java - 使用堆栈(链表)评估 Postfix

Question

我需要使用链表堆栈来计算后缀表达式。我想我需要一些关于算法的帮助。我写 13+ 作为输入，但我得到100作为输出。

PostfixCalculator 类:

public class PostfixCalculator{
    String expression;
    MyStack stack = new MyStack<Double>();

    public PostfixCalculator(String postFixExpression)
    {
         expression = postFixExpression;
    }

    public String calculate()
    {
        String output = "";
        char character = ' ';
        double digit = 0;

        for(int x = 0; x < expression.length(); x++)
        {
            if(Character.isDigit(expression.charAt(x))) {
                    digit = expression.charAt(x);
            }
            character = expression.charAt(x);
            if(expression.charAt(x) == digit)
            {
                stack.push(digit);
            }
            else if(character == '*')
            {
                double tmp = (double) stack.pop() * (double) stack.pop();
                stack.push(tmp);
            }
            else if(character == '/')
            {
                double tmp = (double) stack.pop() / (double) stack.pop();
                stack.push(tmp);
            }
            else if(character == '+')
            {
                double tmp = (double) stack.pop() + (double) stack.pop();
                stack.push(tmp);
            }
            else if(character == '-')
            {
                double tmp = (double) stack.pop() - (double) stack.pop();
                stack.push(tmp);
            }
        }

        while(!stack.isEmpty())
        {
            output = output + (double) stack.pop();
        }

        return output;
    }
}

PostfixCalculatorTest 类:

import java.util.Scanner;

public class PostfixCalculatorTest
{

    public static void main(String[] args)
    {
        Scanner input = new Scanner(System.in);
        System.out.println("Type the postfix expression that you want to evaluate");
        String expression = input.next();
        PostfixCalculator calculator = new PostfixCalculator(expression);
        System.out.println(calculator.calculate());
    }
}

Answer 1

首先这个

if(Character.isDigit(expression.charAt(x))) {
     digit = expression.charAt(x);
}

保存小数ASCII位置 x 处的字符值为 double，对于字符 '1' 为 49，对于 '3' 为 51，因此您得到 100 结果

应该是

digit = Double.parseDouble("" + expression.charAt(x));

即解析 char 以获得它代表的 double 值。

这是一个小变化

character = expression.charAt(x);
if(Character.isDigit(character)) {
    digit = Double.parseDouble("" + character);
    stack.push(digit);
}

那么它将适用于 13+ 并给出 4 作为结果。

这些行可以删除:

character = expression.charAt(x);
if(expression.charAt(x) == digit)
{
    stack.push(digit);
}