Java中一个变量的二次方程求解器

Question

I have this code to solve second degree equations. The user inputs the equation with any variable whose name matches [a-zA-Z], but the program has one issue: if the user inputs an equation with more than one variable in it, such as "x^2+2y-20=0" (which contains two variables, x and y), it will still "solve" it and return "a=1 , b=2 , c=-20".

I want to make it require that the user inputs only one variable, like "x^2+2x-20=0" or "y^2+2y-20=0" (or any other letter, as long as the same letter is used throughout the equation), which would be solved as "a=1 , b=2 , c=-20" also.

Because the program currently doesn't differentiate between variables, it solves an equation with multiple variables as if they were the same variable (character) and that's wrong.

public class ParseEquation {
public static String coeff(String str, String regex) {
    Pattern patt = Pattern.compile(regex);
    Matcher match = patt.matcher(str);
    // missing coefficient default
    String coeff = "+0";
    double value = 0;

   if(match.find()) 
        coeff = match.group(1);
    // always have sign, handle implicit 1
    value= Double.parseDouble((coeff.length() == 1) ? coeff + "1" 
        : coeff);

}
public static String[] quadParse(String arg) {
    String str = ("+" + arg).replaceAll("\\s", "");

    double a1 = Double.parseDouble(coeff(str, "([+-][0-9]*)([a-z A-Z]\\^2)"));
    double b1 = Double.parseDouble(coeff(str, "([+-][0-9]*)([a-z A-Z](?!\\^))"));
    double c1= Double.parseDouble(coeff(str, "([+-][0-9]+)(?![a-z A-Z])"));
    System.out.println("Values are a: " + a1 + " b: " + b1 + " c: " + c1);
    double dis = (Math.pow(b1, 2.0)) - (4 * a1 * c1);
    double d = Math.sqrt(dis);
    double X = 0, Y = 0; 

    if (dis > 0.0 || dis < 0.0) {
        X = (-b1 + d) / (2.0 * a1);
        Y = (-b1 - d) / (2.0 * a1);
        String root1 = Double.toString(X);
        String root2 = Double.toString(Y);
        return new String[]{root1, root2};
    } else if (dis == 0.0) {
        X = (-b1 + 0.0) / (2.0 * a1);//repeated root
        String root2 = Double.toString(X);
        return new String[]{root2};
    }
    return new String[-1];
}
public static void main(String[] args) throws IOException {
    BufferedReader r = new BufferedReader (new InputStreamReader(System.in));
    String s;
    while ((s=r.readLine()) != null) {
        String[] pieces = quadParse(s);
        System.out.println(Arrays.toString(pieces));
    }
}
}

Currently, the above code returns this incorrect result for "x^2+2y-20=0":

Values are a: 1.0 b: 2.0 c: -20.0
[3.58257569495584, -5.58257569495584]

The right result would be to allow the user to input only one variable character in the equation, like "x^2+2x-20=0", else throw an exception.

Answer 1

Before parsing the line into a formula via quadParse(s); , examine the contents of it. Create a length 26 boolean array, where each index represents a variable that you've seen in the equation. Iterate over the input one letter at a time, and update the array if you see a particular variable. If there are multiple variables that you've seen at the end of that process, you can then throw an exception before calling quadParse .