Evaluate a String in Android Studio

Question

I am writing a program, and user input ends up as a mathematical expression inside a string. How do I evaluate it into a double?

I don't have a lot of experience in this language, I am mostly familiar with BASIC(lol). So if anyone can give me the simplest step by step instructions to do this, it would be very much appreciated.

Please give more detail of what you specifically want to do as then it will be easier for people to answer your question. At the moment it is too vague. — haymansfield, Oct 09 '13 at 19:41

score 1 · Answer 1 · edited May 23 '17 at 12:28

1

It is unfortunately not too straightforward in Java. The two top options seem to be using the built-in javascript engine or using the exp4j library.

You can read more about them in these answers: evaluating-a-math-expression-given-in-string-form and java-parse-a-mathematical-expression-given-as-a-string

edited May 23 '17 at 12:28

Community

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1

answered Oct 09 '13 at 19:29

alexroussos

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Thank you, seems understandable enough. I will find out when I try it, lol. – base_ten Oct 10 '13 at 03:02

score 1 · Answer 2 · answered Jun 30 '18 at 15:26

You can try this program written in this answer.

https://stackoverflow.com/a/26227947.

I have copy pasted the code here. The explanation of the code is in the original answer if you follow the link.

public 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();
}`

Great code, I used it to calculate dynamic formulas and worked as expected — IgniteCoders, Apr 15 '19 at 09:31

score 0 · Answer 3 · answered Jun 11 '22 at 16:04

Ok. I guess you basically have a String like String abc="(56+55(6/655)-5522*1222)"; and you want to evaluate this without changing its type. Yes, there is a library available for this.

Library

Update The Library if there is any update

implementation 'com.udojava:EvalEx:2.7'

----------`

import com.udojava.evalex.Expression;
String memory="";


equal.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            memory=editText.getText().toString();  //Get User's Entered Equation 
            Expression expression=new Expression(memory); //This Library Evaluate It
            String abc=expression.eval().toString(); //Insert The Data into A String
            textView.setText(abc); //Show The Data
        }
    });

Evaluate a String in Android Studio

3 Answers3