File I/O

1
File I/O
ifstreams and ofstreams Sections 11.1 11.2
1
2
Introduction

• Up to now we have been
• getting input from the keyboard via cin
• sending output to the screen via cout and cerr
• We now see how to
• get input from a text file
• send output to a text file

2
3
Problem
Using OCD, design and implement a program that
computes the average of a sequence of numbers
stored in a file.
Project 9
3
4
Preliminary Analysis

• Using include ltiostreamgt, a program can read
  from the keyboard via the istream named cin,
• but this is of little benefit when the data we
  need is in a file.
• What we need is a way to somehow establish an
  _________-like connection between our __________
  and the_________________, such that we can then
  read from the file via that connection.

Program
4
5
Behavior

• Our program should display its purpose and then
  display a prompt for the name of the input file,
  which it should then read.
• Our program should open a connection from itself
  to that input file. It should
• then read the numbers from the input file via
  the connection, and compute their sum and count.
  It should then
• close the connection, and compute and display the
  average of the numbers.

Program
data1.txt
8097 93.57265.8 . . . 68.3
5
6
Objects

• Description Type Kind
  Name

purpose, prompt string constant (none) file
name string varying inFileName connection
?? varying fin a number double varying number sum
double varying sum count double varying count aver
age double varying average
6
7
Operations

• Description Library? Name

or gtgt if no blanks in filenames
display a string string ltlt read a string
string getline() open a connection ?? ??to
a file read numbers via ?? ??connection sum
count numbers built-in , , loop close
connection ?? ?? compute average
built-in / display average iostream ltlt
7
8
Algorithm

• 0. Display purpose of program, and prompt for
  input file name.
• 1. Read name of input file from cin into
  inFileName.
• 2. Open connection named fin to file named in
  inFileName.
• 3. Initialize sum, count to zero.
• 4. Loop
• a. Try to read a value from fin into number
• b. If no values were left, terminate
  repetition.
• c. Add number to sum.
• d. Increment count.
• End loop.
• 5. Close fin.
• 6. If count gt 0
• a. Compute average sum / count. b. Display
  count and average with appropriate labels
• Else
• display error message.

8
9
Implementation in C

• To establish connections to an file, the
  ___________ library provides file-stream types
• the ifstream class for input streams
• the ofstream class for output streams
• The programmer uses these types to declare
  file-stream objects for example,
• _______________________
• _______________________

9
10

• To connect such fstream objects to files
• Method 1 Use the open() function member in
  these classes
• ifstream fin
• __________________________________
• ofstream fout
• __________________________________
• Method 2 Use initializing declarations
• ifstream fin("name of file") ofstream
  fout("name of file")

10
11

• Disadvantage of "hard-wiring" actual file) into a
  program the program must be modified when using
  a different file.
• A better alternative Have user enter the file
  name and store it in a string variable for
  example,
• string inFileName cout ltlt "Enter name of file
  " getline(cin, inFileName)
• When attaching an fstream to this file, we must
  use string's _____________ (or c_str()) function
  member to extract the actual file name stored in
  the string object for example,
• ifstream _____________________________
• Similar declarations are used for output streams.

or use gtgt ifno blanks in filenames
11
12

• How to read from / write to a file connected to
  an fstream
• Input Use ____ just like from an ____________
  
• fin gtgt var
• Output Use ____ just like from an ____________
• fout ltlt expr

or getline()
Note For output, we can use any of the format
manipulators such as endl, fixed, setprecison(),
and setw()
ifstream inherits everything from
istream ofstream " "
" ostream
12
13
When we are finished with a file both for
input and output files we should break the
connection between it and the program. Although
this will happen automatically when the
connection reaches the end of its scope, it is a
good idea to do this explicitly with the
fstream's function member ____________ for
example, fin.close() fout.close()
13
14
Objects

• Description Type Kind
  Name

purpose, prompt string constant (none) file
name string varying inFileName connection ifstream
varying fin a number double varying number sum do
uble varying sum count double varying count averag
e double varying average
14
15
Operations

• Description Library Name

display a string string ltlt read a string
string getline() open a connection
fstream declarationto a file of
fstream (or open()) read numbers via
fstream gtgtconnection sum count numbers
built-in , , loop close connection
fstream close() compute average built-in /
display average iostream ltlt
or gtgt
15
16
In Visual Ccreate it in Visual C and
"Add to" project. Must be in same folder as
program.cpp and the .vcproj

• Two More Problems
• Opening a file for input can fail.
• Solution Use the ifstream function member
  _______________.
• ifstream fin(infileName.data()) assert(_______
  ________________)

2. How do we know when we all the data in the
file has been read? The ifstream function member
_________ returns true if an _________________
readfound no data remaining in the file.
Must tryto read and fail
16
17
Coding

