Computing Fundamentals with C

1
C and Object-Oriented Programming File I/O
2
File Streams

• Goals
• Use ifstream objects for disk file input
• Use ofstream objects for disk file output
• Apply the indeterminate loop pattern to process
  data until end of file

3
ifstream objects

• ifstream objects
• allow input from a disk file
• are similar to istream objects cin
• they share the same named operations
• they use the same operator for input ltlt
• must be initialized by the programmer
• can be tested to determine if a disk file
  actually exists

4
ifstream objects continued

• General form to construct ifstream objects
• ifstream object-name ( "file-name" )
• Example code
• ifstream inFile("myfile.dat")
• this example associates the object name inFile
  with the file named myfile.dat in the present
  working directory.

5
Reading 3 numbers

• include ltfstreamgt // for the ifstream class
• include ltiostreamgt // needed for cout
• using namespace std
• int main()
• int n1, n2, n3
• ifstream inFile("input.dat")
• // Extract 3 integers from the file "input.dat"
• inFile gtgt n1 gtgt n2 gtgt n3
• cout ltlt "n1 " ltlt n1 ltlt endl
• cout ltlt "n2 " ltlt n2 ltlt endl
• cout ltlt "n3 " ltlt n3 ltlt endl
• return 0

6
Getting the path right

• It is easy to initialize an ifstream object and
  not have it associated with an actual disk file
• perhaps the file does not exist
• perhaps the path is wrong
• Recall absolute and relative addressing concepts

7
Reading File names

• When the user enters the file name, they may use
  one \ in UNIX it doesnt matter
• string filename
• cout ltlt "Enter file name "
• cin gtgt filename
• ifstream inFile( filename.c_str() )
• Dialogue
• Enter file name in.dat
• The ifstream class needs a C string literal like
  "in.dat", or the c_str() of the string as in
• filename.c.str() // The C string literal

8
The Indeterminate loop pattern Applied to Disk
Files

• The end of file event can be used to terminate
  user input
• cout ltlt "Enter numbers or end of file to
  quit\n"
• cout ltlt "ctrl-D" ltlt endl
• double x
• int n 0
• while(cin gtgt x)
• // process x
• n

9
eof.cpp

• // pre input.dat is in the current directory
• ifstream inFile("input.dat")
• string s
• if(!inFile)
• cout ltlt "Error 'input.dat' not found" ltlt
  endl
• else
• cout ltlt "Before, eof " ltlt inFile.eof() ltlt
  endl
• while(inFile gtgt s)
• cout ltlt "s " ltlt s ltlt endl
• cout ltlt " After, eof " ltlt inFile.eof() ltlt
  endl

10
Processing until End of File

• We often need to extract data from files stored
  on a computer disk.
• We use ifstream objects for this input.
• The next slide shows a program that averages
  numbers from the file input.dat that has

70.0 80.0 90.0 75.0 85.0
11
Read until end of file

• include ltfstreamgt // for class ifstream
• include ltiostreamgt
• using namespace std
• int main()
• ifstream inFile("input.dat")
• double number, sum 0.0
• int n 0
• while(inFile gtgt number)
• sum number
• n
• cout ltlt "Average " ltlt (sum / n) ltlt endl
• return 0
• Output
• Average 70

12
Event Loop with More Complex Disk File Input

• Assume this file data represents four employees
  each line represents one employee
• 40 8.88 1 S Demlow Mary
• 42 7.77 2 M Barrister Harvey
• 0 10.00 3 M Manuala Ho
• 38 9.99 0 S Kline Sue
• Each line of data could be processed until end of
  file.
• Code should work with files of different sizes
  (different numbers of employees).

13
Algorithm to process any number of lines

• For each employee in the file do the following
• Input all data from one line in the file. This
  data represents one employee.
• Use data to construct one weeklyEmp object.
• Use two weeklyEmp member functions grossPay()
  and name()

14
Example Code for eof loop

• cout.fill('') // set as leading character
• while(inFile gtgt hours gtgt rate gtgt exemptions
• gtgt filingStatus gtgt lname gtgt fname )
• emp weeklyEmp(lname", "fname, hours, rate,
• exemptions, filingStatus)
• cout.width(9)
• cout ltlt emp.grossPay()ltlt" "ltlt emp.name() ltlt
  endl
• Output
• 355.20 DEMLOW, MARY
• 334.11 BARRISTER, HARVEY
• 0.00 MANUALA, HO
• 379.62 KLINE, SUE

15
Mixing Numbers and Strings

• Use care when input has a mix of numeric and
  alphanumeric data.
• 40 8.88 1 S Demlow Mary
• 42 7.77 M 2 Barrister Harvey
• Match input order to file data or vice versa
• It is all too easy to

Unexpected character destroys the input stream
16
getline

• Use getline to read in lines of data as one
  string. Here is the function heading
• istream getline(istream is, string s,
• char sentinel '\n')
• Examples
• string name, address
• getline(cin, address) // 1313 MockingB Lane
• getline(inFile, name, '.') // Fatts, Ernest T.
• while( getline(infile, address) )
• // process address

17
ofstream objects

• The files storing large amounts of data are
  typically created by programs that send output to
  those files rather than the screen
• The ofstream class is used to represent a disk
  file for output
• General form
• ofstream object-name ( "file-name" )

18
Sample Program

• include ltfstreamgt // for class ifstream
• include ltiostreamgt
• using namespace std
• include "myfuns" // for decimals
• int main()
• ofstream fout("output.dat")
• fout ltlt "This does not go to the screen" ltlt
  endl
• cout ltlt "This does" ltlt endl
• double x 1.23456789
• fout ltlt x ltlt endl
• decimals(fout, 3)
• fout ltlt x ltlt endl
• decimals(fout, 6)
• fout ltlt x ltlt endl
• return 0