为什么这个 while 循环不显示第一个元素？

Question

我是 C++ 的初学者，想做 this 练习。以下是我的代码，但我不明白为什么它在while循环中不显示下限。

#include <iostream>
#include <iomanip>

using namespace std;

int main()
{
    double low=-0.000001, high=-0.000001, step=0.0, stop, f;
    while (low <= -0.000001 || low > 50000) {
        cout << "Lower limit (0~50,000): "; cin >> low;
        if (low <= -0.000001) {cout << "Must be positive." << endl;}
        else if (low > 50000) {cout << "Out of range." << endl;}
    }
    while (high <= -0.000001 || high > 50000 || high <= low) {
        cout << "Higher limit (0~50,000): "; cin >> high;
        if (high <= -0.000001) {cout << "Must be positive." << endl;}
        else if (high > 50000) {cout << "Out of range." << endl;}
        else if (high <= low) {cout << "Must higher than lower limit." << endl;}
    }
    while (step <= 0 || step > high) {
        cout << "Step (0.000001~" << high << "): "; cin >> step;
        if (step <= 0) {cout << "Must be positive." << endl;}
        else if (step > high) {cout << "Out of range." << endl;}
    }
    cout << endl << "Celsius\t\tFahrenheit" << endl;
    cout << "-------\t\t----------" << endl;
    stop = low;
    while (stop < high) {
        f = 1.8 * stop + 32.0;
        stop += step;
        cout << fixed << setprecision(6) << stop << "\t" << fixed << setprecision(6) << f << endl;
    }
    return 0;
}

例如，如果我输入 1.5 作为下限，我想它的输出应该以 1.5 开头，但它却以 2.0 开头...我该如何解决这个问题？谢谢

Img of Not the outcome I want

Answer 1

第一次 stop 被打印是在第一次增量之后：

while (stop < high) {
    f = 1.8 * stop + 32.0;
    stop += step;                              <- increment
    cout << ... << stop << ... << f << endl;   <- print
}

将打印移到上面，它应该可以正常工作。

旁注：使用调试器和重复 "step over" 命令很容易找到此类问题（也许有一些手表显示您感兴趣的值如何随时间变化）。下面一步一步的执行程序，可以有所启发。

为什么这个 while 循环不显示第一个元素？

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