• /------------------------------------------------
  -------------
• Program to read data values from a text file,
  count them,
• and find their average.
• Input(keyboard) Name of the file
• Input(file) doubles
• Output(screen) Prompts, average of the data
  value with labels, or file-empty
  message
• -------------------------------------------------
  ------------/include ltiostreamgt
  // cin, cout, ...
• ______________________________ // ifstream,
  ofstream, ...
• include ltstringgt //
  stringinclude ltcassertgt //
  assert()using namespace stdint main()
  cout ltlt "\nTo average the numbers in an input
  file," ltlt "\nenter the name of the file
  "
• string inFileName
• getline(cin, inFileName

17
18

• ________________________________ // open the
  connection

________________________________ // verify it
opened double number, sum 0.0 //
variables for int count 0
// computing average
for () // input loop
_____________________ // read
number if (_______________) break // if
none were left, quit sum number
// add it to sum count
// bump up count
// end loop ______________________
_ // close fstream
18
19
if (count gt 0) double average sum /
count cout ltlt "\nThe average of the values
in " ltlt inFileName ltlt " is " ltlt average
ltlt endl else cout ltlt "\n No values
found in file " ltlt inFileName ltlt endl

19
20
Testing

• To test our program, we use a text editor and
  create some easy-to-check input files such as
• 10 20
• 30
• 40
• If we name this particular file test1.txt and
  enter its name when prompted by our program, the
  average value produced should be 25.

Create text files in Visual C
20
21
To average the numbers in an input file, enter
the name of the file test1.txt The average of
the values in test1.txt is 25

• Continue testing using other input files, trying
  to find places where our program breaks down.
• And it is is important to try various kinds of
  files ascending order, descending order, random
  order, files with 1 element, empty files . . .
  because programs that work for some of these may
  fail for others.

21
22

• Once we are confident that our program is
  correct, we can execute it with the data file
  from our problem

To average the numbers in an input file, enter
the name of the file data1.txt The average of
the values in test1.txt is 80.1
22
23
Key Things to Remember

• The fstream library defines two classes
• ifstream, for creating connections between
  programs and input files and
• ofstream, for creating connections between
  programs and output files.
• Both ifstream and ofstream objects are createdin
  a similar fashion.

23
24
If inFileName contains the name of an input file,
and outFileName contains the name of an output
file, then the statements ifstream
fin(inFileName.data()) ofstream
fout(outFileName.data()) define fin and fout as
connections to them.
or ifstream fin fin.open(inFileName.data()
) or ofstream fout fout.open(outFileName
.data())

• Note that the string function member data() (or
  c_str()) must be used to retrieve the actual
  char-acters of the files name from the file
  stream object.

24
25

• If a program tries to open an ifstream to a file
  that doesnt exist, the open is said to fail.
• To check whether or not an ifstream is open, the
  ifstream class provides the is_open() function
  member, which returns true if the ifstream was
  successfully opened, and false if it not.
• Whether or not a file opened correctly should
  always be verified using is_open().

assert(fin.is_open()) or assert(!fin.fail())
or use an if
25
26

• Important Notes About Output Files
• If a program tries to open an ofstream to a file
  that doesnt exist or that can't be found, the
  open operation creates a new __________________
  for output.
• If a program tries to open an ofstream to a file
  that does exist, the open operation (by default)
  ____________ that file of its contents, creating
  a clean file for output.

_____________it!Be careful!
To open an existing file for output without
emptying it, the value iosapp can be given
as a second argument ofstream
fout(outFileName.data(), iosapp) Output will
be appended to whatever is already in the file.
26
27

• For rare occasions where a file is needed for
  both input and output, the fstream class is
  provided
• fstream fInOut(ioFileName.data(),
• iosin iosout)
• This statement defines an fstream named fInOut to
  a file, from which data can be read, and to which
  data can be written.
• The bitwise OR operator () is used to combine
  file open-modes in this manner.

27
28
Summary

• include ltfstreamgt to use file streams.
• To establish a connection to a file whose name is
  given by a string variable fileName
• For input ifstream fin(fileName.data())
• or ifstream fin fin.open(fileName.data
  ())
• For output ofstream fout(fileName.data())
• or ofstream fout fout.open(fileName.da
  ta())

Note data() (or c_str()) must be used to
retrieve the actual characters of the files
name from fileName.
28
29

• To check whether or not an ifstream is open
• assert(fin.is_open()) or
• assert(!fin.fail())
• (or use an if statement)

• Once an ifstream (or ofstream) has been opened,
  it can be read from (or written to) using the
  usual input (or output) operations
• input gtgt, get(), getline(), ...
• output ltlt, put(), ...
• In general, anything that can be done to an
  istream (or ostream) can be done to an ifstream
  (or ofstream).

29
30

• The eof() function member provides a convenient
  way to build input loops
• for ()
• fin gtgt someValue
• if (fin.eof()) break
• // ... process someValue
• Remember that opening a file for output erases
  any contents in the file.

30
31

• Once we are done using an ifstream (or ofstream),
  it should be closed using the close() function
  member fin.close()
• fout.close()
• Most systems limit the number of files a program
  can have open simultaneously, so it is a good
  practice to close a stream when finished with it.
• A closed file can be (re)opened using open()
• fin.open(inFileName.data())
• fout.open(outFileName.data())

31