while loop如何在java中重复将一个数字乘以2，直到它达到100万？

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

你对循环的想法是正确的，但是你的变量有很多问题

1. 第一个问题是，您从用户-testNumber读取了一个变量，但随后（错误地）操作了一个完全不同的变量-newNumber
2. 第二个问题是，您正在测试未更改的变量作为停止条件

你可能希望你的循环是这样的：

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

Answer 2

将数字乘以2后需要更新：

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

此外，您正在使用newNumber和testNumber，并且newNumber似乎没有在任何地方定义

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

您需要选择一个，因为如果您正在更新newNumber，但比较循环中的testNumber，您将创建一个无限循环

你展示的代码不应该编译，除非你在文章中遗漏了什么

Answer 3

在do-while循环中有一个棘手的部分。在这种类型的循环中，首先执行do部分。对于下面的示例，虽然输入已经大于1000000，但它打印1000001

public void doWhileLoop() {

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

因此，在do while循环中执行某些操作之前，最好使用一些保护子句（aka，if-statements）。比如

public void doWhileLoop() {

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

Answer 4

描述这里出了什么问题。我想提供一个完整的例子：

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);
        }
    }
}

Try it online!

---

小心该代码不会处理任何错误的用户输入。例如，如果给它0，它将永远循环，如果给它foo，它将崩溃。如果您想处理用户输入的所有边缘情况，这将实现：

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);
        }
    }
}

Try it online!

Answer 5

您还可以为它创建一个递归方法

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 6

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);
    }

}