Question

我有以下问题：我用Qt IDE编写代码。我被告知，当人们尝试使用其他IDE（如codeblocks或visual studio）编译它时，他们获得的输出是不同的，并且存在功能。有什么想法可以导致这个？我会举个例子：

这是Newton的方法，其函数的根是2.83something。每次我在Qt中运行时，我得到相同的，正确的计算。我在代码块中得到了“nan”，在visual studio中也得到了无关紧要的东西。我不明白，我的代码中某处有错误吗？是什么导致这个？

#include <iostream>
#include <cmath> // we need the abs() function for this program

using namespace std;

const double EPS = 1e-10; // the "small enough" constant. global variable, because it is good programming style

double newton_theorem(double x)
{
    double old_x = x; // asign the value of the previous iteration
    double f_x1 = old_x*old_x - 8; // create the top side of the f(x[n+1] equation
    double f_x2 = 2 * old_x; // create the bottom side
    double new_x = old_x -  f_x1 / f_x2; // calculate f(x[n+1])
    //cout << new_x << endl; // remove the // from this line to see the result after each iteration
    if(abs(old_x - new_x) < EPS) // if the difference between the last and this iteration is insignificant, return the value as a correct answer;
    {
        return new_x;
    }
    else // if it isn't run the same function (with recursion YAY) with the latest iteration as a starting X;
    {
        newton_theorem(new_x);
    }
}// newton_theorem

int main()
{
    cout << "This program will find the root of the function f(x) = x * x - 8" << endl;
    cout << "Please enter the value of X : ";
    double x;
    cin >> x;
    double root = newton_theorem(x);
    cout << "The approximate root of the function is: " << root << endl;

    return 0;
}//main

Answer 1

是的，您遇到未定义的行为：

if(abs(old_x - new_x) < EPS) // if the difference between the last and this iteration is insignificant, return the value as a correct answer;
{
    return new_x;
}
else // if it isn't run the same function (with recursion YAY) with the latest iteration as a starting X;
{
    /*return*/ newton_theorem(new_x); // <<--- HERE!
}

return分支上缺少else。

我们可以尝试解释为什么Qt有效（它的编译器自动将newton_theorem的结果放在返回注册表中，或者共享注册表，或者其他），但事实是任何事情都可能发生即可。有些编译器在后续运行中的行为可能相同，有些可能会一直崩溃。

不同的IDE的输出不同？

1 个答案: