我如何反復將java中的數字乘以2直到達到一百萬？

Question

import java.util.Scanner;

class Main {

    static Scanner userInput = new Scanner(System.in);

    public static void main(String[] args) {
        int testNumber = userInput.nextInt();
        do{
             System.out.println(newNumber * 2);
             newNumber++;
        }while( testNumber < 1000000);
    }
}

Answer 1

將數字乘以2后，需要更新數字：

newNumber = newNumber * 2;
System.out.println(newNumber);

另外，您正在使用newNumber和testNumber並且newNumber似乎沒有在任何地方定義...

}while( ***testNumber***newNumber*** < 1000000);

您需要選擇一個，因為如果要更新newNumber但在循環中比較testNumber ，則會創建一個無限循環。

除非您在發布中遺漏了某些內容，否則顯示的代碼不應編譯。

Answer 2

您對循環有正確的想法，但是變量有多個問題。

1. 您的第一個問題是，您從用戶testNumber讀取了一個變量，但是（錯誤地）您正在操縱一個完全不同的變量newNumber 。
2. 第二個問題是您正在測試未更改的變量作為停止條件。

您可能希望循環如下所示：

    do {
         testNumber = testNumber * 2;
         System.out.println(testNumber);
    } while(testNumber < 1000000);

Answer 3

您也可以為其創建遞歸方法。

public int reachMillion(int num) {
  if(num<=0)
    return -1;          // indicating it is not possible.
  if(num>=1000000)      // Base Condition denoting we have reached 1 million 
    return num;
  return reachMillion(num*2); // recursive part to multiply by 2 until we reach 1 million
}

Answer 4

class Main {
private static Scanner userInput = new Scanner(System.in);
public static void main(String[] args) {
    int newNumber = 0;

    do{
       System.out.println("Enter a positive number: ");

       try{
          newNumber = userInput.nextInt();
       }catch(Exception ignored){  }

       System.out.println("");

    }while(newNumber <= 0);

    System.out.println("-----  " + newNumber + "  multiply by 2 ------");

    while(newNumber <= 1_000_000){
         System.out.print("2 * " + newNumber +" = ");
         newNumber <<= 1;//in some compilers left shift is faster than multiply           
         System.out.println(newNumber);
    }

}

Answer 5

@ brso05很好地描述了這里出了什么問題。 我想提供一個完整的例子：

import java.util.Scanner;



public class Main {

    private static Scanner userInputScanner = new Scanner(System.in);

    public static void main(String[] args) {
        System.out.print("Please input a number: ");

        int userInputNumber = userInputScanner.nextInt();
        System.out.println();

        int newNumber = userInputNumber;

        while (newNumber < 1_000_000) {
            newNumber *= 2; // Take the variable on the left, multiply it by the number on the right, and save it in the variable on the left
            System.out.println(newNumber);
        }
    }
}

在線嘗試！

---

謹防！ 該代碼不會處理任何錯誤的用戶輸入。 例如，如果給它0 ，它將永遠循環，如果給它foo ，它將崩潰。 如果要處理用戶輸入的所有極端情況，可以這樣做：

import java.util.*;



public class Main {

    private static Scanner userInputScanner = new Scanner(System.in);

    public static void main(String[] args) {

        int userInputNumber;

        // 
        while(true) {
            System.out.print("Please input a number: ");
            if (userInputScanner.hasNext()) {
                // The user gave us something, but we don't know if it's a number

                String rawUserInput = userInputScanner.next();

                try {
                    userInputNumber = Integer.parseInt(rawUserInput);

                    // If that previous line runs, the user has given us an integer!
                    System.out.println();
                    if (userInputNumber > 0) {
                        // The user has given a valid number. Break out of the loop and multiply it!
                        break;
                    }
                    else {
                        // The user has given a bad number. Tell them why and ask again.
                        System.out.println("The number has to be greater than 0.");
                    }
                }
                catch (NumberFormatException exception) {
                    // The user has given us something, but it wasn't an integer
                    System.out.println();
                    System.out.println("That is not a number: " + exception.getMessage());
                }
            }
            else {
                // There is no input, so we can't do anything.
                return;
            }
        }
        // Done looping through user input

        int newNumber = userInputNumber;

        while (newNumber < 1_000_000) {
            newNumber *= 2; // Take the variable on the left, multiply it by the number on the right, and save it in the variable on the left
            System.out.println(newNumber);
        }
    }
}

在線嘗試！

Answer 6

do-while循環有一個棘手的部分。 在這種類型的循環中，首先do部分。 對於下面的示例，盡管輸入已經大於1000000，但它會打印1000001。

public void doWhileLoop() {

    int num = 1000001;
    do {
        System.out.println(num);
        num *= 2;
    } while (num < 1000000);
}

因此，在do-while循環中執行某些操作之前，最好先使用一些保護子句（即if-statements ）。 喜歡，

public void doWhileLoop() {

     int num = 1000001;
     if(num >= 1000000) {
        return;
     }
     do {
         System.out.println(num);
         num *= 2;
     } while (num < 1000000);
}