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Chapter 15

Chapter 15. Objectives. You should be able to describe: I/O File Stream Objects and Methods Reading and Writing Character-Based Files Exceptions and File Checking Random File Access File Streams as Function Arguments Common Programming Errors. I/O File Stream Objects and Methods.

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Chapter 15

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  1. Chapter 15

  2. Objectives You should be able to describe: • I/O File Stream Objects and Methods • Reading and Writing Character-Based Files • Exceptions and File Checking • Random File Access • File Streams as Function Arguments • Common Programming Errors

  3. I/O File Stream Objects and Methods • File: Collection of data that is stored together under common name, usually on disk, magnetic tape, or CD • C++ programs stored on disk are examples of files • The stored data in program file is program code that becomes input data to C++ compiler • A C++ program is not usually considered data, and term file, or data file, is typically used to refer only to external files that contain the data used in C++ program

  4. I/O File Stream Objects and Methods (continued) • External name: Unique file name for file • External name is how operating system knows file • Contents of directory or folder are listed by external names • Format of external names: Each computer operating system has its own specifications for external file size • Table 15.1 lists specifications for more commonly used operating systems

  5. I/O File Stream Objects and Methods (continued)

  6. I/O File Stream Objects and Methods (continued) • File naming conventions: • Use descriptive names • Avoid long file names • They take more time to type and can result in typing errors • Manageable length for file name is 12 to 14 characters, with maximum of 25 characters • Choose file names that indicate type of data in file and application for which it is used • Frequently, first eight characters describe data, and an extension describes application

  7. I/O File Stream Objects and Methods (continued) • More file naming conventions • Using DOS convention, the following are all valid computer data file names: prices.dat records info.txt exper1.dat scores.dat math.mem

  8. File Stream Objects • File stream: One-way transmission path that is used to connect file stored on physical device, such as disk or CD, to program • Mode (of file stream): Determines whether path will move data from file into program or from program to file • Input file stream: Receives or reads data from file into program • Output file stream: Sends or writes data to file

  9. File Stream Objects (continued) • Direction (mode) of file stream is defined in relation to program and not file: • Data that goes into program are considered input data • Data sent out from program are considered output data • Figure 15.1 illustrates data flow from and to file using input and output streams

  10. File Stream Objects (continued)

  11. File Stream Objects (continued) • Distinct file stream object must be created for each file used, regardless of file’s type • For program to both read and write to file, both an input and output file stream object are required • Input file stream objects are declared to be of type ifstream • Output file streams are declared to be of type ofstream

  12. File Stream Objects (continued) • Two basic types of files: both store data using binary code • Text (character-based) files: Store each character using individual character code (typically ASCII or Unicode) • Advantage: Allows files to be displayed by word processing program or text editor • Binary-based files: Store numbers in binary form and strings in ASCII or Unicode form • Advantage: Provides compactness

  13. File Stream Methods • Each file stream object has access to methods defined for its respective ifstream or ofstream class, including: • Opening file: connecting stream object name to external file name • Determining whether a successful connection has been made • Closing file: closing connection • Getting next data item into program from input stream • Putting new data item from program onto output stream • Detecting when end of file has been reached

  14. File Stream Methods (continued) • open() method: • Establishes physical connecting link between program and file • Operating system function that is transparent to programmer • Connects file’s external computer name to stream object name used internally by program • Before a file can be opened, it must be declared as either ifstream or ofstream object • File opened for input is said to be in read mode

  15. File Stream Methods (continued) • Example:inFile.open("prices.dat"); • connects external text file named prices.dat to internal program file stream object named inFile • Accesses file using internal object name inFile • Computer saves file under the external name prices.dat • Calling the open() method uses the standard object notation: objectName.open()

  16. File Stream Methods (continued) • fail() method: returns true value if file is unsuccessfully opened, false if open succeeded • Good programming practice is to check that connection is established before using file • In addition to fail() method, C++ provides three other methods, listed in Table 15.2, that can be used to detect file’s status • Program 15.1 illustrates statements required to open file for input including error checking routine to ensure that successful open was obtained

  17. File Stream Methods (continued) • Example of use of fail() method: ifstream inFile; // any object name can be used here inFile.open("prices.dat"); // open the file // check that the connection was successfully // opened if (inFile.fail()) { cout << "\nThe file was not successfully opened" << "\n Please check that the file currently exists." << endl; exit(1); }

  18. File Stream Methods (continued)

  19. File Stream Methods (continued)

  20. File Stream Methods (continued)

  21. File Stream Methods (continued) • Different checking required for output files • If file exists having same name as file to be opened in output mode, existing file is erased and all data lost • To avoid this situation, file is first opened in input mode to see if it exists • If it does, user is given choice of explicitly permitting it to be overwritten (when it is later opened in output mode) • Code used to accomplish this is highlighted in Program 15.2

  22. Embedded and Interactive File Names • Embedding a file name in program causes problems • No provision for user to enter desired file name during program execution • Any changes require modification of open() method and recompile • These problems can be solved by assigning file name to string variable as shown in Programs 15.3a and 15.3b • String variable: Variable that can hold string value

