C++基础入门

A plus sign is output before positive integerommon flag constants:
outStream.setf(ios::right);
Sets right-justification
outStream.setf(ios::left); outStream.setf(ios::dec)
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How Manipulators Work
By function overloading Manipulators are functions that are passed to I/O operators as arguments The functions are then called by the operator Example:
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Output Member Functions
Consider:
outStream.setf(ios::fixed); outStream.setf(ios::showpoint); outStream.precision(2);
Member function precision(x)
Decimals written with ‘x’ digits after decimal
Integers are output in decimal (base 10) notation
outStream.setf(ios::oct); outStream.setf(ios::hex);
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setf() Examples
Common flag constants:
outStream.setf(ios::uppercase);
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Formatting Output
Formatting numeric values for output
Values may not display as you’d expect! cout << “The price is $” << price << endl;
If price (declared double) has value 78.5, you might get: The price is $78.500000 or: The price is $78.5
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How Manipulators Work
Use this manipulator in an expression std::cout << std::endl Implementation of operator << transforms this call Into a call of the passed function with the stream as the argument std:endl(std::cout)
Member function setf()
Allows multitude of output flags to be set
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More Output Member Functions
Consider:
outStream.width(5);
Member function width(x)
Sets width to ‘x’ for outputted value Only affects ‘next’ value outputted Must set width before each value in order to affect all
Set multiple flags with one call:
outStream.setf(ios::fixed | ios::showpoint | ios::right);
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Manipulators
Manipulator defined: “A function called in nontraditional way”
These stmts force all future cout’ed values:
To have exactly two digits after the decimal place
Example: cout << “The price is $” << price << endl;
Now results in the following: The price is $78.50
Member function eof()
inStream.get(next); while (!inStream.eof()) { cout << next; inStream.get(next); } Reads each character until file ends eof() member function returns bool
An uppercase E is used in scientific notation
outStream.setf(ios::showcase);
Show the base of an output number Leading 0 for octal Leading 0x for hexadecimal
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End of File Check with Read
Second method
read operation returns bool value! (inStream >> next)
Expression returns true if read successful Returns false if attempt to read beyond end of file (if the failbit or badbit is set)
In action:
double next, sum = 0; while (inStream >> next) sum = sum + next; cout << “the sum is “ << sum << endl;
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Stream State and Boolean Conditions
operator void * () Returns whether the stream has not run into an error Corresponds to !fail() while (inStream >> next) { … } operator ! () Return whether the stream has run into an error Corresponds to fail() if (! (std::cin >> x)) { … } std::cin >> x; if (std::cin.fail()) { … }
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User-Defined Manipulators
An example of user-defined manipulator
Ignores all characters until end-of-line
#include <istream> #include <limits> template <class charT, class traits> Inline std::basic_istream<charT,traits>& ignoreLine (std::basic_istream<charT,traits>& strm) { // skip until end-of-line strm.ignore(std::numeric_limits<int>::max(), strm.widen('\n')); // return stream for concatenation return strm; }
Can have arguments Placed after insertion operator Do same things as member functions!
In different way
Common to use both ‘together’
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Manipulators
flush and endl defined in <ostream> setw() and setprecision() are in library <iomanip>, std namespace
outStream.setf(ios::scientific);
Floating-point numbers are written in e-notation
outStream.setf(ios::showpoint)
Always include decimal point
outStream.setf(ios::showpos)
CIS 4930 Application Development Using C++
Dr. Kun Suk Kim CISE Department, University of Florida
Input/Output Using Stream Classes
Notice
Web site
/~cis4930sp04ad/
Typical to have ‘varying’ widths To form ‘columns’
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Flags
Recall: member function setf()
Sets condition of output flags