即使我在 if 语句之外定义了变量（简单计算器），我的程序也找不到 if 语句内的变量

Question

我添加了一个 if 语句，它将输入的数字解析为双精度数，以便在第二个 if 语句块中进行计算时使用，如果仅输入数字而不是字母，则程序必须运行，程序似乎无法读取当我输入 double(5.5) 时，效果很好，但当我输入 int(5) 数字时，效果很好。

Scanner numbers = new Scanner(System.in);
Scanner operation = new Scanner(System.in);

double number1 = 0;
double number2 = 0;
String operator;


System.out.print("Enter the operator you would like to choose(+, -, *, /): ");
operator = operation.next();


System.out.print("Enter the first number: ");
String num1 = numbers.nextLine();


System.out.print("Enter your second number: ");
String num2 = numbers.nextLine();


boolean check1 = num1.trim().matches("^[0-9]+$");
boolean check2 = num2.trim().matches("^[0-9]+$");


if (check1 == true && check2 == true){
    number1 = Double.parseDouble(num1);
    number2 = Double.parseDouble(num2);
}else {
    System.out.println("Only enter numbers not letters.");
}


String calculation;
if (operator.equals("+")){
    calculation = (number1 + " + " + number2 + " = " + (number1 + number2));
    System.out.println(calculation);

}else if (operator.equals("-")){
    calculation = (number1 + " - " + number2 + " = " + (number1 - number2));
    System.out.println(calculation);

}else if (operator.equals("*")){
    calculation = (number1 + " * " + number2 + " = " + (number1 * number2));
    System.out.println(calculation);

}else if (operator.equals("/")){
    calculation = (number1 + " / " + number2 + " = " + (number1 / number2));
    System.out.println(calculation);

}else{
    calculation = operator + ":" + " Is not a valid operator!";
    System.out.println(calculation);

}

我认为这可能是 (.) 这就是问题所在， 我输出到控制台

Enter the operator you would like to choose(+, -, *, /): +
Enter the first number: 5.5
Enter your second number: 5.5
Only enter numbers not letters.
0.0 + 0.0 = 0.0 

现在 int 数字工作正常。

Enter the operator you would like to choose(+, -, *, /): +
Enter the first number: 5
Enter your second number: 5
5.0 + 5.0 = 10.0

Answer 1

不要使用多个 Scanner 对象。不需要多个，而且这也可能会产生一些您不想处理的副作用。

由于您正在使用正则表达式，并且其他人建议了其他解决方案，因此我决定向您展示如何使用正则表达式来计算浮点值。正确的表达式是

[-+]?[0-9]*\\.?[0-9]+

或更好...

[-+]?\\d*\\.?\\d+

此表达式计算一个值，该值可能包含可选符号（正或负），后跟零个或多个数字，然后是可选小数点和无限数量的小数位。但是，如果使用小数点，则必须至少出现一位数字。在像 +2. 这样的情况下，仅匹配 +2。

下面的代码是您的解决方案版本，稍加修改：

public static void main(String[] args) {
    final String REGEX = "[-+]?[0-9]*\\.?[0-9]+";
    Scanner scanner = new Scanner(System.in);

    double number1 = 0;
    double number2 = 0;
    String operator;
    
    System.out.print("Enter the operator you would like to choose(+, -, *, /): ");
    operator = scanner.next();

    System.out.print("Enter the first number: ");
    String num1 = scanner.next();

    System.out.print("Enter your second number: ");
    String num2 = scanner.next();
    scanner.close();

    boolean check1 = num1.trim().matches(REGEX);
    boolean check2 = num2.trim().matches(REGEX);

    if (check1 && check2) {
        number1 = Double.parseDouble(num1);
        number2 = Double.parseDouble(num2);
    }else {
        System.out.println("Only enter numbers not letters.");
    }

    String calculation;
    switch (operator) {
        case "+":
            calculation = (number1 + " + " + number2 + " = " + (number1 + number2));
            break;
        case "-":
            calculation = (number1 + " - " + number2 + " = " + (number1 - number2));
            break;
        case "*":
            calculation = (number1 + " * " + number2 + " = " + (number1 * number2));
            break;
        case "/":
            calculation = (number1 + " / " + number2 + " = " + (number1 / number2));
            break;
        default:
            calculation = operator + ":" + " Is not a valid operator!";
            break;
    }
    System.out.println(calculation);
}

主要区别是我只使用了一个 Scanner 对象，并且我用 if/else 替换了嵌套的 if/else 。代码>开关。

下面是一个示例运行

Enter the operator you would like to choose(+, -, *, /): +
Enter the first number: 5.5
Enter your second number: -7.2
5.5 + -7.2 = -1.7000000000000002

其他建议的改进是：

1. 循环直到提供有效的运算符。没有理由等到提供所有输入后才确定由于运算符无效而无法完成计算。
2. 对每个数字执行类似于上面 #1 的操作。
3. 使用BigDecimal进行计算。
4. 设置结果的格式以显示您想要显示的任意数量的小数值。

Answer 2

正则表达式“^[0-9]+$”仅对于整数计算为 true，因为它检查字符串是否由 0 - 9 组成。如果您还想检查双精度数，这可能会有所帮助：https://www.baeldung.com/java-check-string-number

第一个建议这可能是最简单的。用 try catch 包围 Double.parseDouble ，因为如果它不是数字，它会给你一个 NumberFormatException 。