c++小数点后舍入

c++ math.h中有这么几种函数：round，floor，ceil，trunc

round提供四舍五入，floor向下取整，ceil向上取整，trunc截断

例子 

/* round vs floor vs ceil vs trunc */
#include <stdio.h>      /* printf */
#include <math.h>       /* round, floor, ceil, trunc */

int main ()
{
  const char * format = "%.1f 	%.1f 	%.1f 	%.1f 	%.1f
";
  printf ("value	round	floor	ceil	trunc
");
  printf ("-----	-----	-----	----	-----
");
  printf (format, 2.3,round( 2.3),floor( 2.3),ceil( 2.3),trunc( 2.3));
  printf (format, 3.8,round( 3.8),floor( 3.8),ceil( 3.8),trunc( 3.8));
  printf (format, 5.5,round( 5.5),floor( 5.5),ceil( 5.5),trunc( 5.5));
  printf (format,-2.3,round(-2.3),floor(-2.3),ceil(-2.3),trunc(-2.3));
  printf (format,-3.8,round(-3.8),floor(-3.8),ceil(-3.8),trunc(-3.8));
  printf (format,-5.5,round(-5.5),floor(-5.5),ceil(-5.5),trunc(-5.5));
  return 0;
}

Output:

value   round   floor   ceil    trunc
-----   -----   -----   ----    -----
2.3     2.0     2.0     3.0     2.0
3.8     4.0     3.0     4.0     3.0
5.5     6.0     5.0     6.0     5.0
-2.3    -2.0    -3.0    -2.0    -2.0
-3.8    -4.0    -4.0    -3.0    -3.0
-5.5    -6.0    -6.0    -5.0    -5.0

关于有效数字问题，保留几位有效数字以及同时要求保留几位小数的实现：需要用到setprecision，用以保留有效数字，加上fixed，用以保留特定几位小数

例子

// setprecision example
#include <iostream>     // std::cout, std::fixed
#include <iomanip>      // std::setprecision

int main () {
  double f =3.14159;
  std::cout << std::setprecision(5) << f << '
';
  std::cout << std::setprecision(9) << f << '
';
  std::cout << std::fixed;
  std::cout << std::setprecision(5) << f << '
';
  std::cout << std::setprecision(9) << f << '
';
  return 0;
}

Output:

3.1416
3.14159
3.14159
3.141590000

同时保留特定几位小数输出

// modify floatfield
#include <iostream>     // std::cout, std::fixed, std::scientific

int main () {
  double a = 3.1415926534;
  double b = 2006.0;

  std::cout.precision(5);

  std::cout << "default:
";
  std::cout << a << '
' << b << '
' << c << '
';

  std::cout << '
';

  std::cout << "fixed:
" << std::fixed;
  std::cout << a << '
' << b << '
' << c << '
';

return 0;
}

Output:

default:
3.1416
2006
1e-010

fixed:
3.14159
2006.00000
0.00000