Transcript Program Input and the Software Design Process

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.
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Why would we want to read from a file instead of a keyboard?
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Little mistakes.
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Easier to enter in large amounts of data multiple times.
Why would we want to write to a file?
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Allows us to look at the output over and over without a rerun.
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Output of one program can be used as input to another.
Using Files
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If we want file I/O in program we have to do:
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Request the preprocessor to include the header file fstream.
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#include<fstream>
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Use declaration statements to declare the file streams we will use.
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Prepare each file for reaching and writing by using a function named
open.
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Specify the name of the file stream in each input or output statement.
Using Files
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Through the header file fstream, the C++ standard library defines
two data types, ifstream and ostream.
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ifstream represents a stream of characters coming from an input file.
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ostream represents a stream of characters going to an output file.
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ALL the operators we have learned about cout and cin are valid
with these data types.
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ifstream uses (>>) operator, get function, ignore function.
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ostream uses (<<) operator, endl, setw, setprecision, etc..
Declaring File Streams
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Just like we declared int, char, float, etc.. We declare file streams:
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int x;
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float y;
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ifstream myIn;
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ostream myOut;
NOTE: Cannot read and write from the same file using the ifstream
and the ostream.
Opening Files
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Opening a file causes the computer’s operating system to perform
certain actions that allow us to proceed with file I/O.
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Example of opening a file for reading and writing.
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myIn.open(“input.txt”); //Name of the file is irrelevant.
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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
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Associates a stream variable used in your program with a physical
file on disk.
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Open, with an input file, sets the file’s reading marker to the first
piece of data in the file.
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Open, with an output file, checks whether the file already exists.
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If it does:
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If it doesn’t:
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Erases all the old contents of the file. Sets the writing marker to the beginning.
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
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Closing a file causes the operating system to perform certain wrapup activities on it and to break the connection between the stream
variable and the file.
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Close function associated with both the ifstream and ostream.
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Do we always been to close files?
Run-Time Input of File Names
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Open function associated with the ifstream data type requires an
argument that specifies the name of the actual data file.
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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.
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ifstream myIn;
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string fileName;
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cout << “Enter the input file name: “;
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cin >> fileName;
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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!
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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.
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fileName.c_str();
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This function is to allow programmers to call library functions that expect C
strings and not just string strings. =)
Input Failure
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Things can go wrong whenever you input something to a program.\
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Prompt user for int value, but we give it some char.
cin will enter a fail state.
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Any further I/O operations using that stream are considered null
operations.
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Computer doesn’t halt program execution or display an error.
Invalid data is the most common reason for input failure.