'cout'語句不被執行

Question

以下程序是查找不超過400萬的偶數斐波那契項的和。 在這個程序中最後的'cout'語句根本沒有得到執行。爲什麼？請幫助。

#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

我不能執行程序，但我認爲你的程序永遠不會結束，這就是爲什麼你永遠不會去那個聲明。您的外圍while循環將繼續前進unitl c <= 4000000。但是，如果且只有(a + b) <= 4000000，您纔會增加c，所以c永遠不會超過400萬。

爲了解決這個問題，你可以嘗試以下：

#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 
     // ... 
    } 
} 

你怎麼能指望這個循環終止？