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MODULE 18

C++ STREAM FORMATTED I/O 1

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MODULE 5: C FORMATTED I/O

 

 

 

 

My Training Period: xx  hours   The source code for this tutorial is available in C++ Formatted I/O source codes.   The C++ formatted I/O programming skills:   To understand and use various member functions for C++ formatted I/O. To understand and use various stream manipulators for C++ formatted I/O.   18.1  iostream Library In C Formatted I/O you have learned the formatted I/O in C by calling various standard functions.  In this Module we will discuss how this formatted I/O implemented in C++ by using member functions and stream manipulators. If you have completed this C++ Data Encapsulation until C++ Polymorphism, you should be familiar with class object.  In C++ we will deal a lot with classes.  It is readily available for us to use. We will only discuss the formatted I/O here, for file I/O and some of the member functions mentioned in this Module, will be presented in another Module.  The discussion here will be straight to the point because some of the terms used in this Module have been discussed extensively in C Formatted I/O. The header files used for formatted I/O in C++ are: Header file Brief description <iostream> Provide basic information required for all stream I/O operation such as cin, cout, cerr and clog correspond to standard input stream, standard output stream, and standard unbuffered and buffered error streams respectively. <iomanip> Contains information useful for performing formatted I/O with parameterized stream manipulation. <fstream> Contains information for user controlled file processing operations. <strstream> Contains information for performing in-memory formatting or in-core formatting.  This resembles file processing, but the I/O operation is performed to and from character arrays rather than files. <stdiostrem> Contains information for program that mixes the C and C++ styles of I/O.   Table 18.1:  iostream library The compilers that fully comply with the C++ standard that use the template based header files won’t need the .h extension.  Please refer to Namespaces for more information. The iostream class hierarchy is shown below.  From the base class ios, we have a derived class: Class Brief description istream Class for stream input operation. ostream Class for stream output operation.   Table 18.2:  ios derived classes So, iostream support both stream input and output.  The class hierarchy is shown below.

 

18.2  Left and Right Shift Operators

• We have used these operators in most of the previous tutorials for C++ codes.

• The left shift operator (<<) is overloaded to designate stream output and is called stream insertion operator.

• The right shift operator (>>) is overloaded to designate stream input and is called stream extraction operator.

• These operators used with the standard stream object (and with other user defined stream objects) is listed below:

• For file processing C++ uses (will be discussed in another Module) the following classes:

• Stream output program example:

// string output using <<

#include <iostream>

using namespace std;

 

void main(void)

{

cout<<"Welcome to C++ I/O module!!!"<<endl;

cout<<"Welcome to ";

cout<<"C++ module 18"<<endl; 

// endl is end line stream manipulator

// issue a new line character and flushes the output buffer

// output buffer may be flushed by cout<<flush; command

}

 

Output:

 

 

// concatenating <<

#include <iostream>

using namespace std;

 

void main(void)

{

int p = 3, q = 10;

 

cout << "Concatenating using << operator.\n"

     <<"--------------------------------"<<endl;

cout << "70 minus 20 is "<<(70 - 20)<<endl;

cout << "55 plus 4 is "<<(55 + 4)<<endl;

cout <<p<<" + "<<q<<" = "<<(p+q)<<endl;

}

 

Output:

 

• Stream input program example:

#include <iostream.h>

using namespace std;

 

void main(void)

{

int p, q, r;

 

cout << "Enter 3 integers separated by space: \n";

cin>>p>>q>>r;

// the >> operator skips whitespace characters such as tabs,

// blank space and newline.  When eof is encountered, zero (false) is returned.

cout<<"Sum of the "<<p<<","<<q<<" and "<<r<<" is = "<<(p+q+r)<<endl;

}

 

Output:

 

 

 

18.3  get() and getline() Member Functions of Stream Input For the get() function, we have three versions. get() without any arguments, input one character from the designated streams including whitespace and returns this character as the value of the function call.  It will return EOF when end of file on the stream is encountered.  For example: cin.get(); get() with a character argument, inputs the next character from the input stream including whitespace.  It return false when end of file is encountered while returns a reference to the istream object for which the get member function is being invoked.  For example: char ch; ... cin.get(ch); get() with three arguments, a character array, a size limit and a delimiter (default value ‘\n’).  It reads characters from the input stream, up to one less than the specified maximum number of characters and terminates or terminates as soon as the delimiter is read.  For example:

char  namevar[30];

...

cin.get(namevar, 30);

// get up to 29 characters and inserts null

// at the end of the string stored in variable

// namevar.  If a delimiter is found,

// the read terminates.  The delimiter

// is left in the stream, not stored in the array.

4. getline() operates like the third get() and insert a null character after the line in the character array.  It removes the delimiter from the stream, but does not store it in the character array.

