File IO

1
File I/O

• Reading Chapter 3

2
File

• A file is a named area in secondary storage that
  holds a collection of information
• Each file has a name and extension (optional)
• Name can usually be at least eight characters
  long, and usually alpha-numeric characters
• Extension is usually up to three characters long,
  and used to group files by type
• Some operating systems are case-sensitive

readme.txt program.exe lab7.cpp
LAB7.CPP iostream.h report.doc
3
Why file I/O?

• Programs can
• Read data from files instead of keyboard
• Write output to files instead of screen
• Advantages of file input
• Easier to enter data into a file
• Data does not need to be retyped
• Advantages of file output
• Output can be seen over and over again without
  rerunning the program
• The output of one program can be used as input to
  another

4
C file streams

• C allows file I/O through file stream objects

5
C file streams

• The C I/O library defines
• Input file stream (ifstream) stream of
  characters coming into a program from an input
  file
• Output file stream (ofstream) stream of
  characters going from a program to an output file

6
Performing file I/O

• Include the fstream header file
• include ltfstreamgt
• Declare file stream objects
• Open files using file stream objects
• Use file stream objects to read data from a file
  or write data into a file (in the same way cin or
  cout is used)
• Close files

7
Declaring file stream objects

• Declaring file stream objects without file names
• ifstream inFile // input file stream
• ofstream outFile // output file stream
• Declaring file stream objects with file names
• ifstream inFile ("input.dat")
• ofstream outFile ("output.dat")

8
Opening a file

• Files may be opened like
• ifstream inFile
• ofstream outFile
• inFile.open ("input.dat")
• outFile.open ("output.dat")

9
What does a file opening do?

• Associates the file stream object with the
  physical (disk) name of the file
• If the input file does not exist on disk, open is
  not successful
• If the output file does not exist on disk, a new
  file with that name is created
• If the output file already exists, it is
  overwritten
• Places a file reading marker at the very
  beginning of the file, pointing to the first
  character in it

10
Reading and writing with file streams

• Read using the gtgt operator
• inFile gtgt aNumber
• infile gtgt stringVariable gtgt aNumber
• Write using the ltlt operator
• outFile ltlt "Output to the file" ltlt endl
• outFile ltlt aNumber

11
Closing a file

• Files opened with statements like
• ifstream inFile("input.dat")
• ofstream outFile
• outFile.open ("output.dat")
• are closed with
• inFile.close()
• outFile.close()

12
Assignment

• Write a C program that will read the data from
  an input file called data.in and display the data
  in the screen.