  23. Embedded and Interactive File Names (continued)

  24. Embedded and Interactive File Names (continued)

  25. Closing a File • close() method: Breaks connection between file’s external name and file stream object • Object can then be used for another file • Good programming practice is to close files no longer needed • Operating system automatically closes any open files at end of normal program execution • Example: The statement inFile.close(); closes inFile stream’s connection to its current file • close() method takes no argument

  26. Reading and Writing Character-Based Files • Operations similar to reading input from keyboard and writing data to display screen • For writing to file, the cout object is replaced by ofstream object name declared in program • Example: If outFile is declared as object of type ofstream, the following output statement is valid: outFile << descrip << ' ' << price; • The file name directs output stream to file instead of standard display device • Example: Program 15.4

  27. Reading and Writing Character-Based Files (continued) • Program 15.4 output: • File named prices.dat is created and saved by computer as text file (the default file type) • prices.dat is sequential file consisting of the following data: Mats 39.95 Bulbs 3.22 Fuses 1.08 • Actual storage of characters in file depends on character codes used by computer • Output file file contains 36 characters (Figure 15.2)

  28. Reading and Writing Character-Based Files (continued)

  29. Reading from a Text File • Almost identical to reading data from standard keyboard • cin object replaced by ifstream object declared in program • Example: The input statement inFile >> descrip >> price; reads next two items in file and stores them in variables descrip and price • File stream name directs input to come from file stream rather than standard input device stream

  30. Reading from a Text File (continued) • Program 15.5 illustrates how the prices.dat file created in Program 15.4 can be read • Also illustrates method of detecting end-of-file (EOF) marker using good() function (see Table 15.2) • Other methods that can be used for stream input are listed in Table 15.3 • Each method must be preceded by stream object name

  31. Reading from a Text File (continued)

  32. Standard Device Files • C++ supports logical and physical file objects • Logical file object: Stream that connects file of logically related data (data file) to a program • Physical file object: Stream that connects to hardware device such as keyboard or printer • Standard input file: Physical device assigned to program for data entry • Standard output file: Physical device on which output is automatically displayed

  33. Other Devices • The keyboard, display, error reporting, and logging streams are automatically connected to the stream objects named cin, cout, cerr, clog • Requires iostream header file • Other devices can be used if the name assigned by system is known • Example: Most personal computers assign name prn to printer connected to computer • Statement outFile.open("prn") connects printer to ofstream object named outFile

  34. Exceptions and File Checking • Error detection and exception handling are used in C++ programs that access one or more files • General exception handling code (section 7.3) try { // one or more statements, at least one // of which should throw an exception } catch(exceptionDataType parameterName) { // one or more statements } • Program 15.7 illustrates file opening exception handling

  35. Opening Multiple Files • Example: Read the data from character-based file named info.txt, one character at a time, and write this data to file named backup.txt • Essentially, this is a file-copy program • Figure 15.4 illustrates structure of streams needed to produce file copy • Program 15.9 creates backup.txt file as an exact duplicate of info.txt file using procedure described in Figure 15.4

  36. Opening Multiple Files (continued)

  37. Random File Access • File access: Retrieving data from file • File organization: The way data is stored in a file • Sequential organization: Characters in file are stored in sequential manner, one after another • Random Access: Any character in an open file can be read directly without having to read characters ahead of it

  38. Random File Access (continued) • File position marker: Long integer that represents an offset from the beginning of each file • Keeps track of where next character is to be read from or written to • Allows for random access of any individual character • Table 15.4 shows functions used to access and change the file position marker • Program 15.10 illustrates use of seek() and tell() to read and display file in reverse order

  39. Random File Access (continued)

  40. File Streams as Function Arguments • A file stream object can be used as function argument • Function’s formal parameter must be a reference (see Section 6.3) to appropriate stream, either as ifstream& or ofstream& • Example: Program 15.11 • ofstream object named outfile is opened in main() • Stream object is passed to the function inOut() • inOut() is used to write five lines of user-entered text to file

  41. Common Programming Errors • Using file’s external name in place of internal file stream object name when accessing file • Opening file for output without first checking that file with given name already exists • Not checking for preexisting file ensures that file will be overwritten • Not understanding that end of a file is detected only after EOF sentinel has either been read or passed over

  42. Common Programming Errors (continued) • Attempting to detect end of file using character variables for EOF marker • Any variable used to accept EOF must be declared as an integer variable • Using integer argument with the seekg() and seekp() functions • Offset must be a long integer constant or variable • Any other value passed to these functions can result in unpredictable result

  43. Summary • A data file is any collection of data stored in an external storage medium under a common name • A data file is connected to file stream using fstream’s open() method • This function connects file’s external name with internal object name • A file can be opened in input or output mode • An opened output file stream either creates a new data file or erases data in an existing opened file

  44. Summary (continued) • All file streams must be declared as objects of either the ifstream or ofstream classes • In addition to any files opened within a function, the standard stream objects cin, cout, and cerrare automatically declared and opened when a program is run

  45. Summary (continued) • Data files can be accessed randomly using the seekg(),seekp(), tellg(), and tellp() methods • The g versions of these functions are used to alter and query file position marker for input file streams • The p versions do the same for output file streams • Table 15.5 lists the methods supplied by the fstream class for file manipulation

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