循环计算器代码

Question

我有这个计算器代码，使用用户后缀表达式和文件输入，具体取决于用户的偏好，我需要创建一个循环，如果任何后缀表达式不正确，（3 3 +，3 x它已经为用户提供了有关错误的正确错误消息，但我需要它再次为文件输入和键盘输入启动循环，从用户输入k开始或f他们想要的输入。

import java.io.File;
import java.io.FileNotFoundException;
import java.util.Scanner;

public class Test {
public static void main(String[] args) throws FileNotFoundException {
    Scanner Scanner = new Scanner(System.in);
    System.out.println("Press K to enter the input using the keyboard. Press F to read expressions from a file");
    String option = Scanner.nextLine();
    while (!(option.equalsIgnoreCase("K") || option.equalsIgnoreCase("F"))) {
        System.out.println("Invalid option - please choose again");
        option = Scanner.nextLine();
    }
    if (option.equalsIgnoreCase("K")) {

        Scanner Scanner2 = new Scanner(System.in);

        System.out.println("Please enter a postfix expression: ");

        String line = Scanner2.nextLine();

        String[] elements = line.split(" ");

        if (elements.length != 3) {
            System.out.println("Error: Invalid Expression: " + line);

        }

        else {
            try {
                // "try" is a special statement which allows us to deal with
                // "exceptions"

                double num1 = Double.parseDouble(elements[0]);
                double num2 = Double.parseDouble(elements[1]);

                double answer = 0;
                boolean OpperationWrong = false;
                String operation = elements[2];
                switch (operation) {

                case "+": {
                    answer = (num1 + num2);
                    System.out.println("Answer is: = " + answer);
                }
                    break;

                case "-": {
                    answer = (num1 - num2);
                    System.out.println("Answer is: = " + answer);

                }
                    break;

                case "/": {
                    answer = (num1 / num2);
                    System.out.println("Answer is: = " + answer);

                }
                    break;

                case "*": {
                    answer = (num1 * num2);
                    System.out.println("Answer is: = " + answer);
                }
                    break;

                default:
                    OpperationWrong = true;
                }
                String ans;
                if (OpperationWrong) {
                    System.out.println("Please use +,*,/,- for your operator");
                } else {
                    ans = String.valueOf(answer);
                }
            } catch (NumberFormatException e) {
                System.out.println("Invalid Key entered, please enter a number for first 2 elements");
            }
        }
    } else if (option.equalsIgnoreCase("F")) {

        try {
            Scanner Scanner3 = new Scanner(System.in);
            System.out.print("Enter the file name to read expressions from : ");
            File file = new File(Scanner3.nextLine());
            Scanner3 = new Scanner(file);

            while (!(file.exists())) {
                System.out.println("Error: That file does not exist, please re-enter: ");

            }

            String line;

            while (Scanner3.hasNext()) {
                line = Scanner3.nextLine(); // read the next line of text
                                            // from the file
                String[] elements = line.split(" ");
                if (elements.length != 3) {
                    System.out.println("Error: Invalid Expression: " + line);
                } else {
                    try {
                        // "try" is a statement which allows us to deal with
                        // "exceptions"

                        double num1 = Double.parseDouble(elements[0]);
                        double num2 = Double.parseDouble(elements[1]);

                        double answer = 0;
                        boolean OpperationWrong = false;
                        String operation = elements[2];
                        switch (operation) {

                        case "+": {
                            answer = (num1 + num2);
                            System.out.println("Answer is: = " + answer);
                        }
                            break;

                        case "-": {
                            answer = (num1 - num2);
                            System.out.println("Answer is: = " + answer);

                        }
                            break;

                        case "/": {
                            answer = (num1 / num2);
                            System.out.println("Answer is: = " + answer);

                        }
                            break;

                        case "*": {
                            answer = (num1 * num2);
                            System.out.println("Answer is: = " + answer);
                        }
                            break;

                        default:
                            OpperationWrong = true;
                        }
                        String ans;
                        if (OpperationWrong) {
                            ans = " Please use +,*,/,- for your operator";
                        } else {
                            ans = String.valueOf(answer);
                        }
                    } catch (NumberFormatException e) {
                        System.out.println("Invalid expression please enter a number for first 2 elements");
                    }
                }
            }
        } catch (FileNotFoundException exception) {
            System.out.println("The file could not be found");
        }
    }
}
}

