爲什麼我不能在開關中使用相同的變量？

Question

例如，如果我寫這樣的代碼：

double result = 0; 
    switch (operation) { 
     case "divide": 
      double result = (double) first/second; 
      string = Double.toString(result); 
      break; 
     case "multiply": 
      double result = (double) first * second; 
      string = Double.toString(result); 
      break; 
     case "substract": 
      double result = (double) first - second; 
      string = Double.toString(result); 
      break; 
     case "add": 
      double result = (double) first + second; 
      string = Double.toString(result); 
      break; 
    } 

它返回錯誤：可變「結果」在範圍已定義。

我知道如何解決這個問題。我可以在每種情況下創建一個不同的變量，但是這樣會通過必須爲每個變量編寫代碼來延長代碼長度。

如何使這項工作在給定的代碼中書寫？

Answer 1

這是由result已被定義在switch語句頂部的事實引起的，並且您正在聲明中再次聲明它。

嘗試這樣：

switch (operation) { 
    case "divide": 
     double result = (double) first/second; 
     string = Double.toString(result); 
     break; 
    case "multiply": 
     double result = (double) first * second; 
     string = Double.toString(result); 
     break; 
    case "substract": 
     double result = (double) first - second; 
     string = Double.toString(result); 
     break; 
    case "add": 
     double result = (double) first + second; 
     string = Double.toString(result); 
     break; 
} 

換句話說，你有過什麼，只是從你最初的聲明。

---

或者，你可以聲明double，並用它在你的每一個案件：

double result = 0; 
switch (operation) { 
    case "divide": 
     result = (double) first/second; 
     string = Double.toString(result); 
     break; 

    ... 

---

由於亞辛Hajaj在評論中指出，這將是，如果要好得多您在switch語句的末尾使用了string = Double.toString(result);行一次，而不是多次重複。

這將是這個樣子：

double result = 0; 
switch (operation) { 
    case "divide": 
     result = (double) first/second; 
     break; 
    case "multiply": 
     result = (double) first * second; 
     break; 
    ... 
} 
string = Double.toString(result); 

Answer 2

實際上，你可以通過使用括號。

switch (operation) { 
     case "divide": { 
      double result = (double) first/second; 
      string = Double.toString(result1); 
      break; 
     } 
     case "multiply": { 
      double result = (double) first * second; 
      string = Double.toString(result2); 
      break; 
     } 
     default: 
      break; 
    } 

Answer 3

在java中，{}內定義的任何局部變量都可用，直到最後一行，因此您可以在塊內定義之後重用任何地方。所以在你的情況下沒有用來重新定義結果變量。只需在第一行中定義並隨處使用即可。

double result = 0; 
    switch (operation) { 
     case "divide": 
      result = (double) first/second; 
      string = Double.toString(result1); 
      break; 
     case "multiply": 
      result = (double) first * second; 
      string = Double.toString(result2); 
      break; 
     case "substract": 
      result = (double) first - second; 
      string = Double.toString(result3); 
      break; 
     case "add": 
      result = (double) first + second; 
      string = Double.toString(result4); 
      break; 
    } 

Answer 4

becaue您已經定義雙重結果變量，以便做到像下面希望它會工作

double result = 0; <----- already define here 
switch (operation) { 
    case "divide": 
     result = (double) first/second; <------ there is no need to 
                define it again 
     string = Double.toString(result1); 
     break; 
    case "multiply": 
     result = (double) first * second; 
     string = Double.toString(result2); 
     break; 
    case "substract": 
     result = (double) first - second; 
     string = Double.toString(result3); 
     break; 
    case "add": 
     result = (double) first + second; 
     string = Double.toString(result4); 
     break; 
} 

Answer 5

如果要使用公共變量，那麼你可以這樣做。

double result = 0; 
    switch (operation) { 
     case "divide": 
      result = (double) first/second; 
      string = Double.toString(result); 
      break; 
     case "multiply": 
      result = (double) first * second; 
      string = Double.toString(result); 
      break; 
     case "substract": 
      result = (double) first - second; 
      string = Double.toString(result); 
      break; 
     case "add": 
      result = (double) first + second; 
      string = Double.toString(result); 
      break; 
    } 

Answer 6

你不能在你的代碼的一個範圍內聲明相同的變量名，使用全局變量的結果，你也萬一範圍內使用同一個變量。

double result = 0; 
    switch (operation) { 
     case "divide": 
      result = (double) first/second; 
      string = Double.toString(result); 
      break; 
     case "multiply": 
      result = (double) first * second; 
      string = Double.toString(result); 
      break; 
     case "substract": 
      result = (double) first - second; 
      string = Double.toString(result); 
      break; 
     case "add": 
      result = (double) first + second; 
      string = Double.toString(result); 
      break; 
    }