2

当我尝试编译代码时，我收到一条错误消息else without a previous if：

// Fibonacci series using recursion
#include <iostream>
using namespace std;
int fib (int n);

int main()
{
     int n, answer;

     cout << "\n\n\t\tEnter number to find: ";
     cin >> n;
     cout << "\n\n";

     answer = fib(n);

     if(n < 3 && n > 1)
         cout << answer << " is the " << n;
         cout << "nd Fibonacci number\n";
     {
      if(n < 3)
         cout << answer << " is the " << n;
         cout << "st Fibonacci number\n";
      else   
         cout << answer << " is the " << n;
         cout << "rd Fibonacci number\n";
     }
     else
     cout << answer << " is the " << n;
     cout << "th Fibonacci number\n";
     return 0;
}

int fib (int n)
{
     cout << "Processing fib (" << n << ")... ";

     if (n < 3)
     {
         cout << "Return 1!\n";
         return 1;
     }
     else
     {
         cout << "Call fib(" << n-2 << ") ";
         cout << "and fib(" << n-1 << ").\n";
         return( fib(n-2) + fib(n-1));
     }
}
4

8 回答 8

4

当然这是关于键括号的问题：

int main()
{
     int n, answer;

     cout << "\n\n\t\tEnter number to find: ";
     cin >> n;
     cout << "\n\n";

     answer = fib(n);

     if(n < 3 && n > 1) {
         cout << answer << " is the " << n;
         cout << "nd Fibonacci number\n";

         if(n < 3) {
             cout << answer << " is the " << n;
             cout << "st Fibonacci number\n";
         } else {
             cout << answer << " is the " << n;
             cout << "rd Fibonacci number\n";
         }
     }
     else {
         cout << answer << " is the " << n;
         cout << "th Fibonacci number\n";
     }
     return 0;
}
于 2013-07-11T07:54:48.653 回答
3

if 之后缺少的荣誉（大括号）

if(n < 3 && n > 1)
    cout << answer << " is the " << n;
    cout << "nd Fibonacci number\n";
{
    if(n < 3)

方法

 if(n < 3 && n > 1)
 {
     cout << answer << " is the " << n;
 } // end of if 
 cout << "nd Fibonacci number\n"; // always executed
 { // new anonymous block
     if(n < 3)
于 2013-07-11T07:54:49.587 回答
2

（编辑：这是一个完全重新设计的答案。请参阅评论。感谢您并为“争议”感到抱歉。）

{因为您没有}在main.

首先让我们看一下代码的内部部分：

      if(n < 3)
         cout << answer << " is the " << n;
         cout << "st Fibonacci number\n";
      else   
         cout << answer << " is the " << n;
         cout << "rd Fibonacci number\n";

当前的缩进是“错误的”并且具有误导性。如果你将它复制粘贴到代码编辑器中并使用自动格式化（自动缩进就足够了），你会得到：

      if(n < 3)
         cout << answer << " is the " << n;
      cout << "st Fibonacci number\n";
      else   
         cout << answer << " is the " << n;
      cout << "rd Fibonacci number\n";

它向您展示了代码的真正“含义”。为了清楚起见，添加大括号和空行后：

      if(n < 3)
      {
         cout << answer << " is the " << n;
      }

      cout << "st Fibonacci number\n";

      else   
      {
         cout << answer << " is the " << n;
      }

      cout << "rd Fibonacci number\n";

如您所见，只有第一个cout语句受if. 第二个将始终被执行。然后是一个else遵循“普通”、“无条件”语句的语句，而不是“有条件的”语句/块（整个语句块也是一个语句）。

要修复此部分，您必须将所有条件语句包装在大括号中：

      if(n < 3)
      {
         cout << answer << " is the " << n;
         cout << "st Fibonacci number\n";
      }
      else   
      {
         cout << answer << " is the " << n;
         cout << "rd Fibonacci number\n";
      }

或更紧凑的风格：

      if(n < 3) {
         cout << answer << " is the " << n;
         cout << "st Fibonacci number\n";
      } else {
         cout << answer << " is the " << n;
         cout << "rd Fibonacci number\n";
      }

这样完整的块语句是有条件的。

既然“内部”if-else 部分已经固定，让我们来看看“外部”if-else：

     if(n < 3 && n > 1)
         cout << answer << " is the " << n;
         cout << "nd Fibonacci number\n";
     {
      /* ... fixed inner if-else ... */
     }
     else
     cout << answer << " is the " << n;
     cout << "th Fibonacci number\n";

