Streams, Files, and Formatting
Chapter 8
8.1 Standard Input/Output
Streams
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Stream is a sequence of characters
Working with cin and cout
Streams convert internal representations to
character streams
>> input operator (extractor)
<< output operator (inserter)
Streams have no fixed size
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Reading Data >>
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Leading white space skipped
<nwln> also skipped
Until first character is located
cin >> ch;
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Also read character plus white space as a
character
– get and put functions
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CountChars.cpp
// File: CountChars.cpp
// Counts the number of characters and lines in
// a file
#include <iostream>
#include <string>
using namespace std;
#define ENDFILE "CTRL-D" // CTRL-Z for DOS
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CountChars.cpp
int main()
{
const char NWLN = '\n'; // newline character
char next;
int charCount;
int totalChars;
int lineCount;
lineCount = 0;
totalChars = 0;
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CountChars.cpp
cout << "Enter a line or press " << ENDFILE <<
": ";
while (cin.get(next))
{
charCount = 0;
while (next != NWLN && !cin.eof())
{
cout.put(next);
charCount++;
totalChars++;
cin.get(next);
} // end inner while
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CountChars.cpp
cout.put(NWLN);
lineCount++;
cout << "Number of characters in line " <<
lineCount << " is " << charCount << endl;
}
cout << "Enter a line or press " <<
ENDFILE << ": ";
// end outer while
cout << endl << endl <<
"Number of lines processed is " <<
lineCount << endl;
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CountChars.cpp
cout << "Total number of characters is " <<
totalChars << endl;
return 0;
}
Example of a user session:
Enter a line or press CTRL-D: line one.
Line one.
Number of characters is 9
Enter a line or press CTRL-D: line four.
Line four.
Number of characters is 10
Enter a line or press CTRL-D: CTRL-D
Number of lines processed is 2
Total number of characters is 19
8
Stream Functions
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stream.open(fname): opens a file stream
stream.get(ch): extracts one character from stream
stream.put(ch): inserts one character into stream
stream.close(): closes an opened file stream
stream.eof(): return true if end of stream is
reached and false otherwise
stream.fail(): returns true if a precedent open has
failed and false otherwise
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8.2 External Files
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Batch processing
– Read data from a file (not from keyboard)
•  data can be examined
•  don’t need to retype data when you rerun program
–
–
–
–
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Requires use of data files (save to disk)
Batch can be run during off peak use
allows things to be complete at start of day
Helpful is to echo-print (on the screen) read data
Interactive processing
– Real time systems
– Ok for smaller programs
– Programs that complete quickly
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Files
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Naming (system-dependent)
– Directory name e.g. /Myfiles/
– File name e.g. Payroll.txt
– Extensions: .cc
.dat
.out

.txt
How to attach files to the stream
–
–
–
–
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.in
stream object
external file name
internal name
open
Additional functions as part of fstream class
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Files

Declare the stream to be processed need to
#include <fstream>
ifstream ins; // input stream
ofstream outs; // output stream

