否则没有如果

Question

我正在尝试为周一到期的一个项目的计算机编程课程编写代码，而且我对 Java 还很陌生，但我正在尝试编写一个程序，该程序将首先确定用户输入的数字是否是偶数还是奇数，然后确定该数是否为素数。我不确定我的算法是否正确，所以如果有人对我的算法或其他任何程序有任何更正，请说出来，但我真正的问题是程序拒绝编译。每次我尝试时，它都说它有一个没有 if 问题的 else。这是我的命令框的链接：

http://s1341.photobucket.com/user/Emi_Nightshade/media/Capture_zps45f9a2ea.png.html

这是我的代码：

import java.io.*;
import java.util.*;

public class Lesson9p1_ThuotteEmily
{
    public static void main(String args[])
    {
        Scanner kbReader0=new Scanner(System.in);
        System.out.print("\n\nPlease enter an integer. An integer is whole number, and it can be either negative or positive. Please enter your number: ");
        long num=kbReader0.nextLong();

        if(num%2==0)                                     //if and else with braces
        {
           System.out.println("Your integer " + num + " is even.");
        }
        else
        {
            System.out.println("Your integer " + num + " is odd.");
        }

        Scanner kbReader1=new Scanner(System.in);
        System.out.print("\n\nWould you like to know if your number is prime? Please enter yes or no: ");
        String yn=kbReader1.nextLine();

        if(yn.equals.IgnoreCase("Yes"))
        {
            System.out.println("Okay. Give me a moment.");

            {
                if(num%2==0)
                {
                    System.out.println("Your number isn't prime.");
                }
                else if(num==2)
                {
                    System.out.println("Your number is 2, which is the only even prime number in existence. Cool, right?");
                }
                for(int i=3;i*i<=n;i+=2)
                {
                    if(n%1==0)
                    {
                        System.out.println("Your number isn't prime.");
                    }
                }
                else
                {
                    System.out.println("Your number is prime!");
                }
            }
        }
        if(yn.equals.IgnoreCase("No"))
        {
            System.out.println("Okay.");
        }
    }
}

如果有人能帮助我解决这个问题以及我在程序其他地方可能遇到的任何问题，我将不胜感激！谢谢。

Answer 1

在for 循环之后有一个else 语句

for(int i=3;i*i<=n;i+=2)
{
    if(n%1==0)
    {
        System.out.println("Your number isn't prime.");
    }
}
else
{
    System.out.println("Your number is prime!");
}

您可能必须创建一个布尔变量来执行此操作。有多种方法可以做到这一点，但这是我可能会使用的一种

boolean isPrime = true;
for(int i=3;i*i<=n;i+=2)
{
    if(n%1==0)
    {
        isPrime = false;
    }
}
if(isPrime)
{
    System.out.println("Your number is prime!");
}
else
{
    System.out.println("Your number isn't prime.");
}

Answer 2

问题来了

            else
            {
                System.out.println("Your number is prime!");
            }

是在for循环之后，不是if语句，你要切换它。 ：

           if(num%2==0)
            {
                System.out.println("Your number isn't prime.");
            }
            else if(num==2)
            {
                System.out.println("Your number is 2, which is the only even prime number in existence. Cool, right?");
            }
            else
            {
                System.out.println("Your number is prime!");
            }
            for(int i=3;i*i<=n;i+=2)
            {
                if(n%1==0)
                {
                    System.out.println("Your number isn't prime.");
                }
            }

这是功能性但不正确的算法，你可能想要这个：

           if(num%2==0)
            {
                System.out.println("Your number isn't prime.");
            }
            else if(num==2)
            {
                System.out.println("Your number is 2, which is the only even prime number in existence. Cool, right?");
            }

            boolean isPrime = true;
            for(int i=3;i*i<=n;i+=2)
            {
                if(n%i==0)
                {
                    System.out.println("Your number isn't prime.");
                    isPrime = false;
                }
            }
            if (isPrime){
                System.out.println("Your number is prime!");
            }   

Answer 3

方块

else
{
    System.out.println("Your number is prime!");
}

前面没有对应的'if'。

Answer 4

import java.io.*;
import java.util.*;

public class Lesson9p1_ThuotteEmily {
    public static void main(String args[]) {
        Scanner kbReader0 = new Scanner(System.in);
        System.out.print("\n\nPlease enter an integer. An integer is whole number, and it can be either negative or positive. Please enter your number: ");
        long num = kbReader0.nextLong();

        if (num % 2 == 0) // if and else with braces
        {
            System.out.println("Your integer " + num + " is even.");
        } else {
            System.out.println("Your integer " + num + " is odd.");
        }

        Scanner kbReader1 = new Scanner(System.in);
        System.out.print("\n\nWould you like to know if your number is prime? Please enter yes or no: ");
        String yn = kbReader1.nextLine();

        if (yn.equalsIgnoreCase("Yes")) {
            System.out.println("Okay. Give me a moment.");

            {
                if (num % 2 == 0) {
                    System.out.println("Your number isn't prime.");
                } else if (num == 2) {
                    System.out.println("Your number is 2, which is the only even prime number in existence. Cool, right?");
                } else
                    for (int i = 3; i * i <= num; i += 2) {
                        if (num % i == 0) {
                            System.out.println("Your number isn't prime.");
                        } else {
                            System.out.println("Your number is prime!");
                        }
                    }
            }
        }
        if (yn.equalsIgnoreCase("No")) {
            System.out.println("Okay.");
        }
    }
}

包括所有更正。

Answer 5

这在 Java 中不起作用（没有 if 的 2 个 else）

    if (1 == 1){

    }
    else
    {

    }
    else //syntax error here - else without if
    {

    }

这行得通：

                if (1 == 1){

                }
                else
                {
                    if (2 == 2){

                    }
                    else //this works
                    {

                    }
                }