java - 评估代码，区分负数和负数？

Question

我正在计算一个表达式，但在输入负数时遇到了麻烦。由于我的代码的结构，以及减法运算符和负号是同一字符的事实，我的代码不适用于负数。有办法解决这个问题吗？

private float evalNoPB(String s) {

    float tempAns = 0;
    if (s.contains("*") == false && s.contains("/") == false && s.contains("+") == false && s.contains("-") == false) {
        return Float.parseFloat(s);
    }

    if (s.length() - 1 > 0) {
        int i;
        boolean foundPlusMinus = false;
        for (i = s.length() - 1; i > 0; i--) {
            if (s.charAt(i) == '+' || s.charAt(i) == '-') {
                System.out.println(i);
                foundPlusMinus = true;
                break; // keep value of i for substrings 
            }
            foundPlusMinus = false;
        }

        if (foundPlusMinus == false) { // for loop went through and did not find + or -
            for (i = s.length() - 1; i > 0; i--) {
                if (s.charAt(i) == '*' || s.charAt(i) == '/') {
                    System.out.println(i);
                    break; // keep value of i for substrings
                }
            }
        }

        String sub1 = s.substring(0, i);
        System.out.println(sub1);
        String sub2 = s.substring(i + 1, s.length());
        System.out.println(sub2);

        if (s.charAt(i) == '+') {
            tempAns = evalNoPB(sub1) + evalNoPB(sub2);
        } else if (s.charAt(i) == '-') {
            tempAns = evalNoPB(sub1) - evalNoPB(sub2);
        } else if (s.charAt(i) == '*') {
            tempAns = evalNoPB(sub1) * evalNoPB(sub2);
        } else if (s.charAt(i) == '/') {
            float divisorCheck = evalNoPB(sub2);
            if (divisorCheck != 0) {
                tempAns = evalNoPB(sub1) / evalNoPB(sub2);
            } else { // cannot divide by 0 
                throw new IllegalArgumentException("cannot divide by 0");
            }
        }
    }
    return tempAns;

}

Answer 1

一种解决方法是永远不要“减”，只需添加负数(如果需要减去某些内容，请将其乘以 -1 并与另一个数字相加)。

伪代码:

if I come across a - with nothing on either side and not first in string {
add a + to the left of it
}
else {
if first in string {
     add 1* to left of it
}
do stuff that has to do with *, /, or +.
}
if i run across a + {
    check to see if - is to right of it
    if so {
        add together the values but with the value to right of - multiplied by -1
    }
    else {
         add together values
    }
}