Need to open the files
ins.open (inFileName);
outs.open (outFileName);
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Files
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#define associates the name of the stream
with the actual file name
 You can directly give file names
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fail() function - returns true (nonzero) if file
fails to open
Program CopyFile.cpp demonstrates the use
of the other fstream functions
– get , put, close and eof
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CopyFile.cpp
// File: CopyFile.cpp
// Copies file InData.txt to file OutData.txt
#include <cstdlib>
#include <fstream>
using namespace std;
// Associate stream objects with external file
// names
#define inFile "InData.txt"
#define outFile "OutData.txt"
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CopyFile.cpp
// Functions used ...
// Copies one line of text
int copyLine(ifstream&, ofstream&);
int main()
{
// Local data ...
int lineCount;
ifstream ins;
ofstream outs;
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CopyFile.cpp
// Open input and output file, exit on any
// error.
ins.open(inFile);
if (ins.fail ())
{
cerr << "*** ERROR: Cannot open " <<
inFile << " for input." << endl;
return EXIT_FAILURE;
// failure return
} // end if
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CopyFile.cpp
outs.open(outFile);
if (outs.fail())
{
cerr << "*** ERROR: Cannot open " <<
outFile << " for output." << endl;
return EXIT_FAILURE;
// failure return
} // end if
// Copy each character from inData to outData.
lineCount = 0;
do
{
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CopyFile.cpp
if (copyLine(ins, outs) != 0)
lineCount++;
} while (!ins.eof());
// Display a message on the screen.
cout << "Input file copied to output file." <<
endl;
cout << lineCount << " lines copied." << endl;
ins.close();
outs.close();
return 0;
}
// successful return
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CopyFile.cpp
// Copy one line of text from one file to another
// Pre: ins is opened for input and outs for
// output.
// Post:
Next line of ins is written to outs.
//
The last character processed from
//
ins is <nwln>;
//
the last character written to outs
//
is <nwln>.
// Returns: The number of characters copied.
int copyLine (ifstream& ins, ofstream& outs)
{
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CopyFile.cpp
// Local data ...
const char NWLN = '\n';
char nextCh;
int charCount = 0;
// Copy all data characters from stream ins to
//
stream outs.
ins.get(nextCh);
while ((nextCh != NWLN) && !ins.eof())
{
outs.put(nextCh);
charCount++;
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CopyFile.cpp
}
}
ins.get (nextCh);
// end while
// If last character read was NWLN write it
// to outs.
if (!ins.eof())
{
outs.put(NWLN);
charCount++;
}
return charCount;
// end copyLine
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CopyFile.cpp
Program Output
Input file copied to output file.
37 lines copied.
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File Processing
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Newline character
eof() function returns a False if file is not empty
while ( ! ins.eof()) { do stuff }
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Could have used getline():
string line;
linCount = 0;
getline(ins, line);
while(line.lentgh() != 0) {
lineCount++;
outs << line << endl;
getline(ins, line);
}
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8.3 Using External File
Functions
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Payroll Case Study
Program 1: computes salary using hours and rate
Input file: e.g.
Adam Smith
Thomas New
Ayisha Martin
Output file: e.g.
Adam Smith
Thomas New
Ayisha Martin
35.5
7.25<nwln>
40.0 6.50 <nwln>
20.0 8.00 <nwln><eof>
$257.38<nwln>
$260.00<nwln>
$160.00<nwln><eof>
Program 2: (not handled here)
Does further processing on output file of program 1
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Payroll Case Structure Chart
Write the
payroll file
Prepare
files
Process all
employees
Display
payroll
processEmp
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ProcessEmp Structure Chart
Process all
employees
Read employee
first and last
names
Read employee
salary data (hours
and rate)
Compute
salary
Add
salary to
total payroll
Write first
and last names
and salary
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Payroll.cpp
// File: Payroll.cpp
// Creates a company employee payroll file
// computes total company payroll amount
#include
#include
#include
#include
<fstream>
<cstdlib>
"money.h"
"money.cpp"
using namespace std;
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Payroll.cpp
// Associate streams with external file names
#define inFile "EmpFile.txt" // employee file
#define outFile "Salary.txt" // payroll file
// Functions used ...
// PROCESS ALL EMPLOYEES AND COMPUTE TOTAL
money processEmp(istream&, ostream&);
int main()
{
ifstream eds;
ofstream pds;
money totalPayroll;
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Payroll.cpp
// Prepare files.
eds.open(inFile);
if (eds.fail ())
{
cerr << "*** ERROR: Cannot open " << inFile
<< " for input." << endl;
return EXIT_FAILURE;
// failure return
}
pds.open(outFile);
if (pds.fail())
{
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Payroll.cpp
cerr << "***ERROR: Cannot open " << outFile
<< " for output." << endl;
eds.close();
return EXIT_FAILURE; // failure return
}
// Process all employees and compute total
// payroll.
totalPayroll = processEmp(eds, pds);
// Display result.
cout << "Total payroll is " << totalPayroll <<
endl;
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Payroll.cpp
// Close files.
eds.close();
pds.close();
return 0;
}
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Payroll.cpp
// Insert processEmp here.
// Process all employees and compute total
// payroll amount
// Pre: eds and pds are prepared for
// input/output.
// Post: Employee names and salaries are
// written from eds to pds
// and the sum of their salaries is returned.
// Returns: Total company payroll
money processEmp (istream& eds, ostream& pds)
{
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Payroll.cpp
string firstName;
string lastName;
float hours;
money rate;
money salary;
money payroll;
company payroll
//
//
//
//
input: hoursWorked
input: hourly rate
output: gross salary
return value - total
payroll = 0.0;
// Read first employee's data record.
eds >> firstName >> lastName >> hours >> rate;
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Payroll.cpp
while (!eds.eof())
{
salary = hours * rate;
pds << firstName << “ “ <<lastName << “ “ <<
salary << endl;
payroll += salary;
}
// Read next employee's data record.
eds >> firstName >> lastName >> hours >>
rate;
} // end while
return payroll;
// end processEmp
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PayrollFile.cpp
Program Output
Total payroll is $677.38
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8.4 More on Reading String
Data
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Getline - could be used to process an entire
line of data
Use # as a delimiter character
getline (eds, name, ‘#’);
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Advance the newline
getline (eds, name, ‘\n’);
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Use care when choosing cin, get or getline
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Getline() and Ignore()
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getline(istream& in, string& s)
Reads a line from input stream in and stores it in s
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getline(istream& in, string& s, char delimiter)
Reads all characters up to ‘delimiter’ and stores them in s
(delimiter is read but not stored in s!)
ins.ignore(int n, char delimiter)
Extracts (but not store) up to n characters from input
stream ins until the first ‘delimiter’ character is
encountered. Delimiter is extracted but not stored
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8.5 Input/Output Manipulators
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Chapter 5 covered setw width
Also endl has been introduced
boolalpha has been also used to get ‘true’ and
‘false’ from keyboard
Can be used with the cout and <<
#include <iomanip> when using manipulators
Can be used with external files like cin and cout
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I/O Manipulators
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setw(int n): set the width of the next output
setprecesion(int n): sets the precision for (float) numbers (default is 6)
boolalpha: enables true/false for I/O
noboolalpha: disable previous operation
fixed: decimal notation of floats
scientific: scientific notation of floats
showpoint: display decimal point also for integers
noshowpoint: undo previous operation
left: left-adjust output in field
right: right-adjust (default)
skipws: skip white spaces on input (default)
noskipws: undo previous operation
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Formatting with State Flags
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Depending on the setiosflags or unsetiosflags
– Output can be controlled by other format state flag
– Flags are enumerated types
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ios::flagname
Flags e.g.:
– boolalpha, fixed, left, right, showpoint etc
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Examples:
cin.setf(ios::boolalpha);
cout.setf(ios::boolalpha);
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8.6 Common Programming
Errors
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Connecting streams and external files
– Declare stream object and open the file
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Watch use of while loops when processing
– Test values see what you actually have
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Reading past the eof
White space
Newline character
Formatting via flags
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