Java tutorial
//package com.java2s; /* * Copyright (C) Stichting Akvo (Akvo Foundation) * * This file is part of Akvo Caddisfly. * * Akvo Caddisfly is free software: you can redistribute it and/or modify * it under the terms of the GNU General Public License as published by * the Free Software Foundation, either version 3 of the License, or * (at your option) any later version. * * Akvo Caddisfly is distributed in the hope that it will be useful, * but WITHOUT ANY WARRANTY; without even the implied warranty of * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the * GNU General Public License for more details. * * You should have received a copy of the GNU General Public License * along with Akvo Caddisfly. If not, see <http://www.gnu.org/licenses/>. */ public class Main { public static double eval(final String str) { return new Object() { private 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 IllegalArgumentException("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(); switch (func) { case "sqrt": x = Math.sqrt(x); break; case "sin": x = Math.sin(Math.toRadians(x)); break; case "cos": x = Math.cos(Math.toRadians(x)); break; case "tan": x = Math.tan(Math.toRadians(x)); break; default: throw new IllegalArgumentException("Unknown function: " + func); } } else { throw new IllegalArgumentException("Unexpected: " + (char) ch); } if (eat('^')) { x = Math.pow(x, parseFactor()); // exponentiation } return x; } }.parse(); } }