c - 数字的精度，乘以和除以100

Question

我有以下测试用例

#include <stdio.h>

int main() { 

    double x = 3.987;

    printf("x = %lf\n", x);

    printf("(double) (long) (x) = %lf\n", (double) (long) (x));

    printf("(x*100)/100 = %lf\n", (x*100)/100);

    printf("(double) (long) (x*100)/100 = %lf\n", (double) (long) (x*100)/100);

    printf("(double) (long) (x*10)/10 = %lf\n", (double) (long) (x*10)/10);

    return 0;
}

输出是:

x = 3.987000
(double) (long) (x) = 3.000000
(x*100)/100 = 3.987000
(double) (long) (x*100)/100 = 3.980000
(double) (long) (x*10)/10 = 3.900000

我觉得乘以100和除以100会相互抵消？但它实际上降低了精度。这究竟是如何工作的？

Answer 1

在某些地方，您要转换为“long”，它是整数类型。因此，例如在最后一种情况下，您将 3.987 乘以 10 得到 39.87。然后，您将其转换为“长”，因此它变为 39。将其除以 10 后得到 3.9。

你想通过(long)实现什么？