当我运行以下代码时:
#include <stdio.h>
int main()
{
int i = 0;
volatile long double sum = 0;
for (i = 1; i < 50; ++i) /* first snippet */
{
sum += (long double)1 / i;
}
printf("%.20Lf\n", sum);
sum = 0;
for (i = 49; i > 0; --i) /* second snippet */
{
sum += (long double)1 / i;
}
printf("%.20Lf", sum);
return 0;
}
输出是:
4.47920533832942346919
4.47920533832942524555
这两个数字不应该相同吗?更有趣的是,下面的代码:
#include <stdio.h>
int main()
{
int i = 0;
volatile long double sum = 0;
for (i = 1; i < 100; ++i) /* first snippet */
{
sum += (long double)1 / i;
}
printf("%.20Lf\n", sum);
sum = 0;
for (i = 99; i > 0; --i) /* second snippet */
{
sum += (long double)1 / i;
}
printf("%.20Lf", sum);
return 0;
}
产生:
5.17737751763962084084
5.17737751763962084084
那么为什么它们当时不同而现在相同呢?