cout 格式化输出

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C语言里可以用printf(),%f来实现浮点数的格式化输出，用cout呢。。。？
下面的方法是在网上找到的，如果各位有别的办法谢谢留下...

iomanip.h是I/O流控制头文件,就像C里面的格式化输出一样.以下是一些常的:

dec 置基数为10 相当于"%d"
hex 置基数为16 相当于"%X"
oct 置基数为8 相当于"%o"
setfill(c) 设填充字符为c
setprecision(n)   设显示小数精度为n位
setw(n) 设域宽为n个字符
setioflags(ios::fixed)   固定的浮点显示
setioflags(ios::scientific) 指数表示
setiosflags(ios::left) 左对齐
setiosflags(ios::right) 右对齐
setiosflags(ios::skipws 忽略前导空白
setiosflags(ios::uppercase) 16进制数大写输出
setiosflags(ios::lowercase) 16进制小写输出
setiosflags(ios::showpoint) 强制显示小数点
setiosflags(ios::showpos)   强制显示符号

#include <iomanip>
use namespace std;

double d=11.23456;
cout<<d<<endl; //直接输出只能输出6位数，包括整数部分和小数部分
cout<<setprecision(3)<<d<<endl; //精度为3，输出3位数
cout<<setiosflags(ios::fixed)<<d<<endl;//精度为3，定点输出，输出3位小数
cout<<setiosflags(ios::fixed)<<setprecision(7)<<d<<endl;//位数不够，末尾添0

输出结果：
11.2346
11.2
11.23456
11.2345600

C++格式化输出浮点数

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01.#include <iostream>
02.using std::cout;
03.using std::endl;
04.using std::fixed;
05.using std::scientific;
06.
07.int main()
08.{
09.   double x = 0.001234567;
10.   double y = 1.946e9;
11.
12.   cout << "Displayed in default format:" << endl << x << '\t' << y << endl;
13.
14.   cout << "\nDisplayed in scientific format:" << endl << scientific << x << '\t' << y << endl;
15.
16.   cout << "\nDisplayed in fixed format:" << endl << fixed << x << '\t' << y << endl;
17.   return 0;
18.}
#include <iostream>
using std::cout;
using std::endl;
using std::fixed;
using std::scientific;

int main()
{
double x = 0.001234567;
double y = 1.946e9;

cout << "Displayed in default format:" << endl << x << '\t' << y << endl;

cout << "\nDisplayed in scientific format:" << endl << scientific << x << '\t' << y << endl;

cout << "\nDisplayed in fixed format:" << endl << fixed << x << '\t' << y << endl;
return 0;
}

Displayed in default format:
0.00123457 1.946e+009

Displayed in scientific format:
1.234567e-003 1.946000e+009

Displayed in fixed format:
0.001235 1946000000.000000

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01.#include <iostream.h>
02.
03.main(void)
04.{
05. float a=100100.0, b=0.08;
06. cout.setf(ios::right|ios::scientific|ios::showpoint);
07. cout.width(20);
08. cout <<(-a*b);
09.
10. return 0;
11.}
#include <iostream.h>

main(void)
{
float a=100100.0, b=0.08;
cout.setf(ios::right|ios::scientific|ios::showpoint);
cout.width(20);
cout <<(-a*b);
return 0;
}

-8.008000e+003

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01.#include <iostream>
02.#include <iomanip>
03.#include <limits>
04.using std::cout;
05.using std::endl;
06.using std::setprecision;
07.using std::numeric_limits;
08.
09.int main() {
10. const double pi = 3.14;
11. cout << endl;
12.
13. for(double radius = .2 ; radius <= 3.0 ; radius += .2)
14.    cout << "radius = "
15.    << setprecision(numeric_limits<double>::digits10 + 1)
16.    << std::scientific << radius<< " area = "
17.         << std::setw(10) << setprecision(6)<< std::fixed << pi * radius * radi
18.us << endl;
19. return 0;
20.}
#include <iostream>
#include <iomanip>
#include <limits>
using std::cout;
using std::endl;
using std::setprecision;
using std::numeric_limits;

