c++四舍五入函数，向上取整，向下取整函数

对含有小数点的数进行四舍五入是比较普遍的一种需求。在C++中也有类似的取整函数。在C++的头文件中有floor()和ceil()函数。在STL中还有round()函数。这三个函数的作用如下：

函数名称	函数说明	2.1	2.9	-2.1	-2.9
Floor()	不大于自变量的最大整数	2	2	-3	-3
Ceil()	不小于自变量的最大整数	3	3	-2	-2
Round()	四舍五入到最邻近的整数	2	3	-2	-3

从函数说明中可以看出，
（1） Floor()会取不大于自变量的最大整数，这样自变量是3.1或3.9是没有区别的，返回都是3；自变量是-2.1或-2.9也是没有区别的，返回都是-3；
（2） Ceil()会取不小于自变量的最大整数，这样自变量是3.1或3.9，返回都是4；自变量是-2.1或-2.9，返回的都是-2；
（3） Round()函数，才是我们需要的四舍五入的函数，因为它会返回离自变量最近的整数，这个返回的整数可能大于也可能小于原来的数，但是一定是离它最近的那个整数。

double floor (double x);
      float floor (float x);
long double floor (long double x);
     double floor (T x);           // additional overloads for integral types

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/* floor example */ #include <stdio.h> /* printf */ #include <math.h> /* floor */ int main () { printf ( "floor of 2.3 is %.1lf\n", floor (2.3) ); printf ( "floor of 3.8 is %.1lf\n", floor (3.8) ); printf ( "floor of -2.3 is %.1lf\n", floor (-2.3) ); printf ( "floor of -3.8 is %.1lf\n", floor (-3.8) ); return 0; }

Output:

floor of 2.3 is 2.0 floor of 3.8 is 3.0 floor of -2.3 is -3.0 floor of -3.8 is -4.0

double ceil (double x);
      float ceil (float x);
long double ceil (long double x);
     double ceil (T x);           // additional overloads for integral types

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/* ceil example */ #include <stdio.h> /* printf */ #include <math.h> /* ceil */ int main () { printf ( "ceil of 2.3 is %.1f\n", ceil(2.3) ); printf ( "ceil of 3.8 is %.1f\n", ceil(3.8) ); printf ( "ceil of -2.3 is %.1f\n", ceil(-2.3) ); printf ( "ceil of -3.8 is %.1f\n", ceil(-3.8) ); return 0; }

Output:

ceil of 2.3 is 3.0
ceil of 3.8 is 4.0
ceil of -2.3 is -2.0
ceil of -3.8 is -3.0

 double round (double x);
      float round (float x);
long double round (long double x);
     double round (T x);           // additional overloads for integral types

/* round vs floor vs ceil vs trunc */ #include <stdio.h> /* printf */ #include <math.h> /* round, floor, ceil, trunc */ int main () { const char * format = "%.1f \t%.1f \t%.1f \t%.1f \t%.1f\n"; printf ("value\tround\tfloor\tceil\ttrunc\n"); printf ("-----\t-----\t-----\t----\t-----\n"); 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

具体可查看文献：http://www.cplusplus.com/reference/cmath/round/