析因函数不起作用

Question

我正在开发一个Android计算器应用程序并且已经关闭了所有基本功能，但是当我按下阶乘按钮时，它根本不会返回任何内容。我不知道为什么。请帮忙。

评估功能

public static double eval(final String str) {
    return new Object() {
        int pos = -1, ch;

        void nextChar() {
            ch = (++pos < str.length()) ? str.charAt(pos) : -1;
        }

        void prevChar() {
            ch = (--pos < str.length()) ? str.charAt(pos) : -1;
        }

        boolean eat(int charToEat) {
            while (ch == ' ') nextChar();
            if (ch == charToEat) {
                nextChar();
                return true;
            }
            return false;
        }

        double parse() {
            nextChar();
            double x = parseExpression();
            if (pos < str.length()) throw new RuntimeException("Unexpected: " + (char)ch);
            return x;
        }

        // Grammar:
        // expression = term | expression `+` term | expression `-` term
        // term = factor | term `*` factor | term `/` factor
        // factor = `+` factor | `-` factor | `(` expression `)`
        //        | number | functionName factor | factor `^` factor

        double parseExpression() {
            double x = parseTerm();
            for (;;) {
                if      (eat('+')) x += parseTerm(); // addition
                else if (eat('-')) x -= parseTerm(); // subtraction
                else return x;
            }
        }

        double parseTerm() {
            double x = parseFactor();
            for (;;) {
                if      (eat('x')) x *= parseFactor(); // multiplication
                else if (eat('÷')) x /= parseFactor(); // division
                else return x;
            }
        }

        double parseFactor() {
            if (eat('+')) return parseFactor(); // unary plus
            if (eat('-')) return -parseFactor(); // unary minus

            double x;
            int startPos = this.pos;
            if (eat('(')) { // parentheses
                x = parseExpression();
                eat(')');
            } else if ((ch >= '0' && ch <= '9') || ch == '.') { // numbers
                while ((ch >= '0' && ch <= '9') || ch == '.') nextChar();
                x = Double.parseDouble(str.substring(startPos, this.pos));
            } else if (ch >= 'a' && ch <= 'z' || ch >= 'A' && ch <= 'Z') { // functions
                while (ch >= 'a' && ch <= 'z'|| ch >= 'A' && ch <= 'Z') nextChar();
                String func = str.substring(startPos, this.pos);
                x = parseFactor();
                if (func.equals("sin")) x = Math.sin(x);
                else if (func.equals("cos")) x = Math.cos(x);
                else if (func.equals("tan")) x = Math.tan(x);
                else if (func.equals("log")) x = Math.log(x);
                else if (func.equals("ln")) x = Math.log10(x);
                else if (func.equals("Abs")) x = Math.abs(x);
                //else if (func.equals("fact")) x = CombinatoricsUtils.factorial((int) x);
                else throw new RuntimeException("Unknown function: " + func);
            }

            else if (ch == '√') { // functions
                while (ch == '√') nextChar();
                String func = str.substring(startPos, this.pos);
                x = parseFactor();
                if (func.equals("√")) x = Math.sqrt(x);
                else throw new RuntimeException("Unknown function: " + func);
            }

            else if (ch == '!') { // functions
                int strLength = str.length();
                String func = str.substring(strLength - 1);
                x = parseFactor();
                if (func.equals("!")) x = CombinatoricsUtils.factorial((int) x);
                else throw new RuntimeException("Unknown function: " + func);
            }

            else {
                throw new RuntimeException("Unexpected: " + (char)ch);
            }

            if (eat('^')) x = Math.pow(x, parseFactor());
            else if (eat('³')) x = Math.pow(x, 3);
            else if (eat('²')) x = Math.pow(x, 2); // exponentiation

            return x;
        }
    }.parse();

Answer 1

我用一个简单的测试程序尝试了你的代码：

public static void main(String[] args) {
    System.out.println(eval("!5"));
}

我收到了StackOverflowError，因为您的代码没有跳过!

更改

    else if (ch == '!') { // functions
        int strLength = str.length();
        String func = str.substring(strLength - 1);
        x = parseFactor();

到

    else if (ch == '!') { // functions
         while (ch == '!') nextChar();
         String func = str.substring(startPos, this.pos);
         x = parseFactor();

为我纠正了这个问题。

如果你不想在感叹号上面循环，你仍然需要跳过它：

    else if (ch == '!') { // functions
         eat('!');  // this is missing in your current implementation
         x = CombinatoricsUtils.factorial((int) parseFactor());

目前我没有解释为什么“事实”的代码不起作用 - 如果我取消注释它们似乎有效（至少对我而言）。