C sqrt == long int

Question

我不知道为什么我有以下代码：

int main() {
    long int height_cat, number_worker_cats, number_helper_cats, height_tree;
    bool flag;
    scanf("%ld%ld", &height_cat, &number_worker_cats);
    for (number_helper_cats = 1; ; ++number_helper_cats) {
        for (height_tree = 1; (long int)pow(number_helper_cats + 1, height_tree) <= height_cat; ++height_tree) {
            if ((long int)(pow(number_helper_cats + 1, height_tree) - height_cat) == 0 && (long int)(pow(number_helper_cats, height_tree) - number_worker_cats) == 0) {
                flag = true;
                break;
            }
        }
        if (flag) {
            break;
        }
    }
 printf("%ld, %ld\n", number_helper_cats, height_tree);
}

我正在搜索number_helper_cats和height_tree (number_helper_cats +1)^height_tree = height_cat和number_helper_cats^height_tree = number_worker_cats，其中height_cat和number_worker_cats是integers。

例如，如果height_cat = 216和number_worker_cats = 125，代码将从number_helper_cats = 5和height_tree = 3 (5+1)^3 = 216和5^3 = 125停止。

但如果我有以下代码它不起作用，永远循环，为什么？

int main() {
    long int height_cat, number_worker_cats, number_helper_cats, height_tree;
    bool flag;
    scanf("%ld%ld", &height_cat, &number_worker_cats);
    for (number_helper_cats = 1; ; ++number_helper_cats) {
        for (height_tree = 1; pow(number_helper_cats + 1, height_tree) <= height_cat; ++height_tree) {
            if ((long int)(pow(number_helper_cats + 1, height_tree)) == height_cat &&
                        (long int)(pow(number_helper_cats, height_tree)) == number_worker_cats) {
                flag = true;
                break;
            }
        }
        if (flag) {
            break;
        }
    }
printf("%ld, %ld\n", number_helper_cats, height_tree);
}

一切都是long int，每个height_cat和number_worker_cats for testcase都是true，另一个例子是height_cat = 5764801，number_worker_cats = 1679616，number_helper_cats = 6和height_tree = 8，因为（6 + 1）^ 8 = 5764801 ，6 ^ 8 = 1679616.但是第一个代码再次运行良好，第二个代码永远循环。 pow是精确的我的意思是6 ^ 3 = 216和5 ^ 3 = 125对吧？ ：P

Answer 1

pow的结果是double，double数字在很多情况下都不准确。要使用double测试相等性，常见的方法是

if (abs(pow(number_helper_cats + 1, height_tree) - height_cat)) < 0.001); // 0.001 is an arbitrary small number
{
     ...
}

要测试<=，请使用pow(number_helper_cats + 1, height_tree) <= height_cat + 0.001。

但是，我必须提到你的代码不能用我的gcc 4.7.2产生你在问题中提到的无限循环。你的所有循环都正常结束。