Program Input and the Software Design Process

1. Program Input and the Software Design Process Robert reaves

2. 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.

3. Using Files • If we want file I/O in program we have to do: • Request the preprocessor to include the header file fstream. • #include<fstream> • 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.

4. Using Files • Through the header file fstream, the C++ standard library defines two data types, ifstreamand ostream. • ifstreamrepresents a stream of characters coming from an input file. • ostreamrepresents 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..

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

6. 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 ifstreamwhile the other the ostream.

7. 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.

8. 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?

9. Run-Time Input of File Names • Open function associated with the ifstreamdata 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. • ifstreammyIn; • string fileName; • cout << “Enter the input file name: “; • cin >> fileName; • myIn.open(fileName); // Compile-time error

10. 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. =)

11. 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.