让我们再次使用代码格式化程序：

     if(n < 3 && n > 1)
         cout << answer << " is the " << n;
     cout << "nd Fibonacci number\n";
     {
         /* ... fixed inner if-else ... */
     }
     else
         cout << answer << " is the " << n;
     cout << "th Fibonacci number\n";

真正的含义现在应该很清楚了（这里使用紧凑的样式）：

     if(n < 3 && n > 1) {
         cout << answer << " is the " << n;
     }

     cout << "nd Fibonacci number\n";

     {
         /* ... fixed inner if-else ... */
     }

     else {
         cout << answer << " is the " << n;
     }

     cout << "th Fibonacci number\n";

中间的有趣块（大括号内的代码但不直接跟在if/后面else）实际上是一个匿名块，它只是引入了一个内部范围（内部定义的变量在关闭之后将不再存在}）。它可以看作是一个简单的语句（无条件），就像cout << "nd Fibonacci number\n";它上面的一样。

再一次，修复很明显：

     if(n < 3 && n > 1) {
         cout << answer << " is the " << n;
         cout << "nd Fibonacci number\n";

         /* ... fixed inner if-else ... */

     } else {
         cout << answer << " is the " << n;
         cout << "th Fibonacci number\n";
     }
于 2013-07-11T07:54:36.243 回答
2

试试这个：

int main()
{
     int n, answer;

     cout << "\n\n\t\tEnter number to find: ";
     cin >> n;
     cout << "\n\n";

     answer = fib(n);

     if(n < 3 && n > 1) 
     {
         cout << answer << " is the " << n;
         cout << "nd Fibonacci number\n";

         if(n < 3) 
         {
             cout << answer << " is the " << n;
             cout << "st Fibonacci number\n";
         } 
         else 
         {
             cout << answer << " is the " << n;
             cout << "rd Fibonacci number\n";
         }
     }
     else
     {
         cout << answer << " is the " << n;
         cout << "th Fibonacci number\n";
     }
     return 0;
}

确保您if的 s 和elsees 相应地位于大括号内。

于 2013-07-11T07:55:33.800 回答
2

您没有在某些带有多个语句的 if else 子句中添加括号。不要在现实世界的编码中这样做。更改代码如下：

// Fibonacci series using recursion
#include <iostream>
using namespace std;
int fib (int n);

int main()
{
     int n, answer;

     cout << "\n\n\t\tEnter number to find: ";
     cin >> n;
     cout << "\n\n";

     answer = fib(n);

     if(n < 3 && n > 1)
      {
         cout << answer << " is the " << n;
         cout << "nd Fibonacci number\n";

      if(n < 3)
     {
         cout << answer << " is the " << n;
         cout << "st Fibonacci number\n";
     }
      else   
      {
         cout << answer << " is the " << n;
         cout << "rd Fibonacci number\n";
       }
     }
     else {
     cout << answer << " is the " << n;
     cout << "th Fibonacci number\n";
     }
     return 0;
}

int fib (int n)
{
     cout << "Processing fib (" << n << ")... ";

     if (n < 3)
     {
         cout << "Return 1!\n";
         return 1;
     }
     else
     {
         cout << "Call fib(" << n-2 << ") ";
         cout << "and fib(" << n-1 << ").\n";
         return( fib(n-2) + fib(n-1));
     }
}
于 2013-07-11T09:24:00.980 回答
1

您忘记在内部 if 上使用 {}。它应该是

if(n < 3)
{
   cout << answer << " is the " << n;
   cout << "st Fibonacci number\n";
}
else
{   
    cout << answer << " is the " << n;
    cout << "rd Fibonacci number\n";
}
于 2013-07-11T07:55:49.050 回答
1

我相信您在 if(n < 3) 之后缺少大括号，因此条件仅适用于下面的行。然后编译器点击'else'......

于 2013-07-11T07:55:53.220 回答
0

你关于大括号的所有答案都是正确的，但我需要的代码是这样的：

if (n > 3)
{
    cout << answer << " is the " << n;
    cout << "th Fibonacci number\n";
}
else 
{
    if(n == 3)
    {
        cout << answer << " is the " << n;
        cout << "rd Fibonacci number\n";
    } 
    else
    {
       if(n < 3 && n > 1)
       {
           cout << answer << " is the " << n;
           cout << "nd Fibonacci number\n";
       }
       else
       {
           cout << answer << " is the " << n;
           cout << "st Fibonacci number\n";
       }

    }
}
于 2013-07-11T09:55:59.803 回答