为什么我会收到一个错误的编译错误：'else'没有先前的'if'？

Question

当我尝试编译代码时，出现错误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));
     }
}

Answer 1

肯定是关键括号的问题：

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;
}

Answer 2

你的if

之后缺少荣誉（花括号）

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

装置

 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)   

Answer 3

（编辑：这是一个完全重做的答案。请参阅评论。谢谢你，抱歉“争议”。）

因为您未在{中正确使用大括号（“大括号”，}和main）。

首先让我们来看看代码的内部部分：

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

目前的缩进是“错误的”并且具有误导性。如果将其复制粘贴到代码编辑器中并使用自动格式化（auto- 缩进就足够了），您将获得：

      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”。为了清晰起见，在添加括号和空白行之后：

      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，cout仅限第一个if语句。第二个将永远执行。然后是一个else，它遵循“普通”，“无条件”语句，而不是“条件”语句/块（一个语句块整体上也是一个语句 ）。

要修复此部分，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";
      }

使完整的块语句受到限制。

现在“内部”if-else部分已修复，让我们采用“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";

现在应该清楚真正的意义（在这里使用紧凑的风格）：

     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";

中间的滑稽块（括号内的代码但不是直接跟在if / else之后）实际上是一个匿名块，它只引入了一个内部范围（内部定义的变量不存在）结束后}）。它可以看作是一个简单的陈述（无条件），就像它上面的cout << "nd Fibonacci number\n";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";
     }

Answer 4

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;
}

确保您的if和else因此在大括号内。

Answer 5

您没有在带有多个语句的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));
     }
}

Answer 6

你忘了在内部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";
}

Answer 7

我相信你在if（n＆lt; 3）之后缺少大括号，因此条件仅适用于下面的行。然后编译器命中'else'......

Answer 8

关于花括号的所有答案都是正确的，但我需要的代码就像这些人一样：

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";
       }

    }
}