package 表达式求值;
import java.util.Stack;
/*
 * 中序表达式求值实现
 */
public class CenterExpression {
 public double evaluate(String expression){ //传入中序表达式
  char [] ex = expression.toCharArray();
  Stack num = new Stack<>();
  Stack ops = new Stack<>();
  for(int i = 0; i < ex.length; i++){  //循环将表达式依次入栈
   char c = ex[i];
   if(c < '9' && c > '0'){
    num.push(Double.parseDouble(Character.toString(c)));
   }
   else if(c == '('){
    ops.push('(');
   }
   else if(c == ')'){
    while(true){
     char op = ops.pop();
     if(op == '('){
      break;
     }
     else{
      switch(op){
      case '+':num.push(num.pop()+num.pop());break;
      case '-':num.push(num.pop()-num.pop());break;
      case '*':num.push(num.pop()*num.pop());break;
      case '/':num.push(num.pop()/num.pop());break;
      default:break;
      }
     }
    }
   }
   else if(ops.empty() && (c == '+' || c == '-' || c == '*' || c == '/')){
    ops.push(c);
   }
   else if(!ops.isEmpty() && (c == '+' || c == '-' || c == '*' || c == '/')){
    char op =ops.peek();
    while((op == '*' || op == '/') && (c == '+' || c == '-')){
     op = ops.pop();
     switch(op){
     case '+':num.push(num.pop()+num.pop());break;
     case '-':num.push(num.pop()-num.pop());break;
     case '*':num.push(num.pop()*num.pop());break;
     case '/':num.push(num.pop()/num.pop());break;
     default:break;
     }
     if(ops.isEmpty()){
      break;
     }
     else{
      op = ops.peek();
     }
    }
    ops.push(c);
   }
  }
  while(!ops.isEmpty()){  //处理剩余可以按计算机扫描顺序处理的表达式
   char op =ops.pop();
   switch(op){
   case '+':num.push(num.pop()+num.pop());break;
   case '-':num.push(num.pop()-num.pop());break;
   case '*':num.push(num.pop()*num.pop());break;
   case '/':num.push(num.pop()/num.pop());break;
   default:break;
   }
  }
  return num.pop();
 }
 public static void main(String [] args){
  CenterExpression exp = new CenterExpression();
  System.out.println(exp.evaluate("1*2+5*3"));
 }
}