'cout'语句不被执行

Question

以下程序是查找不超过400万的偶数斐波那契项的和。 在这个程序中最后的'cout'语句根本没有得到执行。为什么？请帮助。

#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

我不能执行程序，但我认为你的程序永远不会结束，这就是为什么你永远不会去那个声明。您的外围while循环将继续前进unitl c <= 4000000。但是，如果且只有(a + b) <= 4000000，您才会增加c，所以c永远不会超过400万。

为了解决这个问题，你可以尝试以下：

#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; 
} 

Answer 2

while (c <= 4000000) 
{ 
    // ... 
    if ((a + b) <= 4000000) 
    { 
     c = a + b; // i.e. <= 4000000 
     // ... 
    } 
} 

你怎么能指望这个循环终止？