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std::cout << "HELLO"; std::cout << '\n'; std::cout << 123;
Course List http://www.c-jump.com/bcc/
Standard Output cout, cerr
Standard Input cin
Formatting
Error handling
File I/O (later)
network communication
encryption
data compression
persistent storage of objects
character-oriented input and output
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Output stream buffer:
| H | E | L | L | O | \n | 1 | 2 | 3 | ... | ... |
Input stream buffer:
| 4 | 5 | 6 | \n | 7 | 8 | 9 | \n | ... | ... |
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Objects are C++ variables of user-defined types. They are declared as
typename object_name;
For example, if Shape3D is 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
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
*/
std::cout is a pre-defined object
We need to
#include <iostream>
to bring std::cout into our program
std::cout is attached to the standard 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 to myfile.txt
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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 );
std::cin is a pre-defined object
We need to have
#include <iostream>
to bring std::cin into our program
std::cin is attached to a standard input device
Example of I/O redirection from file or another device:
CMD> myprogram < myfile.txt
The above command gets standard input from myfile.txt
Formatted input means
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;
}
Unformatted input means
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;
}
Acronym EOF stands for condition known as end 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.
EOF is a symbol that becomes defined after
#include <iostream>
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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
*/
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There are three possible ways to format floating point values:
cout << value // default
cout << fixed << value
cout << scientific << value
In fixed and scientific notations precision specifies exactly how many digits to display after the decimal point, even if they are trailing decimal zeros.
#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
*/
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#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;
}
#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
#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;
}
Manipulator std::setw( n ) determines minimum output width n.
Requires header
#include <iomanip>
If output is shorter than the field width n, the output is padded with fill characters.
#include <iostream>
#include <iomanip>
using namespace std;
int main () {
cout << setw( 40 ) << "Hello";
return 0;
}
/*
Output:
Hello
*/
Fill character can be changed by setfill manipulator:
#include <iostream>
#include <iomanip>
using namespace std;
int main () {
cout << setfill( '?' ) << setw( 40 ) << "Hello";
return 0;
}
/*
Output:
???????????????????????????????????Hello
*/
left appends fill characters at the end.
right inserts fill characters at the beginning.
#include <iostream>
#include <iomanip>
using namespace std;
int main () {
cout << left << setfill( '?' ) << setw( 40 ) << "Hello";
return 0;
}
/*
Output:
Hello???????????????????????????????????
*/
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
*/
Scientific notation (exponential notation) adjusts specified decimal point to the left or to the right according to the specified value of exponent e.
The e-suffix represents times 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
Normalized scientific notation expects absolute part A(*) of the number A×10b to 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 part A is also known as mantissa.
#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
*/
The following are integer output manipulators:
boolalpha: use symbolic representation of true and false when inserting or extracting bool values. By default, bool values inserted and extracted as numeric values 1 and 0.
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 are sticky: they persist until another manipulator is applied.
The data type manipulators can be applied to both input and output streams.
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 program cin_setf.cpp ( download ) illustrates integer base/prefix manipulation and rudimentary format-related error recovery.
See also cin.setf() and cin.unsetf() documentation.
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 for 1234.56789 is 1234.57,
fixed makes it 1234.56789.
scientific: insert floating-point values in scientific format with an exponent field. For example,
default format for 1234.56789 is 1234.567,
scientific makes it 1.234568e+03.
Note: the above manipulators are sticky: they persist until another manipulator is applied.
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
