C++ Stream I/O

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C++ Stream I/O

1. Stream Input/Output

Standard Outputcout,cerr
Standard Inputcin
Formatting
Error handling
File I/O (later)

2. Stream I/O Applications

network communication
encryption
data compression
persistent storage of objects
character-oriented input and output

3. Stream Output Concept


    std::cout << "HELLO";
    std::cout << '\n';
    std::cout << 123;

Output stream buffer:

...

4. Stream Input Concept

Input stream buffer:

...


    int one;
    int two;
    std::cin >> one;
    std::cin >> two;

5. Using C++ Objects

Objects are C++variablesofuser-defined types. They are declared as
```
    typename object_name;
```

For example, ifShape3Dis a user-defined type, then we could try


    Shape3D widget;               // create object named "widget"

    widget.display( parameters ); // call member function with parameters

    widget.reset( );              // call member function without parameters

Dot-notation is used to access functions built into objects.

Alternatively, some objects may allow use of arithmetic operators. For example,


    std::cout << widget;          // display widget on console output device

6. Standard Output Stream


    typename      object name
    ------------  -----------
    std::ostream  std::cout;  // cout is a predefined object

Usage example:


    #include <iostream> // C++ standard library header

    using std::cout;    // cout declared inside std namespace

    int main()
    {
        int x = 65;
        cout << x;      // display value of number 65
        cout.put( x );  // display character 'A'
        return 0;
    }

    /* This program prints:
    65A
    */

7. Standard Output Stream, Cont.

std::coutis a pre-defined object
We need to
```
    #include <iostream>
```
to bringstd::coutinto our program
std::coutis attached to thestandard output device, which can be console display, file, or printer.
I/O redirection to file or printer is made easy:
```
CMD> myprogram > myfile.txt
```
The above command sends standard output tomyfile.txt

8. Formatted Output

C++ data types have OS- and hardware-specific internal representations...
- ...If we display anumeric valueusingformatted output, its value is properlyconvertedto a corresponding sequence of characters.
- ...If we display abool, we couldconvertits value to words"true"or"false".
Formatted Outputhandles value-to-text conversions for us:

7 8 9 ; 0 . 5 ; t r u e

For example,


#include <iomanip>
#include <iostream>
using std::cout;
using std::boolalpha;

int main( )
{
     int i = 789;
     double d = 0.5;
     bool b = true;

     cout << i;
     cout << ';';
     cout << d;
     cout << ';';
     cout << boolalpha << b;
     return 0;
}
/* Program output:
789;0.5;true
*/

9. Standard Input Stream


    typename      object name
    ------------  -----------
    std::istream  std::cin;    // cin is a predefined object

Usage:


    // formatted input of C++ variables
    int x;
    cin >> x;

    // unformatted input of single characters
    char x = cin.get( );

    // line-based input of text strings
    string text;
    getline( cin, text );

10. Standard Input Stream, Cont.

std::cinis a pre-defined object
We need to have
```
    #include <iostream>
```
to bringstd::cininto our program
std::cinis attached to astandard input device
Example of I/O redirection from file or another device:
```
    CMD> myprogram < myfile.txt
```
The above command gets standard input frommyfile.txt

11. Formatted Input

Formatted inputmeans

reading expected type and format
automatic whitespace processing


        #include <iostream>

        int main( )
        {
            int one;
            int two;
            std::cout << "Please enter two integers: ";
            std::cin >> one >> two;
            return 0;
        }

12. Unformatted I/O Example

Unformatted inputmeans

reading individual characters
discovering data type and format as you read


        #include <iostream>

        int main()
        {
            int onechar; // using int because char cannot represent EOF

            while ( ( onechar = std::cin.get() ) != EOF ) {
                std::cout.put( onechar );
            }

            return 0;
        }

13. What is EOF?

AcronymEOFstands for condition known asend of file.
When reading from a file, the condition occurs naturally at the end of file.
To keep things consistent, the condition is also triggered when user enters
```
    CTRL+Z (on Windows machine)

    CTRL+D (on Unix machine)
```
on the keyboard.
EOFis asymbolthat becomes defined after
```
    #include <iostream>
```

