Text Files and Streams

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Chapter 11
Text Files and Streams
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Outline

• Streams and Stream Variables (Objects)
• Input (read) from a File
• Header Technique
• End-of-File (EOF) Technique
• How a Text File is Stored
• Output (write) to a File
• Streams as Function Parameters
• Inheritance and Stream Parameters
• More Member Functions for Streams

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Streams

• What Is a Stream?
• Sequence of data (bytes) coming from a source
  waiting to be moved to a destination
• Predefined Streams
• Predefined by the stream libraries for everyones
  use
• cin
• Stream of bytes coming from keyboard to your
  program
• cout
• Stream of bytes going from your program to
  monitor

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Stream Variables Are Objects

• Instances of a Class
• Lots of different I/O classes defined in C
• istream ostream general input output
  streams
• cin is an istream, and cout is an ostream
• ifstream ofstream input output file
  streams
• Objects More Complex than Variables
• Contain 2 kinds of things
• Attributes member variables (data describing
  the stream)
• Methods member functions (actions on that
  data)
• Each object contains different data in its
  member variables than all the other objects
  usually

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Stream Variables (Objects)

• Stream objects are declared just like variables
• Example ifstream In_File
• Using an objects Member Functions
• Syntax object_name.member_function(arguments)
• Have to specify
• Which object youre talking about
• Which of that objects functions youre using
• What information that function needs to do its
  job
• example
• In_File.open("Data_File.txt")
• Talking about ifstream object In_File
• Using that objects open function
• Passing it the disk pathname of the file to be
  opened

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We Want To

• be able to read from, write to, control disk
  files as streams
• Open any disk file so it works like a stream
  for reading or writing
• Provide the names of our disk files when we write
  the program or ask for a files name at run time
• Be able to detect problems (file not found, tried
  to read past end-of-file, etc.)
• Create new files, read files, overwrite existing
  files, append new data onto the end of existing
  files
• Pass a file to a function for it to process
• Have more than one file open at the same time

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Input from a File Stream
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Input from a File - Basic Steps

• // 1. Include the library fstream
• include ltfstreamgt
• int main()
• // 2. Open a file for input
• ifstream exams_in // declares the file stream
  object
• exams_in.open(exam.dat) // attaches stream
  object to an file
• // 3. Read from the file stream
• exams_in gtgt num_exams
• ... ...
• // 4. Close the file stream
• exams_in.close() // detaches the stream from
  the file exam.dat

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Example
Header of exam.dat tells how many
scores follow
Specify the path (directory)
exams_in.open ("C\\csci1010\\exam.dat")
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Protecting against a nonexistent file

• Using cerr and abort

fail() returns 1 if the open() function failed
cerr is predefined in iostream
terminates the program we need include ltcstdlibgt

• Using the assert function

• Primarily designed for use in debugging
• Causes immediate program termination
• Issues general error message
• we need include ltcassertgt

Things are okay if the open operation did NOT fail
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Input from a File End-of-File (EOF)

• Not convenient to provide a header at the
  beginning of a file
• Might miscount when the file contains a large
  number of data groups (records)
• Addition / deletion requires the header to be
  changed
• The program detects when an attempt to read past
  the end of the file has occurred

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Example
eof() returns 1 only when an input operation has
already attempted to read past the end of the file
ws is whitespace manipulator that skip over
whitespace characters ( , \t, \n)
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Structure of a Text File
(\n characters)
(EOF)
(current position pointer)

• Text lines separated by newline (\n) characters
• Can be read written just like any other
  character
• File ended by EOF condition (not a character)
• wage_in gtgt name gtgt hours gtgt rate
• wage_in gtgt ws

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Example Other ways to do this
At EOF, the gtgt operator returns 0 (false) So
well just read the next datum we want
while (wage_in gtgt name gtgt hours gtgt rate) pay
hours rate cout ltlt name ltlt ltlt pay ltlt
endl
skips any whitespace, then checks for EOF
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More about Function eof ()

• It only works if program has bumped its nose -
  trying to read past end of the file
• If you read the last character of a file, but
  dont try to read past that, eof () returns
  false (meaning not yet)
• This is also true of the gtgt operator
• Most useful with fstream classs member functions
• They dont automatically return a detectable
  value,so you dont know whether they hit EOF
  without testing

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Entering File Names Interactively

• What if we dont know the file name when we write
  the program?
• We get the file name from somewhere (like the
  user), and put it into a string object

But the open() function does NOT know from C
strings because it uses (old) C-language string
have to use .c_str() to extract the (old)
C-language string from a C string
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Output to a File Stream
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Output (Write) to Files

• Writing to a New File (or Replacing an Existing
  Files Contents)
• ofstream fout // declares a output stream
• fut.open ("Output.txt", iostrunc) // attaches
  the file
• if (!fout.fail())
• fout ltlt "This replaces contents of Output.txt
  ltlt endl
• fout.close () // detaches and save the file
• Appending to an Existing File
• ofstream fout
• fout.open ("Output.txt", iosapp)
• if (!fout.fail())
• fout ltlt "This goes onto the end of Output.txt"

default
appendonto the end
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Updating Files
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More about Opening Closing Files

• string Word
• ifstream fin ("In1.txt") // open In1.txt when
  declaring fin
• ofstream fout ("Out1.txt")
• while (fin gtgt Word)
• fout ltlt Word ltlt endl
• fin.close () // Finished with
  In1.txt
• fout.close () // Finished with
  Out1.txt
• fin.open("In2.txt") // Now open In2.txt
  for reading
• fout.open ("Out2.txt, // And append to
  Out2.txt
• iosapp)
• while (fin gtgt Word)
• fout ltlt Word ltlt endl

change file settings (e.g., whether to start
writing to an output file at the beginning or
append onto the end) by using specially-defined
const values as the 2nd parameter to the open
call.
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Passing a File to Functions (1)

• What to Pass?
• file name
• file stream object
• If we use file name, function has to open
  and close file in addition to doing its real job
• Using file stream object usually makes more sense
• Caller takes care of opening file, verifying that
  its there, etc.
• Function just does its thing on the file
  whichever file it is

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Passing Files to Functions (2)

• RULE file stream objects MUST be passed
  by reference, not by value
• Even if contents of disk file itself may not
  change, files member functions must be able to
  change current position pointer
• Member functions called from your function
  must have direct access to the file/stream
  object which is only possible when passed by
  reference
• If we passed it by value
• Function would get a copy of file/stream object
• Any changes would get made to the copy, not the
  original
• When function returned, copy would vanish, and
  so would any changes

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Example
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Inheritance and Stream Parameters
ios
istream
ostream
iostream
ifstream
ofstream
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More fstream Member Functions

• For use when gtgt ltlt wont do
• Single-character I/O
• char Ch
• ifstream File_In
• ofstream File_Out
• File_In.get(Ch) // Even gets whitespace chars.
• File_Out.putback(Ch) // at the front
• Text-line input
• string Buffer
• ifstream File_In
• getline (File_In, Buffer)
• Gets characters from In_File into Buffer,
  stopping at next '\n'
• Does not copy the '\n' into the buffer
• Leaves current position pointer pointing after
  the '\n'