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Java 进行四则运算
四则运算的基础原理是将中缀表达式转换成为后缀表达式。然后进行计算。
转换思路和原理,可以参考将中缀表达式转化为后缀表达式
import java.math.BigDecimal; import java.util.ArrayList; import java.util.List; import java.util.Stack; import java.util.regex.Pattern; public class Arithmetic { //操作符stack private Stack<String> operator = new Stack<String>(); //后缀表达式 private List<String> postFix = new ArrayList<String>(); private Pattern operatorPattern = Pattern.compile("[\\d\\.]"); private Pattern arithmeticPattern = Pattern.compile("[\\(\\)\\+\\-\\*\\/]"); //将中缀表达式换为后缀表达式 public void parse(String s) { s = replace(s); int j = 0; for (int i = 0; i < s.length(); i++) { String temp = s.substring(i, i + 1); if (operatorPattern.matcher(temp).matches()) { continue; } if (arithmeticPattern.matcher(temp).matches()) { j = process(j, i, s, temp); } else if (" ".equals(temp)) { return; } } if (j < s.length()) { postFix.add(s.substring(j, s.length())); } while (!this.operator.isEmpty()) { postFix.add(operator.pop()); } } private String replace(String s) { return s.replaceAll("\\s", ""); } private int process(int startIndex, int currentIndex, String str, String word) { if (startIndex != currentIndex) { postFix.add(str.substring(startIndex, currentIndex)); } addOperator(word); startIndex = currentIndex + 1; if (startIndex > str.length()) { startIndex = str.length(); } return startIndex; } public void addOperator(String operator) { if ("(".equals(operator)) { } else if(")".equals(operator)) { while (!this.operator.isEmpty()) { String temp = this.operator.pop(); if ("(".equals(temp)) { break; } else { postFix.add(temp); } } return; } else if (!this.operator.isEmpty()) { while(!this.operator.isEmpty()) { String temp = this.operator.peek(); if (needPop(temp, operator)) { this.postFix.add(this.operator.pop()); } else { break; } } } this.operator.add(operator); } public boolean needPop(String inStackTop, String current) { return getLevel(current.charAt(0)) <= getLevel(inStackTop.charAt(0)); } public int getLevel(char operator) { switch (operator) { case '+': return 1; case '-': return 1; case '*': return 2; case '/': return 2; default: return -1; } } public BigDecimal compute() { Stack<BigDecimal> stack = new Stack<BigDecimal>(); for (int i=0; i < this.postFix.size(); i++) { if (arithmeticPattern.matcher(postFix.get(i)).matches()) { BigDecimal bd2 = stack.pop(); BigDecimal bd1 = stack.pop(); BigDecimal temp = compute(postFix.get(i).charAt(0), bd1, bd2); stack.add(temp); } else { stack.add(new BigDecimal(postFix.get(i))); } } return stack.pop(); } private BigDecimal compute(char operator, BigDecimal bd1, BigDecimal bd2) { switch (operator) { case '+': return bd1.add(bd2); case '-': return bd1.subtract(bd2); case '*': return bd1.multiply(bd2); case '/': return bd1.divide(bd2);//应当使用bd1.divide(divisor, scale, roundingMode); default: return null; } } public static void main(String[] args) { Arithmetic arithmetic = new Arithmetic(); arithmetic.parse("9+(3-1)*3+10/2"); System.out.println(arithmetic.postFix); System.out.println(arithmetic.compute()); arithmetic = new Arithmetic(); arithmetic.parse("9+(3-1)*3+10/2+1000-200-(100+5-9/3)"); System.out.println(arithmetic.postFix); System.out.println(arithmetic.compute()); arithmetic = new Arithmetic(); arithmetic.parse("9 + (3 - 1) * 3 + 10 / 2 + 1000 - 200 - ( 100 + 5 - 9 / 3)"); System.out.println(arithmetic.postFix); System.out.println(arithmetic.compute()); } }
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