用 int 替换 String 中的字符，以便对其进行评估 - Java

Question

我正在编写的一个与算术级数有关的应用程序需要一些帮助。基本上，用户输入一个公式 - 其中包含一个变量 n - 以及他们想要计算的术语范围。然后应用程序应该计算每个案例，将 n 替换为正在计算的当前项，从第一项开始直到最后一项，最后显示完整序列。例如，如果我将其输入为公式：

安=n + 4

并且在术语范围内：

从 1 到 5

输出应该是：

A1 = 5.0  //because it is n + 4 and the current term n is 1, so 1 + 2
A2 = 6.0  //n + 4, n = 2 so 3 + 2, and so on...
A3 = 7.0  
A4 = 8.0
A5 = 9.0  //n reached 5, so the calculation stops there.
Complete sequence: ( 5.0 , 6.0 , 7.0 , 8.0 , 9.0 )

结果定义为 double，因为它并不总是整数（例如 An = n + 1/2）。

为了计算数学表达式，我使用 @Boann's eval method，就是这个：

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

        void nextChar() {
            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 *= 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') { // functions
                while (ch >= 'a' && ch <= 'z') nextChar();
                String func = str.substring(startPos, this.pos);
                x = parseFactor();
                if (func.equals("sqrt")) x = Math.sqrt(x);
                else if (func.equals("sin")) x = Math.sin(Math.toRadians(x));
                else if (func.equals("cos")) x = Math.cos(Math.toRadians(x));
                else if (func.equals("tan")) x = Math.tan(Math.toRadians(x));
                else throw new RuntimeException("Unknown function: " + func);
            } else {
                throw new RuntimeException("Unexpected: " + (char) ch);
            }

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

            return x;
        }
    }.parse();
}

为了完成这项工作（因为上面的方法不接受我的自定义变量 n），我正在尝试使用 replace() 方法将字符 n 替换为整数 i - 表示当前项 - 在 for 循环中定义：

    for (i = firstTerm; i < lastTerm; ) {
    String strResult = strFormula.replace('n', (char) i); //the Result string, given after replacing the character "n" from the formula with the variable "i" so it can be evaluated

    termsAndResults.add("A" + i + " = " + eval(strResult) + "\n"); //add the current term and its result (e.g. "A3 = 8", "A4 = 10") to a list
    allResults.add(String.valueOf(eval(strResult))); //add the result to a list so each of them can be displayed later
    i++; //move onto the next term
    if (i == lastTerm) { 
        textResults.setText("  " + termsAndResults.toString().replace('[', ' ').replace(']', ' ').replace(',', ' ').trim() + "\nComplete sequence: (" + allResults.toString().replace('[', ' ').replace(']', ' ').trim() + ", ...)"); //display each term, its results and finally the complete sequence. All those replace and trim methods are there just so there won't be any "," or "[" and "]" when the results are displayed
    }

但是每当我输入包含 "n" 的公式时，应用程序就会崩溃。 Logcat:

com.example.APTesting E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.APTesting, PID: 22158
java.lang.RuntimeException: Unexpected: [and a square character which doesn't show up here as text][2]
    at com.example.APTesting.MessageFragment.parseFactor(MessageFragment.java:96)
    at com.example.APTesting.MessageFragment.parseTerm(MessageFragment.java:66)
    at com.example.APTesting.MessageFragment.parseExpression(MessageFragment.java:57)
    at com.example.APTesting.MessageFragment.parse(MessageFragment.java:45)
    at com.example.APTesting.MessageFragment.eval(MessageFragment.java:103)
    at com.example.APTesting.MessageFragment.access0(MessageFragment.java:20)
    at com.example.APTesting.MessageFragment.onClick(MessageFragment.java:149)
    at android.view.View.performClick(View.java:7161)
    at android.view.View.performClickInternal(View.java:7133)
    at android.view.View.access00(View.java:804)
    at android.view.View$PerformClick.run(View.java:27416)
    at android.os.Handler.handleCallback(Handler.java:883)
    at android.os.Handler.dispatchMessage(Handler.java:100)
    at android.os.Looper.loop(Looper.java:241)
    at android.app.ActivityThread.main(ActivityThread.java:7617)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:941)

This是没有出现的字符。通过单击错误消息，问题似乎来自 termsAndResults.add("A" + i + " = " + eval(strResult) + "\n")，这可能是由于将 'n' 替换为 i 错误消息来自 eval() 方法 - throw new RuntimeException("Unexpected: " + (char) ch);。我尝试了类似问题的解决方案，但弹出了几乎相同的错误。我是初学者。任何帮助将不胜感激。

Answer 1

请试试这个..

String strResult = strFormula.replace('n','i').trim().replace(" ","")

Answer 2

将 i 的值转换为 char 会产生不可打印的字符。例如，如果 i 为 1，则结果字符将成为 header 控制字符的开始。这就是您看到带有问号的框的原因。

您需要整数 i 的字符串值。只需使用

strFormula.replace("n", String.valueOf(i))

用 int 替换 String 中的字符，以便对其进行评估 - Java

Replace a character from a String with an int so it can be evaluated - Java

java

android

replace

eval