user for the name of the file to read.  ifstream myIn;  string fileName;  cout << “Enter the input file name: “;  cin >> fileName;  myIn.open(fileName); // Compile-time error

Run-Time of File Names  Why do we get a compile-time error?  Open function does not expect an argument of type string, but instead it expects a C string.  We will talk more about C strings later, just KNOW that open doesn’t take string types. =)  Lucky for us our string type does provide a function for us!  c_str, value returning function.  c_str function returns a C string that is equivalent to the one contained in the variable used to call it.  fileName.c_str();  This function is to allow programmers to call library functions that expect C strings and not just string strings. =)

Input Failure  Things can go wrong whenever you input something to a program.\  Prompt user for int value, but we give it some char.  cin will enter a fail state.  Any further I/O operations using that stream are considered null operations.  Computer doesn’t halt program execution or display an error.  Invalid data is the most common reason for input failure.