java 使用堆栈（LinkedList）将中缀转换为前缀

Converting infix to prefix using a Stack (LinkedList)

提问by Noxob

I am trying to write a method that converts infix to prefix and to do that i want to read a sting reverse and use a stack. When i execute this code i am getting an exception atcharacter = expression.charAt(limit);how can i fix that code?

我正在尝试编写一种将中缀转换为前缀的方法，为此我想读取一个反向并使用堆栈。当我执行此代码时，我收到一个异常，character = expression.charAt(limit);我该如何修复该代码？

My input was1+3and the error i got was:

我的输入是1+3 ，我得到的错误是：

Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 3 at java.lang.String.charAt(Unknown Source) at PrefixTranslator.translate(PrefixTranslator.java:27) at PrefixTranslatorTest.main(PrefixTranslatorTest.java:11)

线程“main”中的异常 java.lang.StringIndexOutOfBoundsException：字符串索引超出范围：3 at java.lang.String.charAt(Unknown Source) at PrefixTranslator.translate(PrefixTranslator.java:27) at PrefixTranslatorTest.main(PrefixTranslatorTest.java) :11)

PrefixTranslator Class:

PrefixTranslator 类：

public class PrefixTranslator
{

static private String expression;
private MyStack<Character> stack = new MyStack<Character>();

//Constructor
public PrefixTranslator(String infixExpression)
{
        expression = infixExpression;
}//End of constructor



public String translate()
{
    //Declare Method Variables
    String input = "";
    String output = "";
    char character = ' ';
    char nextCharacter = ' ';


    for(int limit = expression.length(); limit > 0 ; limit--)
    {
        character = expression.charAt(limit);

        if(isOperator(character))
        {
            output = output + character + " ";
        }   
        else if(character == '(')
        {
            stack.push(character);
        }
        else if(character == ')')
        {
            while(!stack.top().equals('('))
                output = output + stack.pop() + " ";
            stack.pop();
        }
        else
        {
            if(Character.isDigit(character) && (limit + 1) < limit && Character.isDigit(expression.charAt(limit+1)))
            {
                stack.push(character);
                stack.push(expression.charAt(limit+1));
            }
            else if(Character.isDigit(character))
            {
                stack.push(character);
            }   
            else
            {
                output = output + character;
            }
        }
    }//End of for

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

    return output;
}//End of translate method

//Check priority on characters
public static int precedence(char operator)
{
    if(operator == '+' || operator =='-')
        return 1;
    else if(operator == '*' || operator == '/')
        return 2;
    else
        return 0;
}//End of priority method

public boolean isOperator(char element)
{
    if(element == '*' || element == '-' || element == '/' || element == '+')
        return true;
    else
        return false;
}//End of isOperator method

}//End of class

PrefixTranslatorTest Class:

PrefixTranslatorTest 类：

import java.util.Scanner;

public class PrefixTranslatorTest{

    public static void main(String[] args)
    {
        Scanner input = new Scanner(System.in);
        System.out.println("Enter the expression that you want to convert to prefix.");
        String expression = input.next();
        PrefixTranslator translator = new PrefixTranslator(expression);
        System.out.println(translator.translate());


    }

}

回答by cassiomolin

The first index of Java arrays is 0and the method length()returns the number of elements of array. If you need the last element's index, use length() - 1.

Java 数组的第一个索引是0，该方法length()返回数组的元素数。如果您需要最后一个元素的索引，请使用 length() - 1.

So instead of

所以代替

for (int limit = expression.length(); limit > 0; limit--)

use

利用

for (int limit = expression.length() - 1; limit >= 0; limit--)

回答by ?smet Alkan

For the given input 1+3expression.length()returns 3and you can refer to indexes 0-2of that string. So, your loop shouldn't be:

对于给定的输入1+3expression.length()返回3，您可以参考0-2该字符串的索引。所以，你的循环不应该是：

for(int limit = expression.length(); limit > 0 ; limit--)

And it should be

它应该是

for(int limit = expression.length() - 1; limit >= 0 ; limit--)