14. Unformatted I/O Summary

Unformattedis typically viewed as
- low-level I/O
- highest efficiency
- individual character processing
Input normally finishes atend-of-filemarker,EOF
Input proceeds regardless of multiple lines of text
Input has no automatic whitespace recognition


// Unformatted output
std::ostream std::cout;     // predefined

cout.put( c );              // single byte

cout.write( array, count ); // array of bytes



// Unformatted input
std::istream std::cin;      // predefined

c = cin.get( );             // single byte

cin.read( array, count );   // array of bytes

15. Formatted I/O

Floating-point values by default use fixed-point notation:


    #include <iostream>
    int main()
    {
      const double PI = 3.1415926;
      std::cout << PI << '\n';
      return 0;
    }
    /*
    Output: 3.14159
    */

16. Floating-point Precision

Floating-point values are printed withexact fraction part,as specified by theprecision.
Indefault notationformat, the precision specifiesmax number of meaningful digitsto display bothbefore and afterthe decimal point.
Thedefault precisionis 6 digits.
This means that the value is rounded to the best approximation while using 6 digits.
If necessary, the default format may change to scientific notation to preserve most accurate representation.

Precision can be changed by callingprecision()member function of the stream to improve data accuracy:


#include <iostream>
void main()
{
  const double PI = 3.1415926;
  std::cout.precision( 7 );
  std::cout << PI << '\n';
}
/*
Output: 3.141593
*/

17. Floating-point Manipulators

There are three possible ways to format floating point values:


    cout << value // default

    cout << fixed << value

    cout << scientific << value

Infixedandscientificnotations precision specifies exactlyhow many digits to displayafter the decimal point, even if they are trailing decimal zeros.

18. Floating-point Example


    #include <iostream>
    using namespace std;
    int main()
    {
        const double PI = 3.1415926;
        cout.precision( 7 );
        cout << fixed << "fixed format: " << PI << '\n';
        cout << scientific << "scientific format: " << PI << '\n';
        // back to default format:
        cout.unsetf( ios_base::fixed );
        cout.unsetf( ios_base::scientific );
        cout << "default format: " << PI << '\n';
        return 0;
    }
    /*
    Output:
    fixed format: 3.1415926
    scientific format: 3.1415926e+000
    default format: 3.141593
    */

19. C++ String

std::stringstores and manipulates text data
Add
```
    #include <string>
```
to bringstd::stringtype into our program


#include <iostream>
#include <string>
using namespace std;

int main()
{
    int qty;
    string units;

    // Formatted input of text:
    cout << "Qty and units: ";
    cin >> qty;
    cin >> units;

    cout << "You entered: ";
    cout << qty;
    cout << ' ';
    cout << units;
    cout << '\n';
    return 0;
}

20. Line-based Input, std::getline


    #include <iostream>
    #include <string>
    using namespace std;

    int main()
    {
        string text_line;

        // Line-based input of text:
        cout << "Type something: ";
        getline( cin, text_line );

        cout << "You typed: ";
        cout << text_line;
        cout << '\n';

        return 0;
    }

21. String Stream


    #include <cassert>
    #include <string> 
    #include <sstream> 
    int main()
    {
        const double PI = 3.1415926;
        double value;
        std::stringstream  buffer;  // text buffer
        buffer.precision( 8 );      // increase default precision (*)
        buffer << PI;               // formatted output  
        buffer >> value;            // formatted input
        assert( PI == value );
        std::string text; 
        text = buffer.str( );  // returns std::string  
        buffer.str( "" );      // clear buffer  
        return 0;
    }

(*) try commenting out precision change and see what happens

22. Low-level I/O using String Stream


    #include <cassert>
    #include <iostream>
    #include <sstream>
    using namespace std;
    int main()
    {
        stringstream buffer;
        int onechar; // because char cannot represent EOF
        cout << "Enter some text: ";
        while ( ( onechar = cin.get() ) != EOF ) {
            if ( onechar == '\n' ) {
                break; // exit loop at the end of the line

