如何使用带有哨兵的while循环重复输入

Question

我需要有关在此代码上放置 while 循环的位置的帮助。几周前我刚开始学习 Java。我想写一个哨兵值-1，如果输入，程序将退出。只要用户输入不是-1，就一直要求重复程序。

当我将 while 循环 "while(currentPop != -1)" 放在第一个问题“输入当前人口”下方时，程序成功运行了它的第一道课程。但是，它并没有回到第一个问题。相反，它直接进入第二个问题，“输入出生率：”。

我应该如何继续并确保第一个问题在循环运行后不断被问到？

谢谢大家！

import java.util.Scanner;

public class Population
{
    public static void main(String[] args)
    {
        Scanner scan = new Scanner(System.in);
        double birthRate, deathRate;
        double currentPop = 0; 
        double newPop;
        long years;

        System.out.print("Enter current population or -1 to exit: ");
        currentPop = scan.nextDouble();

        System.out.print("Enter birth rate: ");
        birthRate = scan.nextDouble();

        System.out.print("Enter death rate: ");
        deathRate = scan.nextDouble();
        newPop = 0;
        years = 0;          

        System.out.println("===================");
        System.out.println("YEAR     POPULATION");
        System.out.println("===================");

        if (birthRate > deathRate) {
            System.out.printf("0   %,15d\n", (int)currentPop);
            double growthRate = (birthRate - deathRate);
            double doublingTime = Math.log(2) / 
                                Math.log(1 +(growthRate/100)); 
            for (years = 1; years <= (doublingTime+1); years++) {
                newPop = ((growthRate/100) * currentPop) + currentPop;
                currentPop = newPop;
                System.out.printf("%,d   %,15d\n",years,(int)currentPop);
            }
            System.out.printf("\nIt will take %,d years to reach double " 
                            + "the population of %,d\n\n",
                              (int)(doublingTime + 1),(int)currentPop);   
        } else if (birthRate < deathRate) {
            System.out.printf("0   %,15d\n", (int)currentPop);
            double growthRate = (birthRate - deathRate);
            double decreaseTime = Math.log(1/currentPop) 
                                  / Math.log(1 + (growthRate/100));
            for (years = 1; years < (1 + decreaseTime) ; years++) {
                newPop = ((growthRate/100) * currentPop) + currentPop;
                currentPop = newPop;
                System.out.printf("%,d   %,15d\n",years,(int)currentPop);
            }
            System.out.printf("\nPopulation will be zero in %,d years.\n",
                             (int)decreaseTime + 1);
        } else if(birthRate == deathRate) {
        System.out.printf("0   %,15d\n", (int)currentPop);
        double growthRate = (birthRate - deathRate);
        double decreaseTime = Math.log(1/currentPop) 
                              / Math.log(1 + (growthRate/100));
        for (years = 1; years < (1 + decreaseTime) ; years++) {
            newPop = ((growthRate/100) * currentPop) + currentPop;
            currentPop = newPop;
            System.out.printf("%,d   %,15d\n",years,(int)currentPop);
        }
        System.out.printf("\nPopulation is stable.");
    }
}

}

Answer 1

要实现while，循环首先要确定终止条件，直到条件达到终止条件（即false），它的语法可以是

while(true) { //your code} 

Answer 2

这是你想要的代码 -

import java.util.Scanner;

public class Population
{
    public static void main(String[] args)
    {
        Scanner scan = new Scanner(System.in);
        double birthRate, deathRate;
        double currentPop = 0; 
        double newPop;
        long years;

        while(true) {
            System.out.print("Enter current population or -1 to exit: ");
            currentPop = scan.nextDouble();
            if(currentPop == -1)
                break;

            System.out.print("Enter birth rate: ");
            birthRate = scan.nextDouble();

            System.out.print("Enter death rate: ");
            deathRate = scan.nextDouble();
            newPop = 0;
            years = 0;          