Answer 1

少数事情

看起来您可以通过非常强大的方式利用这些功能来解决您的问题。我还注意到你可能想仔细看看你如何使用和命名你的变量。

功能

函数或方法是任何编程语言中非常重要的一部分，当你可以执行某些事情并发现自己想要再次某事时，这就是你可以做到的确定你想要一个函数或方法的位置。

您已完成上述难题，因为您的代码可以完成您希望它执行的操作。现在你想再次这样做，所以我们可以把这个动作想象成一个函数。

如果用户按下＆＃39; X＆＃39;您可能希望添加退出该功能的功能。或类似的东西，如果你选择实施它，我会把这个练习留给你。

变量

有些情况下，如果您已经有权访问变量，则初始化变量，对我来说最明显的是Scanner。当您声明并初始化Scanner Scanner = new Scanner(System.in)时，只要您有权访问Scanner变量，就可以访问System.in（另一个提示是使用小写字母命名变量，其中Scanner和Scanner稍后不明确关于你是否引用变量或类）。后来做Scanner Scanner2 = new Scanner(System.in)是不必要的。

您可以对变量进行一些其他清理工作，但我再次将它们留给您。

代码

import java.io.File;
import java.io.FileNotFoundException;
import java.util.Scanner;

public class Test {
    public static void main(String[] args) throws FileNotFoundException {
        Scanner scanner = new Scanner(System.in);
        boolean quitting = false;
        while (!quitting){
            quitting = processInput(scanner);
        }
    }

    public static boolean processInput(Scanner scanner){
        System.out.println("Press K to enter the input using the keyboard. Press F to read expressions from a file");
        String option = scanner.nextLine();
        while (!(option.equalsIgnoreCase("K") || option.equalsIgnoreCase("F"))) {
            System.out.println("Invalid option - please choose again");
            option = scanner.nextLine();
        }
        if (option.equalsIgnoreCase("K")) {
            System.out.println("Please enter a postfix expression: ");

            String line = scanner.nextLine();

            String[] elements = line.split(" ");

            if (elements.length != 3) {
                System.out.println("Error: Invalid Expression: " + line);

            }
            else {
                try {
                    // "try" is a special statement which allows us to deal with
                    // "exceptions"

                    double num1 = Double.parseDouble(elements[0]);
                    double num2 = Double.parseDouble(elements[1]);

                    double answer = 0;
                    boolean OpperationWrong = false;
                    String operation = elements[2];
                    switch (operation) {

                    case "+": {
                        answer = (num1 + num2);
                        System.out.println("Answer is: = " + answer);
                    }
                        break;

                    case "-": {
                        answer = (num1 - num2);
                        System.out.println("Answer is: = " + answer);

                    }
                        break;

                    case "/": {
                        answer = (num1 / num2);
                        System.out.println("Answer is: = " + answer);

                    }
                        break;

                    case "*": {
                        answer = (num1 * num2);
                        System.out.println("Answer is: = " + answer);
                    }
                        break;

                    default:
                        OpperationWrong = true;
                    }
                    String ans;
                    if (OpperationWrong) {
                        System.out.println("Please use +,*,/,- for your operator");
                    } 
                    else {
                        ans = String.valueOf(answer);
                    }
                } catch (NumberFormatException e) {
                    System.out.println("Invalid Key entered, please enter a number for first 2 elements");
                }
            }
        } 
        else if (option.equalsIgnoreCase("F")) {
            try {
                System.out.print("Enter the file name to read expressions from : ");
                File file = new File(scanner.nextLine());
                Scanner fileScanner = new Scanner(file);

                while (!(file.exists())) {
                    System.out.println("Error: That file does not exist, please re-enter: ");

                }

                String line;

                while (fileScanner.hasNext()) {
                    line = fileScanner.nextLine(); // read the next line of text
                                                // from the file
                    String[] elements = line.split(" ");
                    if (elements.length != 3) {
                        System.out.println("Error: Invalid Expression: " + line);
                    } 
                    else {
                        try {
                            // "try" is a statement which allows us to deal with
                            // "exceptions"

                            double num1 = Double.parseDouble(elements[0]);
                            double num2 = Double.parseDouble(elements[1]);

                            double answer = 0;
                            boolean OpperationWrong = false;
                            String operation = elements[2];
                            switch (operation) {

                            case "+": {
                                answer = (num1 + num2);
                                System.out.println("Answer is: = " + answer);
                            }
                                break;

                            case "-": {
                                answer = (num1 - num2);
                                System.out.println("Answer is: = " + answer);

                            }
                                break;

                            case "/": {
                                answer = (num1 / num2);
                                System.out.println("Answer is: = " + answer);

                            }
                                break;

                            case "*": {
                                answer = (num1 * num2);
                                System.out.println("Answer is: = " + answer);
                            }
                                break;

                            default:
                                OpperationWrong = true;
                            }
                            String ans;
                            if (OpperationWrong) {
                                ans = " Please use +,*,/,- for your operator";
                            } 
                            else {
                                ans = String.valueOf(answer);
                            }
                        } catch (NumberFormatException e) {
                            System.out.println("Invalid expression please enter a number for first 2 elements");
                        }
                    }
                }
            } catch (FileNotFoundException exception) {
                System.out.println("The file could not be found");
            }
        }
        return false;
    }
}

