IDG | four arithmetic expressions

Analysis

First, convert the infix expression to the suffix expression (inverse polish expression), and then use the stack for calculation.

Parentheses and decimals are not considered.

Code

import java.util.LinkedList;import java.util.List;import java.util.Stack;public class ExpCal {public static double calc(String exp) {if (exp == null || exp.length() <= 0) {throw new IllegalArgumentException();}char[] c = exp.toCharArray();Stack<Character> s = new Stack<Character>();List<String> reversePolishNotation = new LinkedList<String>();for (int i = 0; i < c.length; ++i) {if (c[i] == '+' || c[i] == '-' || c[i] == '*' || c[i] == '/') {while (!s.isEmpty() && compOp(s.peek(), c[i]) >= 0) {reversePolishNotation.add(String.valueOf(s.pop()));}s.push(c[i]);} else {StringBuilder sb = new StringBuilder();while (i < c.length && c[i] >= '0' && c[i] <= '9') {sb.append(c[i++]);}reversePolishNotation.add(sb.toString());--i;}}while (!s.isEmpty()) {reversePolishNotation.add(String.valueOf(s.pop()));}Stack<Double> num = new Stack<Double>();for (String e : reversePolishNotation) {if (e.equals("+")) {num.push(num.pop() + num.pop());} else if (e.equals("-")) {double a = num.pop();double b = num.pop();num.push(b - a);} else if (e.equals("*")) {num.push(num.pop() * num.pop());} else if (e.equals("/")) {double a = num.pop();double b = num.pop();num.push(b / a);} else {num.push(Double.parseDouble(e));}}return num.pop();}private static int compOp(char a, char b) {return getPri(a) - getPri(b);}private static int getPri(char c) {switch (c) {case '+':case '-':return 1;case '*':case '/':return 2;default:return 0;}}public static void main(String[] args) {System.out.println(calc("4*3+2*5-8/8-2*6/3+2/1-4"));}}

IDG | four arithmetic expressions