cout 格式化输出

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 6
pricision

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)