Lecture 11 File IO

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Lecture 11File I/O

• ENGR17 Engineering Programming
• Section 1
• Fall 2001

10/15/01
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Outline

• Open a sequential access data file
• Write information to a sequential access data
  file
• Close a sequential access data file
• Read information from a sequential access data
  file
• Test for the end of an input file

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Fields, Records, and Data Files

• Field
• A single item of information about a person,
  place, or thing
• Example Name
• Record
• One or more related fields that contain all of
  the necessary data about a specific person,
  place, or thing
• Example Name, USD ID, Grade
• Data file
• A collection of related records
• Three types sequential, random, binary
• Example ENGR17 Grade File (contains mostly Fs)

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Fields, Records, and Data Files
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Sequential Access Data Files

• Each record in the file is both stored and
  retrieved in consecutive order (sequentially)
• Advantage
• Easy to create
• Drawback
• Records can be processed only in the order in
  which they are stored
• Work best for processing either a small file
  (lt100 records) or a large file whose records are
  always processed in consecutive order

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Sequential Access Data Files
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Components of File Access

• Steps to perform when reading from or writing to
  a sequential access file
• Create File Objects
• Open File in appropriate mode
• Read
• Write
• Append
• Check if file open failed
• If it failed, end the function
• Read To or Write From File
• Close File

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Opening a Sequential Access Data File

• Must use include ltfstream.hgt
• Use the ifstream and ofstream classes to create
  input or output file objects, respectively
• Input data files Contents are read by a program
• Output data files files to which a program
  writes data
• After creating file objects, use the C open
  function to open the file for either either
  input, output, or append

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Creating File Objects
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open Function Modes
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Opening Files with the open Function
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Record Indicator

• Used to keep track of the next record either to
  read or write in a data file

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Test If File Open Was Successful

• Use the C fail function, defined in both the
  ifstream and ofstream classes
• fail function returns
• Value 1 (for True) if the open failed
• Value 0 (for False) if the open did not fail
• Usually done immediately after opening the file

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Test If File Open Was Successful
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Write to a Sequential Access Data File

• Syntax for number or string
• object ltlt data ltlt endl
• Each record is typically written on a separate
  line in the file to distinguish one from another
• Just like printing output to the console using
  cout

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Read from a Sequential Access Data File

• Syntax for reading a single number
• object gtgt data
• Syntax for reading an entire line (string)
• object.getline(data,sizeof(data))
• Using getline() reads the entire contents of the
  line up to the newline character
• Just like reading from cin
• Use eof to stop reading at end of file

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The eof Function

• Used only when reading a file
• Used to determine if the record indicator is at
  the end of the input file
• If a program attempts to read past the end of the
  file, the eof function returns 1 (for True)
  otherwise, it returns 0 (for False)
• Used when the number of records in the file is
  unknown

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Closing a File

• close function is defined in both the ifstream
  and ofstream classes
• Syntax
• object.close ()
• Not closing an open data file can result in loss
  of data

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Example 1

• Write the numbers 0 to n-1 to a data file.
• include ltiostream.hgt
• include ltfstream.hgt
• void Write_Num_File(int n)
• void main ()
• Write_Num_File(5)

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Example 1

• void Write_Num_File(int n)
• int i
• char FileName20 "Data.txt"
• ofstream DataFile
• DataFile.open(FileName)
• if (DataFile.fail())
• cout ltlt "Unable to write to " ltlt FileName ltlt
  endl
• return
• for (i0 iltn i)
• DataFile ltlt i ltlt endl
• DataFile.close()

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Example 2

• Read numbers from a data file.
• include ltiostream.hgt
• include ltfstream.hgt
• void Read_Num_File()
• void main ()
• void Read_Num_File()

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Example 2

• void Read_Num_File()
• int value
• char FileName20 "Data.txt"
• ifstream DataFile
• DataFile.open(FileName)
• if (DataFile.fail())
• cout ltlt "Unable to read from " ltlt FileName ltlt
  endl
• return
• DataFile gtgt value
• while(!DataFile.eof())
• cout ltlt value ltlt endl
• DataFile gtgt value
• DataFile.close()

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Example 3

• Write n strings to a data file.
• include ltiostream.hgt
• include ltfstream.hgt
• void Write_String_File(int n)
• void main ()
• Write_String_File(5)

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Example 3

• void Write_String_File(int n)
• int i
• char FileName20 "Data.txt"
• ofstream DataFile
• DataFile.open(FileName)
• if (DataFile.fail())
• cout ltlt "Unable to write to " ltlt FileName ltlt
  endl
• return
• for (i0 iltn i)
• DataFile ltlt "Test String " ltlt i ltlt endl
• DataFile.close()

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Example 4

• Read strings from a data file.
• include ltiostream.hgt
• include ltfstream.hgt
• void Read_String_File()
• void main ()
• void Read_String_File()

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Example 4

• void Read_String_File()
• char buffer100
• char FileName20 "Data.txt"
• ifstream DataFile
• DataFile.open(FileName)
• if (DataFile.fail())
• cout ltlt "Unable to read from " ltlt FileName ltlt
  endl
• return
• DataFile.getline(buffer,sizeof(buffer))
• while(!DataFile.eof())
• cout ltlt buffer ltlt endl
• DataFile.getline(buffer,sizeof(buffer))
• DataFile.close()

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Example 5

• Append test scores to a data file

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Contents of scores.dat
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Summary

• Components of file access
• Creating file objects
• How to open a sequential access data file
• How to test if a file opened successfully
• How to write records to a sequential file
• How to read records from a sequential file
• How to find the end of a file
• How to close a sequential access data file

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Example

• Average score function
• Write a function that reads scores stored in the
  scores.dat file, calculates the average score,
  and returns the average

scores.dat
97.2 87 73 84.5 72.3
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