package com.coding.basic.stack.expr;
import java.util.List;
import java.util.Stack;
/**
* 用两个栈实现中序表达式求值
* @author chenming E-mail:cm_20094020@163.com
* @version 创建时间:2017年5月7日 下午4:17:24
*/
public class InfixExpr {
String expr = null;
public InfixExpr(String expr) {
this.expr = expr;
}
public float evaluate() {
TokenParser parser = new TokenParser();
List<Token> tokens = parser.parse(this.expr);
Stack<Token> opStack = new Stack<>();
Stack<Float> numStack = new Stack<>();
for(Token token : tokens){
if (token.isOperator()){
while(!opStack.isEmpty()
&& !token.hasHigherPriority(opStack.peek())){
Token prevOperator = opStack.pop();
Float f2 = numStack.pop();
Float f1 = numStack.pop();
Float result = calculate(prevOperator.toString(), f1,f2);
numStack.push(result);
}
opStack.push(token);
}
if(token.isNumber()){
numStack.push(new Float(token.getIntValue()));
}
}
while(!opStack.isEmpty()){
Token token = opStack.pop();
Float f2 = numStack.pop();
Float f1 = numStack.pop();
numStack.push(calculate(token.toString(), f1,f2));
}
return numStack.pop().floatValue();
}
private Float calculate(String op, Float f1, Float f2){
if(op.equals("+")){
return f1+f2;
}
if(op.equals("-")){
return f1-f2;
}
if(op.equals("*")){
return f1*f2;
}
if(op.equals("/")){
return f1/f2;
}
throw new RuntimeException(op + " is not supported");
}
}