int main() {
const double pi = 3.14;
cout << endl;

for(double radius = .2 ; radius <= 3.0 ; radius += .2)
    cout << "radius = "
    << setprecision(numeric_limits<double>::digits10 + 1)
    << std::scientific << radius<< " area = "
         << std::setw(10) << setprecision(6)<< std::fixed << pi * radius * radi
us << endl;
return 0;
}

radius = 2.0000000000000001e-001 area =   0.125600
radius = 4.0000000000000002e-001 area =   0.502400
radius = 6.0000000000000009e-001 area =   1.130400
radius = 8.0000000000000004e-001 area =   2.009600
radius = 1.0000000000000000e+000 area =   3.140000
radius = 1.2000000000000000e+000 area =   4.521600
radius = 1.3999999999999999e+000 area =   6.154400
radius = 1.5999999999999999e+000 area =   8.038400
radius = 1.7999999999999998e+000 area = 10.173600
radius = 1.9999999999999998e+000 area = 12.560000
radius = 2.1999999999999997e+000 area = 15.197600
radius = 2.3999999999999999e+000 area = 18.086400
radius = 2.6000000000000001e+000 area = 21.226400
radius = 2.8000000000000003e+000 area = 24.617600

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01.#include <iostream>
02.#include <iomanip>
03.#include <string>
04.
05.using namespace std;
06.
07.int main( ) {
08.
09.   ios_base::fmtflags flags = cout.flags( );
10.
11.   double pi = 3.14285714;
12.
13.   cout << "pi = " << setprecision(5) << pi << '\n';
14.
15.   cout.flags(flags);
16.}
#include <iostream>
#include <iomanip>
#include <string>

using namespace std;

int main( ) {

ios_base::fmtflags flags = cout.flags( );

double pi = 3.14285714;

cout << "pi = " << setprecision(5) << pi << '\n';

cout.flags(flags);
}

pi = 3.1429

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01.#include <iostream>
02.#include <iomanip>
03.#include <math.h>
04.using namespace std;
05.int main()
06.{
07.   double root2 = sqrt( 2.0 );
08.   int places;
09.
10.   cout << setiosflags( ios::fixed)
11.        << "Square root of 2 with precisions 0-9.\n"
12.        << "Precision set by the "
13.        << "precision member function:" << endl;
14.
15.   for ( places = 0; places <= 9; places++ ) {
16.      cout.precision( places );
17.      cout << root2 << '\n';
18.   }
19.
20.   cout << "\nPrecision set by the "
21.        << "setprecision manipulator:\n";
22.
23.   for ( places = 0; places <= 9; places++ )
24.      cout << setprecision( places ) << root2 << '\n';
25.
26.   return 0;
27.}

将 cout 的 flag 保存到变量, 以便修改后的恢复

    ostream::fmtflags old = cout.flag() ;        // 无参将返回当前 flag 值
    cout.flag(old) ;                            // 恢复到原先保存的值

将 bool 值以 literals 输出

cout <<"numeric : " <<true <<" or " <<false <<endl ;              // 1 or 0
cout <<"literals : " <<boolalpha <<true <<" or " <<false <<endl ; // true or false
cout <<"literals : " <<boolalpha <<0 <<endl ;                     // 0    原因: 0 在cout中不等价于 false

一旦我们使用 boolalpha 将改变 cout 对 bool 值的输出格式. 此后的 cout 都会将 bool 输出为 literals.

将 bool 值以 numeric 输出

    cout <<"numeric : " <<noboolalpha <<true <<" or " <<false <<endl ;// 1 or 0

从此以后, cout 对 bool 值的输出将恢复 numeric 格式

指定 Integral Values 的 Base

    const int ival = 17 ;        // 'ival' is constant, so value never change
    cout <<"oct : " <<oct <<ival <<endl ;        // 21 : 8 进制
    cout <<"dec : " <<dec <<ival <<endl ;        // 17 : 10 进制
    cout <<"hex : " <<hex <<ival <<endl ;        // 11 : 16 进制
    cout <<"hex : " <<hex <<17.01 <<endl ;        // 17.01 : 不受影响

如 boolalpha 一样, oct, dec, hex 也是 persistent. 一旦改变, 将影响后续的输出格式.