            } else if ( isalpha( onechar ) ) {
                buffer << "alpha: " << char( onechar ) << '\t' << onechar <<'\n';

            } else if ( isdigit( onechar ) ) {
                buffer << "digit: " << char( onechar ) << '\t' << onechar <<'\n';

            } else {
                buffer << "other: " << char( onechar ) << '\t' << onechar <<'\n';
            }
        }
        cout << buffer.str() << '\n'; // str() returns string
        return 0;
    }

23. Field Width

Manipulatorstd::setw( n )determines minimum output widthn.
Requires header
```
    #include <iomanip>
```

If output is shorter than thefield widthn, the output is padded withfill characters.


    #include <iostream>
    #include <iomanip>
    using namespace std;
    int main () {
        cout << setw( 40 ) << "Hello";
        return 0;
    }
    /*
    Output:
                                     Hello
    */

24. Fill Character

Fill charactercan be changed bysetfillmanipulator:


    #include <iostream>
    #include <iomanip>
    using namespace std;
    int main () {
        cout << setfill( '?' ) << setw( 40 ) << "Hello";
        return 0;
    }
    /*
    Output:
    ???????????????????????????????????Hello
    */

25. Field Alignment

leftappends fill characters at the end.

rightinserts fill characters at the beginning.


    #include <iostream>
    #include <iomanip>
    using namespace std;
    int main () {
        cout << left << setfill( '?' ) << setw( 40 ) << "Hello";
        return 0;
    }
    /*
    Output:
    Hello???????????????????????????????????
    */

26. Alignment of Numeric Fields

Adjusting numeric fields:


    #include <iostream>
    #include <iomanip>
    using namespace std;
    int main() {
      int x = -1234;
      cout << setw( 10 ) << internal << x << '\n';
      cout << setw( 10 ) << left     << x << '\n';
      cout << setw( 10 ) << right    << x << '\n';
      return 0;
    }
    /*
    Output:
    -     1234
    -1234
       -1234
    */

27. Scientific Notation

Scientific notation(exponential notation) adjusts specifieddecimal pointto the left or to the right according to the specified value of exponente.

Thee-suffix representstimes ten raised to the power. For example,

        1e-2 ==   0.01 == 1×10^-2

        1e-1 ==   0.10 == 1×10^-1

        1e-0 ==   1.00 == 1×10⁰

        1e+0 ==   1.00 == 1×10⁰

        1e+1 ==  10.00 == 1×10¹

        1e+2 == 100.00 == 1×10²

28. Normalized Scientific Notation

Normalized scientific notationexpects absolute partA^(*)of the numberA×10^bto be in the range
- 1 <=A< 10

Try the following program to convert numbers from scientific notation to ordinary decimal notation:


    #include <iostream>
    #include <iomanip>
    using namespace std;

    int main() {
        for (;;) {
            double dbl;
            cin >> dbl;
            cout << setw( 14 ) << showpoint << dbl << '\n';
        }
        return 0;
    }

________________

(*) the absolute partAis also known asmantissa.

29. More Manipulator Examples


#include <iostream>
#include <iomanip>
using namespace std;
int main() {
/*         5600 */ cout << setw( 14 )               << 56e2 << '\n';
/*      5600.00 */ cout << setw( 14 ) << showpoint  << 56e+2 << '\n';
/*2.345678e+002 */ cout << setw( 14 ) << scientific << 234.5678 << '\n';
/*          255 */ cout << setw( 14 )               << 255 << '\n';
/*           ff */ cout << setw( 14 ) << hex        << 255 << '\n'; 
/*          377 */ cout << setw( 14 ) << oct        << 255 << '\n';
/*            1 */ cout << setw( 14 )               << true << '\n';
/*         true */ cout << setw( 14 ) << boolalpha  << true << '\n';
/*  1234.567890 */ cout << setw( 14 ) << fixed      << 1234.56789 << '\n';
/*     1234.568 */ cout << setw( 14 ) << fixed << setprecision(3) << 1234.56789 << '\n';
    return 0;
}
/*
Output:
          5600
       5600.00
 2.345678e+002
           255
            ff
           377
             1
          true
   1234.567890
      1234.568
*/

