本机尝试在java中生成表达式eval函数。

Question

我对自己提出了一个挑战，在Java上创建函数'eval()‘(来自python)，而不需要导入任何额外的库。但当我面对"-20-10“时我被困住了。

当找到运算符时，我的代码基本上将字符串分成两部分(a和b)。

String equ = "-20-10" // The first "-" is found
String a = "";
String b = "20-10";

然后，字符串a和b进入递归循环。但在这种情况下，a将为“(空)”并输出错误。

String equ = "20-10";
String a = "20";
String b = "10";

这个错误与程序的逻辑有关，我似乎无法理解，需要指导。

private String calc(String equ) {
        String result = equ;
        if(equ.contains("+")) {
            System.out.println("+ found");
            String a = equ.substring(0, equ.indexOf('+'));
            System.out.println("+ a:"+a);
            String b = equ.substring(equ.indexOf('+')+1);
            System.out.println("+ b:"+b);
            a = calc(a);
            System.out.println(a+" + "+b);
            b = calc(b);
            result = String.valueOf(Double.valueOf(a)+Double.valueOf(b));
            System.out.println("+ result:"+result);
        }
        else if (equ.contains("-")) {
                System.out.println("- found:");
                String a = equ.substring(0, equ.indexOf('-'));
                System.out.println("- a:"+a);
                String b = equ.substring(equ.indexOf('-')+1);
                System.out.println("- b:"+b);
                a = calc(a);
                System.out.println(a+" - "+b);
                b = calc(b);
                result = String.valueOf(Double.valueOf(a)-Double.valueOf(b));
                System.out.println("- result:"+result);
        }
        else if (equ.contains("/")) {
            System.out.println("/ found:");
            String a = equ.substring(0, equ.indexOf('/'));
            System.out.println("/ a:"+a);
            String b = equ.substring(equ.indexOf('/')+1);
            System.out.println("/ b:"+b);
            a = calc(a);
            System.out.println(a+" / "+b);
            b = calc(b);
            result = String.valueOf(Double.valueOf(a)/Double.valueOf(b));
            System.out.println("/ result:"+result);
        }
        else if(equ.contains("*")) {
            System.out.println("* found:");
            String a = equ.substring(0, equ.indexOf('*'));
            System.out.println("* a:"+a);
            String b = equ.substring(equ.indexOf('*')+1);
            System.out.println("* b:"+b);
            a = calc(a);
            System.out.println(a+" * "+b);
            b = calc(b);
            result = String.valueOf(Double.valueOf(a)*Double.valueOf(b));
            System.out.println("* result:"+result);
        }
        return result;
    }

Answer 1

浏览问题是，当您用calc()调用"-20"时，它将字符串分离到""和"20"，所以当tou调用Double.valueOf("")时，他抛出了一个NumberFormatException。要解决此问题，只需将正负if状态更改为：

if(equ.substring(1).contains("+")) {
            System.out.println("+ found");
            String a = equ.substring(0, equ.lastIndexOf('+'));
            System.out.println("+ a:"+a);
            String b = equ.substring(equ.lastIndexOf('+')+1);
            System.out.println("+ b:"+b);
            a = calc(a);
            System.out.println(a+" + "+b);
            b = calc(b);
            result = String.valueOf(Double.valueOf(a)+Double.valueOf(b));
            System.out.println("+ result:"+result);
        }
        else if (equ.substring(1).contains("-")) {
                System.out.println("- found:");
                String a = equ.substring(0, equ.lastIndexOf('-'));
                System.out.println("- a:"+a);
                String b = equ.substring(equ.lastIndexOf('-')+1);
                System.out.println("- b:"+b);
                a = calc(a);
                System.out.println(a+" - "+b);
                b = calc(b);
                result = String.valueOf(Double.valueOf(a)-Double.valueOf(b));
                System.out.println("- result:"+result);
        }

这使得函数将"-20"看作是"-20"，而不是""减去"20"。