如何添加一個循環到這個程序並讓它保持運行直到程序爲真？

Question

如何在此程序中添加循環以便程序保持執行，然後在用戶輸入介於100和999之間的有效數字時結束？

import java.util.*; 

public class numLoop{ 
public static void main(String[] arg){ 
    Scanner sc = new Scanner(System.in); 
    int number = 0; 

    try{ 
    Scanner reader = new Scanner(System.in); 
    System.out.print("Enter a number between 100 and 999: "); 
    number = reader.nextInt(); 

    if(number > 999 || number < 100) 
     System.out.println("\nYour number was invalid."); 

    else 
     System.out.println("\nThe number " + number + " was vaild."); 

     } 
     catch(InputMismatchException ime){ 
      System.out.println("You didn't enter a number."); 

    } 
} 
} 

Answer 1

把你整個的try/catch一個while循環中：

while (number < 100 || number > 999) 
{ 
    // ... 
} 

或者使用while (true) { }或for (;;) { }把它放在一個無限循環，然後添加一個條件break如果輸入一個有效的數字：

if (number < 100 || number > 999) 
{ 
    // handle invalid entry 
} 

else 
{ 
    // handle valid entry 

    break; // exit loop 
} 

Answer 2

使用while循環可以實現以下邏輯

（僞）

Input a number 
while (number is not valid) 
    Tell them something went wrong 
    Input another number 
Valid number was entered, do something about it 

while循環本身看起來像

while (condition) { 
    // code to run while condition is true 
} 

Answer 3

import java.util.*; 

public class numLoop{ 
public static void main(String[] arg){ 
    Scanner sc = new Scanner(System.in); 
    int number = 0; 

    try{ 
    Scanner reader = new Scanner(System.in); 
    System.out.print("Enter a number between 100 and 999: "); 
    number = reader.nextInt(); 

    while(number > 999 || number < 100){ 
     System.out.println("\nYour number was invalid."); 
     number = reader.nextInt(); 
    } 

    System.out.println("\nThe number " + number + " was vaild."); 


     }catch(InputMismatchException ime){ 
      System.out.println("You didn't enter a number."); 

    } 
} 
} 

Answer 4

您還可以使用一段時間（真）中斷循環是這樣的：

import java.util.*; 

    public class numLoop{ 
    public static void main(String[] arg){ 
     Scanner sc = new Scanner(System.in); 
     int number = 0; 
    while(true){ 
     try{ 
     Scanner reader = new Scanner(System.in); 
     System.out.print("Enter a number between 100 and 999: "); 
     number = reader.nextInt(); 

     if(number > 999 || number < 100) 
      System.out.println("\nYour number was invalid."); 

     else 
      System.out.println("\nThe number " + number + " was vaild."); 
      break; 
      } 
      catch(InputMismatchException ime){ 
       System.out.println("You didn't enter a number."); 

     } 
    } 
    } 
    } 

被警告不是每個人都是這樣的事情，這是很好的代碼練習冰。

Answer 5

三個小行...您現有的try前添加

while(true) // you could (and should) add braces (for the look of it). 
      // could also have used for(;;) 

這裏

else 
    System.out.println("\nThe number " + number + " was vaild."); 

那麼你需要一個

else { 
    System.out.println("\nThe number " + number + " was vaild."); 
    break; // End the infinite loop. 
}