30. Manipulators for Integer Data Types

The following areinteger output manipulators:
- boolalpha: use symbolic representation oftrueandfalsewhen inserting or extractingboolvalues. By default,boolvalues inserted and extracted as numeric values1and0.
- dec: insert or extract integer values in decimal (base 10) format.
- hex: insert or extract integer values in hexadecimal (base 16) format, such as "0xFF" or simply "FF".
- oct: insert or extract integer values in octal format, e.g. "077"
- showbase: insert a prefix that reveals the base of an integer value.
- noshowbase: reverse back to baseless integer output format.
Note: the above manipulators aresticky: they persist until another manipulator is applied.
The data type manipulators can be applied to bothinputandoutputstreams.

31. cin interpretation of 0 and 0x prefixes


    cin.unsetf( ios::dec ); // Interpret 0 and 0x as hexadecimal and octal prefixes
    cin.unsetf( ios::oct ); // Ignore octal prefix, interpret 077 as decimal 77

    cin.unsetf( ios::dec );
    cin.setf( ios::oct );   // All integers now expected to be octal base

    cin.unsetf( ios::dec );
    cin.setf( ios::hex );   // All integers now expected to be base-16:

Sample programcin_setf.cpp(download) illustrates integer base/prefix manipulation and rudimentary format-related error recovery.
See also<noindex><a href="http://www.cplusplus.com/reference/iostream/ios_base/setf.html" target="_blank"><tt>cin.setf()</tt></a></noindex>and<noindex><a href="http://www.cplusplus.com/reference/iostream/ios_base/unsetf.html" target="_blank"><tt>cin.unsetf()</tt></a></noindex>documentation.

32. Manipulators for Floating-point Data Types

showpoint: insert a decimal point unconditionally in a generated floating-point field.
fixed: insert floating-point values in fixed-point format. For example,
- default format for1234.56789is1234.57,
- fixed makes it1234.56789.
scientific: insert floating-point values in scientific format with an exponent field. For example,
- default format for1234.56789is1234.567,
- scientificmakes it1.234568e+03.

Note: the above manipulators aresticky: they persist until another manipulator is applied.

33. C++ Standard Library Objects


    typename           object name
    ------------       -----------
    std::ostream       std::cout; // predefined object
    std::istream       std::cin;  // predefined object
    std::string        text;      // text data
    std::stringstream  buffer;    // text buffer

34. Stream Functionality

http://www.c-jump.com/CIS60/PresentationIOstreams/lecture.html

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2011-12-12 10:11
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C++ Stream I/O

C++ Stream I/O

1. Stream Input/Output

2. Stream I/O Applications

3. Stream Output Concept

4. Stream Input Concept

5. Using C++ Objects

6. Standard Output Stream

7. Standard Output Stream, Cont.

8. Formatted Output

9. Standard Input Stream

10. Standard Input Stream, Cont.

11. Formatted Input

12. Unformatted I/O Example

13. What is EOF?

14. Unformatted I/O Summary

15. Formatted I/O

16. Floating-point Precision

17. Floating-point Manipulators

18. Floating-point Example

19. C++ String

20. Line-based Input, std::getline

21. String Stream

22. Low-level I/O using String Stream

23. Field Width

24. Fill Character

25. Field Alignment

26. Alignment of Numeric Fields

27. Scientific Notation

28. Normalized Scientific Notation

29. More Manipulator Examples

30. Manipulators for Integer Data Types

31. cin interpretation of 0 and 0x prefixes

32. Manipulators for Floating-point Data Types

33. C++ Standard Library Objects

34. Stream Functionality

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