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.}