Answer 2

尝试使用while（true）循环并在输入可接受时中断。

一个简单的例子：

int input=0;
while(true){
input =new Scanner(System.in).nextInt(); 
if(input<=0){
System.out.println("Illegal input!");
continue;
}
break; 
}

Answer 3

NB： - 您应该在JAVA中使用小写字母作为变量名称，例如Scanner scanner。 - 您应为每个来源使用一个扫描程序，例如System.in或new File("name of file")。 - 对于键盘扫描仪（System.in），建议使用static扫描仪变量，如：private static Scanner scanner = new Scanner(System.in);。 - 当您使用具有路径文件Scanner(File source)

的扫描仪时，应关闭扫描仪

如果您想从头开始重新循环，可以尝试使用这样的无限循环while(true)：

public class Test {
// lower case of name variable
private static Scanner scanner = new Scanner(System.in);
public static void main(String[] args) throws FileNotFoundException {
while(true){
System.out.println("Press K to enter the input using the keyboard. Press F to read expressions from a file");
String option = scanner.nextLine();
while (!(option.equalsIgnoreCase("K") || option.equalsIgnoreCase("F"))) {
    System.out.println("Invalid option - please choose again");
    option = scanner.nextLine();
}
if (option.equalsIgnoreCase("K")) {

    System.out.println("Please enter a postfix expression: ");

    String line = scanner.nextLine();

    String[] elements = line.split(" ");

    if (elements.length != 3) {
        System.out.println("Error: Invalid Expression: " + line);

    }

    else {
        try {
            // "try" is a special statement which allows us to deal with
            // "exceptions"

            double num1 = Double.parseDouble(elements[0]);
            double num2 = Double.parseDouble(elements[1]);

            double answer = 0;
            boolean OpperationWrong = false;
            String operation = elements[2];
            switch (operation) {

            case "+": {
                answer = (num1 + num2);
                System.out.println("Answer is: = " + answer);
            }
                break;

            case "-": {
                answer = (num1 - num2);
                System.out.println("Answer is: = " + answer);

            }
                break;

            case "/": {
                answer = (num1 / num2);
                System.out.println("Answer is: = " + answer);

            }
                break;

            case "*": {
                answer = (num1 * num2);
                System.out.println("Answer is: = " + answer);
            }
                break;

            default:
                OpperationWrong = true;
            }
            String ans;
            if (OpperationWrong) {
                System.out.println("Please use +,*,/,- for your operator");
            } else {
                ans = String.valueOf(answer);
            }
        } catch (NumberFormatException e) {
            System.out.println("Invalid Key entered, please enter a number for first 2 elements");
        }
    }
} else if (option.equalsIgnoreCase("F")) {

    try {
        System.out.print("Enter the file name to read expressions from : ");
        File file = new File(scanner.nextLine());
        Scanner scanner1 = new Scanner(file);

        while (!(file.exists())) {
            System.out.println("Error: That file does not exist, please re-enter: ");

        }

        String line;

        while (scanner1.hasNext()) {
            line = scanner1.nextLine(); // read the next line of text
                                        // from the file
            String[] elements = line.split(" ");
            if (elements.length != 3) {
                System.out.println("Error: Invalid Expression: " + line);
            } else {
                try {
                    // "try" is a statement which allows us to deal with
                    // "exceptions"

                    double num1 = Double.parseDouble(elements[0]);
                    double num2 = Double.parseDouble(elements[1]);

                    double answer = 0;
                    boolean OpperationWrong = false;
                    String operation = elements[2];
                    switch (operation) {

                    case "+": {
                        answer = (num1 + num2);
                        System.out.println("Answer is: = " + answer);
                    }
                        break;

                    case "-": {
                        answer = (num1 - num2);
                        System.out.println("Answer is: = " + answer);

                    }
                        break;

                    case "/": {
                        answer = (num1 / num2);
                        System.out.println("Answer is: = " + answer);

                    }
                        break;

                    case "*": {
                        answer = (num1 * num2);
                        System.out.println("Answer is: = " + answer);
                    }
                        break;

                    default:
                        OpperationWrong = true;
                    }
                    String ans;
                    if (OpperationWrong) {
                        ans = " Please use +,*,/,- for your operator";
                    } else {
                        ans = String.valueOf(answer);
                    }
                } catch (NumberFormatException e) {
                    System.out.println("Invalid expression please enter a number for first 2 elements");
                }
            }
        }
//            close your Scanner
     scanner1.close();

    } catch (FileNotFoundException exception) {
        System.out.println("The file could not be found");
    }
}
}
}
}