• A program examples:

// end of file controls depend on system

// Ctrl-z followed by return key - IBM PC, Ctrl-d - UNIX and MAC

#include <iostream>

using namespace std;

 

void main(void)

{

char p;

 

cout <<"Using member functions get(), eof() and put()\n"

     <<"---------------------------------------------"<<endl;

cout<<"Before any input, cin.eof() is "<<cin.eof()<<endl;

cout<<"Enter a line of texts followed by end of file control: "<<endl;

 

while((p = cin.get()) !=EOF)

    cout.put(p);

cout<<"\nAfter some text input, cin.eof() is "<<cin.eof()<<endl;

}

 

Output:

 

 

// another get() version

#include <iostream>

using namespace std;

 

const int SIZE = 100;

 

void main(void)

{

char bufferOne[SIZE], bufferTwo[SIZE];

 

cout <<"Enter a line of text:"<<endl;

cin>>bufferOne;

// store the string in array bufferOne

// just the first word in the array string, then the

// first whitespace encountered

cout<<"\nThe line of text read by cin>> was:"<<endl;

cout<<bufferOne<<endl;

cin.get(bufferTwo, SIZE);

// the rest of the string

cout<<"The line of text read by cin.get(bufferTwo,SIZE) was:"<<endl;

cout<<bufferTwo<<endl;

}

 

Output:

 

 

// getline() example

#include <iostream>

using namespace std;

 

const SIZE = 100;

 

void main(void)

{

char buffer[SIZE];

 

cout<<"Read by cin.getline(buffer, SIZE)\n"

    <<"--------------------------------\n"<<"Enter a line of text:"<<endl;

cin.getline(buffer, SIZE);

cout<<"The line of text entered is: "<<endl;

cout<<buffer<<endl;

}

 

Output:

 

• ignore() member function skips over a designated number of characters (default is one character) or terminates upon encountering a designated delimiter (default is EOF).  For example:

cin.ignore();        // gets and discards 1 character.

 

cin.ignore(5);       // gets and discards 5 characters.

 

cin.ignore(20,’\n’);

// gets and discards up to 20 characters or until

// newline character whichever comes first.

• putback() member function places the previous character obtained by a get() on an input stream back onto that stream.  For example:

char chs;

...

cin.putback(chs);

// put character back in the stream

• peek() member function returns the next character from an input stream, but does not remove the character from the stream.  For example:

char chs;

...

chs = cin.peek();

// peek at character

• Unformatted I/O performed with read() and write() member functions.  They simply input or output as raw byte.

• The read() member function extracts a given number of characters into an array and the write() member function inserts n characters (nulls included).  For example:

char texts[100];

...

cin.read(texts, 100);

// read 100 characters from input stream and don’t append ‘\0’

• Program example:

// using read(), write() and gcount() member functions

#include <iostream>

using namespace std;

 

const int SIZE = 100;

 

void main(void)

{

char buffer[SIZE];

 

cout<<"Enter a line of text:"<<endl;

cin.read(buffer,45);

cout<<"The line of text entered was: "<<endl;

cout.write(buffer, cin.gcount());

// the gcount() member function returns

// the number of unformatted characters last extracted

cout<<endl;

}

 

Output:

 

 

 

18.4  Stream Manipulators

• Used to perform formatting, such as:

1. Setting field width.

2. Precision.

3. Unsetting format flags.

4. Flushing stream.

5. Inserting newline in the output stream and flushing the stream.

6. Inserting the null character in the output stream.

7. Skipping whitespace.

8. Setting the fill character in field.

18.4.1  Stream Base

• For stream base we have:

• Program example:

// using hex, oct, dec and setbase stream manipulator

#include <iostream>

using namespace std;

#include <iomanip>

 

void main(void)

{

int p;

 

cout<<"Enter a decimal number:"<<endl;

cin>>p;

cout<<p<< " in hexadecimal is: "

<<hex<<p<<'\n'

<<dec<<p<<" in octal is: "

<<oct<<p<<'\n'

<<setbase(10) <<p<<" in decimal is: "

<<p<<endl;

cout<<endl;

}

 

Output:

 

 

 

 

 

 

 

 

 

 

 

18.4.2  Floating-point Precision

• Used to control the number of digits to the right of the decimal point.

• Use setprecision() or precision().

• precision 0 restores to the default precision of 6 decimal points.

// using precision and setprecision

#include <iostream>

using namespace std;

#include <iomanip>

// using C++ wrappers to access C function

#include <cmath>

 

void main(void)

{

double theroot = sqrt(11.55);

 

cout<<"Square root of 11.55 with various"<<endl;

cout<<"           precisions"<<endl;

cout<<"---------------------------------"<<endl;

cout<<"Using 'precision':"<<endl;

for(int poinplace=0; poinplace<=8; poinplace++)

{

cout.precision(poinplace);

cout<<theroot<<endl;

}

cout<<"\nUsing 'setprecision':"<<endl;

for(int poinplace=0; poinplace<=8; poinplace++)

cout<<setprecision(poinplace)<<theroot<<endl;

}

 

Output:

 

 

18.4.3  Field Width

• Sets the field width and returns the previous width.  If values processed are smaller than the field width, fill characters are inserted as padding.  Wider values will not be truncated.

• Use width() or setw().  For example:

cout.width(6);  // field is 6 position wide

• A program example:

// using width member function

#include <iostream>

using namespace std;

 

void main(void)

{

       int p = 6;

       char string[20];

 

       cout<<"Using field width with setw() or width()"<<endl;

       cout<<"----------------------------------------"<<endl;

       cout<<"Enter a line of text:"<<endl;

       cin.width(7);

       while (cin>>string)

       {

              cout.width(p++);

              cout<<string<<endl;

              cin.width(7);

              // use ctrl-z followed by return key or ctrl-d to exit

       }

}

 

Output:

 

 

tenouk C++ formatted I/O tutorial

 

 

 

 

 

 

Further C++ formatted I/O reading:

 

1. The source code for this tutorial is available in C++ Formatted I/O source codes.

2. Visual C++ .NET programming tutorials.

3. Check the best selling C / C++, Object Oriented and pattern analysis books at Amazon.com.

 

 

 

 

 

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C++ Stream Formatted Input/Output:  Part 1 | Part 2