爲什麼我得到一個編譯錯誤，說錯誤：'else'沒有以前的'if'？

Question

當我嘗試編譯代碼我得到那個說else without a previous if錯誤：

// 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

（編輯：。這是一個完全重新設計的答案看評論謝謝你，對不起了「爭議」。 ）

因爲你沒有正確地在你的main使用大括號（ 「大括號」，{和}）。

首先讓我們的代碼的這個內部：

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

即當前縮進是「錯誤」和誤導。如果您將其複製粘貼到一個代碼編輯器，並使用自動格式化（自動 - 縮進就足夠了），你會得到：

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

這表明你真正的代碼的「意義」。加括號和空白線清晰後：

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

     cout << "st Fibonacci number\n"; 

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

     cout << "rd Fibonacci number\n"; 

正如你所看到的，只有第一個cout語句由if條件。第二個將始終執行。然後出現一個else，它遵循「普通」，「無條件」語句，而不是「條件化」語句/語句塊（作爲整體，語句塊也是語句）。

爲了解決這個問題的一部分，必須用所有的條件語句中括號：

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

或在更緊湊的風格：

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

使得全塊語句是條件。

現在， 「內部」 的if-else部分是固定的，讓我們的 「外部」 的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"; 

讓我們再次使用一個代碼格式化：

 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"; 

實含義現在應該清楚（使用簡潔的風格在這裏）：一個人在中間

 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"; 

有趣塊（內碼的文胸ces但不直接跟在if/else之上）實際上是一個匿名塊，它只引入一個內部作用域（在關閉}之後，內部定義的變量不再存在）。它可以看作是一個簡單的陳述（無條件的），就像它上面的cout << "nd Fibonacci number\n";一樣。

再次，解決方法是顯而易見的：

 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 2

當然這是一個關於關鍵括號問題：後

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 3

你丟失的榮譽（大括號）您如果

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

表示

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 4

試試這個：

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; 
} 

確保您if S和else ES是相應的大括號內。

Answer 5

您忘記在內部if上使用{}。它應該是

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 6

我相信你失蹤後，大括號，如果（N < 3），所以條件只適用於線下。然後編譯器命中'else'......

Answer 7

你沒有在多語句的某些if else子句中添加括號。不要在真實世界的編碼中做到這一點。如下 更改代碼：

// 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 8

所有你對大括號的答案是正確的，但我需要的代碼是這樣的人：

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"; 
     } 

    } 
}