c++ - 'cout' 语句未被执行

Question

下面的程序是求不超过四百万的偶数斐波那契数项之和。 该程序中的最后一个“cout”语句根本没有执行。为什么？请帮忙。

#include <iostream>

using namespace std;

int main()
{
    int a, b, c, sum, sum1, sum2;
    a = 1;
    b = 2;
    sum2 = 0;

    cout << b << endl;
    c = a + b;

    sum1 = c;

    while (c <= 4000000)
    {
        a = b;
        b = c;
        if ((a + b) <= 4000000)
        {
            c = a + b;
            if (c%2 == 0)
            {
                sum2 = sum2 + c;
                cout << c << endl;
            }
        }
    }

    cout << "The sum of even fibonacci numbers not greater than 4 million is: " << (sum1 + sum2); //Not being executed
    return 0;
}

Answer 1

我无法执行该程序，但我认为您的程序永远不会结束，这就是为什么您永远无法执行该语句的原因。你的外衣while循环将继续进行直到 c <= 4000000 .但是，您只增加 c当且仅当 (a + b) <= 4000000 , 所以 c永远不会超过 400 万。

要解决此问题，您可以尝试以下操作:

#include <iostream>

using namespace std;

int main()
{
    int a, b, c, sum, sum1, sum2;
    a = 1;
    b = 2;
    sum2 = 0;   

    cout << b << endl;
    c = a + b; 

    sum1 = c;

    while (c <= 4000000)
    {
        a = b;
        b = c;
        c = a + b; //Update c regardless.
        if (c <= 4000000)
        {           
            if (c%2 == 0)
            {
                sum2 = sum2 + c;
                cout << c << endl;
            }
        }
    }


    cout << "The sum of even fibonacci numbers not greater than 4 million is: " << (sum1 + sum2); //Not being executed
    return 0;
}