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

Question

I would like some help with an app I'm programming which has to do with Arithmetic Progression. Basically the user inputs a formula - which contains a variable n - and the range of terms they want to calculate. Then the app should calculate every case, replacing n with the current term being calculated, starting from the first term until it reaches the last, and finally displaying the complete sequence. For example, if I input this as the formula:

An = n + 4

And in the range of terms:

from 1 to 5

The output should be:

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 )

The result is defined as a double , as it won't always be an integer (eg An = n + 1/2 ).

In order to evaluate the math expression, I'm using @Boann's eval method , which is this one:

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();
}

And in order to make this work (as the method above won't accept my custom variable n ), I'm trying to use the replace() method to replace the character n with an integer i - representing the current term - which is defined in a for loop:

    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
    }

But the app just crashes whenever I input a formula containing "n". 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$1.parseFactor(MessageFragment.java:96)
    at com.example.APTesting.MessageFragment$1.parseTerm(MessageFragment.java:66)
    at com.example.APTesting.MessageFragment$1.parseExpression(MessageFragment.java:57)
    at com.example.APTesting.MessageFragment$1.parse(MessageFragment.java:45)
    at com.example.APTesting.MessageFragment.eval(MessageFragment.java:103)
    at com.example.APTesting.MessageFragment.access$300(MessageFragment.java:20)
    at com.example.APTesting.MessageFragment$3.onClick(MessageFragment.java:149)
    at android.view.View.performClick(View.java:7161)
    at android.view.View.performClickInternal(View.java:7133)
    at android.view.View.access$3500(View.java:804)
    at android.view.View$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 is the character that doesn't show up. By clicking on the error messages, the issue seems to come from termsAndResults.add("A" + i + " = " + eval(strResult) + "\n") , which probably happens due to replacing 'n' with i The error message comes from within the eval() method - throw new RuntimeException("Unexpected: " + (char) ch); . I tried the solutions from similar questions but pretty much the same error pops up. I'm a beginner. Any help would be appreciated.

Answer 1

Casting those values of i to a char results in a non-printable character. For example, if i is 1, then the resulting character will become the start of header control character. That's the reason you see that box with a question mark in it.

You need the string value of the integer i . Just use

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

Answer 2

Please try with this..

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