Objectives

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Chapter 15
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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

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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

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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

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I/O File Stream Objects and Methods (continued)
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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

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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

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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

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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

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File Stream Objects (continued)
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File Stream Objects (continued)

• Distinct file stream object must be created for
  each file used, regardless of files 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

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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

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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

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File Stream Methods (continued)

• open() method
• Establishes physical connecting link between
  program and file
• Operating system function that is transparent to
  programmer
• Connects files 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

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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()

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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 files status
• Program 15.1 illustrates statements required to
  open file for input including error checking
  routine to ensure that successful open was
  obtained

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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 ltlt "\nThe file was not successfully
  opened"
• ltlt "\n Please check that the file currently
  exists."
• ltlt endl
• exit(1)

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File Stream Methods (continued)
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File Stream Methods (continued)
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File Stream Methods (continued)
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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

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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

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Embedded and Interactive File Names (continued)
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Embedded and Interactive File Names (continued)
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Closing a File

• close() method Breaks connection between files
  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 streams connection to its current
  file
• close() method takes no argument

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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 ltlt descrip ltlt ' ' ltlt price
• The file name directs output stream to file
  instead of standard display device
• Example Program 15.4

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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)

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Reading and Writing Character-Based Files
(continued)
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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 gtgt descrip gtgt 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

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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

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Reading from a Text File (continued)
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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

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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

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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

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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

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Opening Multiple Files (continued)
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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

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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

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Random File Access (continued)
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File Streams as Function Arguments

• A file stream object can be used as function
  argument
• Functions 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

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Common Programming Errors

• Using files 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

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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

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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
  fstreams open() method
• This function connects files 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

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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 cerr are automatically declared and opened
  when a program is run

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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