Question

我正在研究我的代码而且我已经陷入困境。我的问题是评估从键盘输入的后缀表示法。

这是我的代码：

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.NoSuchElementException;

    interface Stack<E> {

        // The elements of the Stack are any kind of objects

        // Access methods:

        public boolean isEmpty ();
            // Returns true only if the stack is empty.

        public E peek ();
            // Returns the element on the top od the stack.

        // Transformation methods:

        public void clear ();
            // Clears the stack.

        public void push (E x);
            // Adds x on the top of the stack.

        public E pop ();
            // Removes and returns the element on the top.
    }

    class ArrayStack<E> implements Stack<E> {
        private E[] elems;
        private int depth;

        @SuppressWarnings("unchecked")
        public ArrayStack (int maxDepth) {
            // Creating new empty stack
            elems = (E[]) new Object[maxDepth];
            depth = 0;
        }


        public boolean isEmpty () {
             // Returns true only if the stack is empty.

            return (depth == 0);
        }


        public E peek () {
            // Returns the element on the top od the stack.
            if (depth == 0)
                throw new NoSuchElementException();
            return elems[depth-1];
        }


        public void clear () {
            // Clears the stack.
            for (int i = 0; i < depth; i++)  elems[i] = null;
            depth = 0;
        }


        public void push (E x) {
            // Adds x on the top of the stack.
            elems[depth++] = x;
        }


        public E pop () {
            // Removes and returns the element on the top.
            if (depth == 0)
                throw new NoSuchElementException();
            E topmost = elems[--depth];
            elems[depth] = null;
            return topmost;
        }



    }





    public class PostFixEvaluation {


        static int evaluatePostfix(char [] izraz, int n)
        {
            ArrayStack e;
            char ch;
            int op1,op2,result=0;
            int i=0;
            while(i<n)
            {
               if(Character.isDigit(izraz[i]))
               {
                  ch=izraz[i];
                  e.push(ch); 
               }
                else
               {
                   ch=izraz[i];
                   op1 =(int)e.pop();
                   op2 =(int)e.pop();
                   if(ch=='+')
                   {
                   result=op1+op2;               
                   }
                    if(ch=='-')
                   {
                   result=op1-op2;               
                   }
                    if(ch=='/')
                   {
                   result=op1/op2;               
                   }
                    if(ch=='*')
                   {
                   result=op1*op2;               
                   }

                   e.push(result); 
               }
                i++;

            }
            return result;
        }


        public static void main(String[] args) throws Exception{

            BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

            String expression = br.readLine();
            char exp[] = expression.toCharArray();

            int rez = evaluatePostfix(exp, exp.length);
            System.out.println(rez);

            br.close();

        }

    }

我的问题在于evaluatePostfix函数。我无法使用推送和弹出功能，因为它给了我这个错误：

PostFixEvaluation.java:100：错误：变量e可能尚未初始化 e.push（CH）; ^ PostFixEvaluation.java:105：错误：变量e可能尚未初始化 op1 =（int）e.pop（）; ^ 注意：PostFixEvaluation.java使用未经检查或不安全的操作。

有人可以帮我解决这个问题吗？

static int evaluatePostfix(char [] izraz, int n)
    {
        ArrayStack e;
        char ch;
        int op1,op2,result=0;
        int i=0;
        while(i<n)
        {
           if(Character.isDigit(izraz[i]))
           {
              ch=izraz[i];
              e.push(ch); 
           }
            else
           {
               ch=izraz[i];
               op1 =(int)e.pop();
               op2 =(int)e.pop();
               if(ch=='+')
               {
               result=op1+op2;               
               }
                if(ch=='-')
               {
               result=op1-op2;               
               }
                if(ch=='/')
               {
               result=op1/op2;               
               }
                if(ch=='*')
               {
               result=op1*op2;               
               }

               e.push(result); 
           }
            i++;

        }
        return result;
    }

Answer 1

在函数e

中初始化evaluatePostfix

ArrayStack<Character> e = new ArrayStack(10); // maxDepth = 10

后缀表示法评估

1 个答案: