c浮点数怎么四舍五入

#include <stdio.h> /* printf */

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

inline int myIntRound(double dInput)

{

    if(dInput >= 0.0f)

    {

        return ((int)(dInput + 0.5f));

    }

    return ((int)(dInput - 0.5f));

}

double d[]={-1.5, -0.7, -0.5, -0.3, 0.3, 0.5, 0.7, 1.5, 1.9};

int print_doubles(double d[], int n)

{

    printf("original
");

    for(int i=0;i<n;i++)

        printf (" %f", d[i]);

    printf("
");

}

//直接int会舍弃浮点部分，只取整数

int print_int_doubles(double d[], int n)

{

    printf("int
");

    for(int i=0;i<n;i++)

        printf (" %f", double(int(d[i])));

    printf("
");

}

int print_round_doubles(double d[], int n)

{

    printf("round
");

    for(int i=0;i<n;i++)

        printf (" %f", round(d[i]));

    printf("
");

    for(int i=0;i<n;i++)

        printf (" %d", int(round(d[i])));

    printf("
");

}

int print_ceil_doubles(double d[], int n)

{

    printf("ceil
");

    for(int i=0;i<n;i++)

        printf (" %f", ceil(d[i]));

    printf("
");

}

int print_floor_doubles(double d[], int n)

{

    printf("floor
");

    for(int i=0;i<n;i++)

        printf (" %f", floor(d[i]));

    printf("
");

}

int print_trunc_doubles(double d[], int n)

{

    printf("trunc
");

    for(int i=0;i<n;i++)

        printf (" %f", trunc(d[i]));

    printf("
");

}

//和round

int print_myint_doubles(double d[], int n)

{

    printf("myint
");

    for(int i=0;i<n;i++)

        printf (" %d", myIntRound(d[i]));

    printf("
");

}

int main ()

{

    print_doubles(d, sizeof(d)/sizeof(d[0]));

    print_int_doubles(d, sizeof(d)/sizeof(d[0]));

    print_round_doubles(d, sizeof(d)/sizeof(d[0]));

    print_floor_doubles(d, sizeof(d)/sizeof(d[0]));

    print_ceil_doubles(d, sizeof(d)/sizeof(d[0]));

    print_trunc_doubles(d, sizeof(d)/sizeof(d[0]));

    print_myint_doubles(d, sizeof(d)/sizeof(d[0]));

    return 0;

}

/*

:! ./round

original

 -1.500000 -0.700000 -0.500000 -0.300000 0.300000 0.500000 0.700000 1.500000 1.900000

int

 -1.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 1.000000 1.000000

round

 -2.000000 -1.000000 -1.000000 -0.000000 0.000000 1.000000 1.000000 2.000000 2.000000

 -2 -1 -1 0 0 1 1 2 2

floor

 -2.000000 -1.000000 -1.000000 -1.000000 0.000000 0.000000 0.000000 1.000000 1.000000

ceil

 -1.000000 -0.000000 -0.000000 -0.000000 1.000000 1.000000 1.000000 2.000000 2.000000

trunc

 -1.000000 -0.000000 -0.000000 -0.000000 0.000000 0.000000 0.000000 1.000000 1.000000

myint

 -2 -1 -1 0 0 1 1 2 2

 */

测试后可以发现，直接int转换不行，会丢失浮点部分。 要用+0.5或-0.5方式。 或者用round函数