            System.out.println("===================");
            System.out.println("YEAR     POPULATION");
            System.out.println("===================");

            if (birthRate > deathRate) {
                System.out.printf("0   %,15d\n", (int)currentPop);
                double growthRate = (birthRate - deathRate);
                double doublingTime = Math.log(2) / 
                                    Math.log(1 +(growthRate/100)); 
                for (years = 1; years <= (doublingTime+1); years++) {
                    newPop = ((growthRate/100) * currentPop) + currentPop;
                    currentPop = newPop;
                    System.out.printf("%,d   %,15d\n",years,(int)currentPop);
                }
                System.out.printf("\nIt will take %,d years to reach double " 
                                + "the population of %,d\n\n",
                                  (int)(doublingTime + 1),(int)currentPop);   
            } else if (birthRate < deathRate) {
                System.out.printf("0   %,15d\n", (int)currentPop);
                double growthRate = (birthRate - deathRate);
                double decreaseTime = Math.log(1/currentPop) 
                                      / Math.log(1 + (growthRate/100));
                for (years = 1; years < (1 + decreaseTime) ; years++) {
                    newPop = ((growthRate/100) * currentPop) + currentPop;
                    currentPop = newPop;
                    System.out.printf("%,d   %,15d\n",years,(int)currentPop);
                }
                System.out.printf("\nPopulation will be zero in %,d years.\n",
                                 (int)decreaseTime + 1);
            } else if(birthRate == deathRate) {
                System.out.printf("0   %,15d\n", (int)currentPop);
                double growthRate = (birthRate - deathRate);
                double decreaseTime = Math.log(1/currentPop) 
                                      / Math.log(1 + (growthRate/100));
                for (years = 1; years < (1 + decreaseTime) ; years++) {
                    newPop = ((growthRate/100) * currentPop) + currentPop;
                    currentPop = newPop;
                    System.out.printf("%,d   %,15d\n",years,(int)currentPop);
                }
                System.out.printf("\nPopulation is stable.");
            }
        }
        scan.close();
    }
}

Answer 3

我认为您应该在第一个问题和其余代码之前添加while(currentPop != -1) {。在该行之后记得设置currentPop = 0 和代码} 的末尾。 编辑： 而且当然 在第一个问题之后 if (currentPop == -1) break;

Answer 4

退出程序

直接从main方法返回

System.out.print("Enter current population or -1 to exit: ");
currentPop = scan.nextDouble();

if (currentPop == -1.0) {
    return;
}

System.out.print("Enter birth rate: ");

通过退出或重启循环程序

在循环构造体中使用break 或continue

while (true) {
    System.out.print("Enter current population or -1 to exit: ");
    currentPop = scan.nextDouble();

    if (currentPop == -1) {
        break;
    } else if (currentPop <= 0) {
        System.out.println("Population must be positive");
        continue; // restart the loop
    }

    System.out.print("Enter birth rate: ");

    ...
}
System.out.println("Done!");

一个不接受-1的函数

您可以使用方法抽象出重复的任务（多个输入）

public static double getValue(Scanner s, String msg) {
  double value = -1;
  while (value == -1) {
    System.out.print(msg);
    value = s.nextDouble();
  }
  return value;
}

在main方法中

currentPop = getValue(scan, "Enter current population: ");
birthRate = getValue(scan, "Enter birth rate: ");

Answer 5

While 循环只会重复它们包含的代码。所以这段代码：

System.out.print("Enter current population or -1 to exit: ");
currentPop = scan.nextDouble();

while (currentPop != -1) {
    System.out.print("Enter birth rate: ");
    birthRate = scan.nextDouble();

    // the rest of your code
}

只会重复while循环中的内容。在这种情况下，它只会询问出生率。

你想要的实际代码是：

while (true) {
    System.out.print("Enter current population or -1 to exit: ");
    currentPop = scan.nextDouble();

    if (currentPop == -1) {
        break;
    }

    System.out.print("Enter birth rate: ");
    birthRate = scan.nextDouble();

    // the rest of your code
}

让我们分解一下。

如果您希望再次询问当前人口，则需要将该行放在 while 循环中。

我不知道你是否熟悉break，但你说你是新手。 break 所做的是退出它所在的任何循环。这与 while (true) 一起意味着 while 循环将永远运行，除非调用 if 语句。希望对您有所帮助！