显示表明 Integer Values 的 Base

    cout <<showbase ;                            // Show base when printing integral values
    cout <<"oct : " <<oct <<ival <<endl ;        // 21 : 8 进制
    cout <<"dec : " <<dec <<ival <<endl ;        // 017 : 10 进制
    cout <<"hex : " <<hex <<ival <<endl ;        // 0x11 : 16 进制
    cout <<"hex : " <<hex <<17.01 <<endl ;        // 17.01 : 不受影响
    cout <<noshowbase ;                            // Reset state of the stream

若想改变16进制字母的大小, 可以结合 uppercase/nouppercase

    cout <<showbase <<uppercase ;
    cout <<"hex : " <<hex <<15 <<endl ;            // 0XF 大写形式
    cout <<nouppercase ;
    cout <<"hex : " <<hex <<15 <<endl ;            // 0xf 小写形式

showbase 与 noshowbase 的作用周期也是 persistent

对于 float/double 型, 有三种格式化控制

一: 输出精度 precision : by default is 6pricision
控制了至多一共会输出多少个数字.
当要输出的数字多余指定的值时, 将发生四舍五入(rounded);
当要输出的数字少于指定的值时, 则实际输出的数字个数将少于指定值.
// cout.pricision(4) ;                         // 等价于 cout <<setprecision(4) ;
cout <<setprecision(4) <<12.345678 <<endl ;    // 12.35  rounded!
cout <<setprecision(10) <<12.345678 <<endl ;   // 12.345678 其实内部发生了 rounded, 而结果正好进位, 与原值相同
cout <<cout.precision() <<endl ;               // 输出当前精度

二: 表现形式 notation : 'very large and very small values are printed using scientific notation. other values use fixeddecimal.'
notation 控制了输出的形式 : 科学计数法(scientific) 和定点小数(fixed)
float f = 101 / 6.0 ;
cout <<fixed <<f <<endl ;           // 16.83334 : 小数点后共6位
cout <<scientific <<f <<endl ;      // 1.683333e+001 : 小数点后共6位
恢复到初始状态
cout.unsetf(ostream::floatfield) ;  // Retrieve to default handling for notation
cout <<f <<endl ;                   // 16.8333 : 所有数字共6位

三: 输出十进制浮点 'By default, when the fractional part of a floating-point value is 0, the decimal point is not displayed. The showpoint manipulator forces the decimal point ot be printed.'
cout <<10.0 <<endl ;                // 10
cout <<showpoint <<10.0 <<endl ;    // 10.0000
cout <<noshowpoint <<endl ;         // Revert to default handling of decimal  

输出填充 Padding the Output

setw to specify the minimum space for the next numeric or string value.

cout <<setw(10) <<12.3 <<endl ;     // ______12.3
cout <<setw(10) <<12 <<3 <<endl ;   // ________123

cout <<setw(3) <<12.345 <<endl ;    // If the total output is more than 3, it can be extended

left to left-justify the output.

    cout <<left ;                                 // left-justify
    cout <<setw(5) <<12 <<setw(5) <<34 <<endl ;   // 12___34___

right to right-justify the output. Output is right-justified bu default.

cout <<right ;                                // By default
cout <<setw(5) <<12 <<setw(5) <<34 <<endl ;   // 12___34___

internal controls placement of the sign on negative value. internal left-justifies the sign and right-justifies the value, padding any intervening space with blanks.(if setfill not set)
cout <<internal ;               // By default
cout <<setw(5) <<-12 <<endl ;   // 12___34___

setfill lets us specify an alternative character to use when padding the output. By default, the value is a space.
cout <<setfill('*') ;          // By default
cout <<setw(5) <<12 <<endl ;   // 12___34___

Header Files

Manipulators Defined in <iomanip>

setfill(char ch)        Fill whitespace with 'ch'
setprecision(int n)     Set floating-point precision to 'n'
setw(int w)             Read or write value to 'w' characters
setbase(int b)          Output integers in base 'b'(only 'b' is 8/10/16 could the function work)

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