java堆栈下溢

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我正在完成这项任务并在线程中不断收到异常"main" java.lang.RuntimeException: Stack Underflow at Stack.pop(Postfix.java:74)在 Postfix.eval(Postfix.java:221)at Postfix.main(Postfix.java:112)

im work this assignment and keep getting Exception in thread "main" java.lang.RuntimeException: Stack Underflow at Stack.pop(Postfix.java:74) at Postfix.eval(Postfix.java:221)at Postfix.main(Postfix.java:112)

不知道为什么我查看堆栈并正确写入，我看不出为什么它在 (3*4)/5 时弹出的问题

dont know why i look at the stack and write it correct , i cant see problem why it pop when (3*4)/5

import java.io.IOException; class CharStack { private final int STACKSIZE= 80; private int top; private char[] items; public CharStack(){ items = new char[STACKSIZE]; top =-1; } public boolean empty() { if(top==-1){ return true; } return false; } public char pop() { if(empty()){ throw new RuntimeException("Stack Underflow"); } return items[top--]; } public void push(char symb) { if(top == STACKSIZE -1) { throw new RuntimeException("Stack Overflow"); } items[++top] =symb; } public char peek() { if(empty()){ throw new RuntimeException("Stack Underflow"); } return items[top]; } } class Stack { private final int STACKSIZE= 80; private int top; private double[] items; public Stack(){ items = new double[STACKSIZE]; top =-1; } public void push(double x) { if(top == STACKSIZE -1) { throw new RuntimeException("Stack Overflow"); } items[++top] =x; } public double pop(){ if(empty()){ System.out.print(top); throw new RuntimeException("Stack Underflow"); } return items[top--]; } public double peek() { if(empty()){ throw new RuntimeException("Stack Underflow"); } return items[top]; } boolean empty() { if(top==-1){ return true; } return false; } } public class Postfix { public final static int MAXCOLS = 80; public static void main(String[] args) throws IOException { String infix, pfix; System.out.println("Enter a infix String: "); infix = readString().trim(); System.out.println("The original infix expr is: " + infix); pfix = postfix(infix); System.out.println("The Postfix expr is: " + pfix); System.out.println("The value is : " + eval(pfix)); } // end main public static boolean isOperand(char x) { if(x == '+') { return false; } else if(x == '-') { return false; } else if (x == '*') { return false; } else if (x == '/') { return false; } else if ( x== '$') { return false; } return true; } public static int operPrecedence(char oper) { if(oper == '+'||oper == '-' ) { return 1; } else if (oper == '*' || oper == '/') { return 2; } else if (oper == '$') { return 3; } return 0; } public static boolean precedence(char top, char symb) { if ((top != '('||top != ')')&&symb == '(') { return false; } if (top == '(' && (symb != '('||symb != ')') ) { return false; } else if((top != '('||top != ')')&&symb ==')' ) { return true; } int opcode1, opcode2; opcode1 =operPrecedence(top) ; opcode2 =operPrecedence(symb) ; if(opcode1>=opcode2){ return true; } return false; } public static String readString() throws IOException { char[] charArray = new char[80]; int position = 0; char c; while ((c = (char) System.in.read()) != '\n') { charArray[position++] = c; } return String.copyValueOf(charArray, 0, position); // turns a character array into a string, starting between zero and position-1 }// end read string public static double eval(String infix) { char c; int position; double opnd1, opnd2, value; Stack opndstk = new Stack(); for (position = 0; position < infix.length(); position++) { c = infix.charAt(position); if (Character.isDigit(c)) // operand-convert the character represent of // the digit into double and push it into the // stack { opndstk.push((double) Character.digit(c, 10)); } else { // operator opnd2 = opndstk.pop(); opnd1 = opndstk.pop(); value = oper(c, opnd1, opnd2); opndstk.push(value); } // else } // end for return opndstk.pop(); }// end eval public static String postfix(String infix) { int position, outpos = 0; char symb; char[] postr = new char[MAXCOLS]; CharStack opstk = new CharStack(); for (position = 0; position < infix.length(); position++) { symb = infix.charAt(position); if (isOperand(symb)) { postr[outpos++] = symb; } else { while (!opstk.empty() && precedence(opstk.peek(), symb)) { postr[outpos++] = opstk.pop(); } // end while if (symb != ')') { opstk.push(symb); } else { opstk.pop(); } } // end else } // end for while (!opstk.empty()) { postr[outpos++] = opstk.pop(); } return String.copyValueOf(postr, 0, outpos); }// end pos public static double oper(char symb, double op1, double op2) { double value = 0; switch (symb) { case '+': value = op1 + op2; break; case '-': value = op1 - op2; break; case '*': value = op1 * op2; break; case '/': value = op1 / op2; break; case '$': value = Math.pow(op1, op2); break; default: throw new RuntimeException("illegal operator: " + symb); }// end switch return value; }// end oper }

推荐答案

您遇到的至少部分问题是您的 isOperand 方法.字符 ( 和 ) 不是操作数，但是，当它们传递给这个方法时，它会返回 true.为了快速测试，我在方法的末尾添加了以下几行:

At least part of the problem you're having is your isOperand method. The characters ( and ) are not operands, however, when they are passed to this method, it would return true. For a quick test, I added the following lines to the end of the method:

else if (x == '(') { return true; } else if (x == ')') { return true; }

您的示例输入 (3*4)/5) 运行成功.但是，这会破坏您的后缀输出，因为它将括号从后缀版本中删除，而是打印 34*5/，我猜您不想要.

And your example input, (3*4)/5) runs successfully. However, this breaks your postfix output, as it leaves the brackets out of the postfix version and instead prints 34*5/, which I am guessing you don't want.

然后，根据我收到的错误消息，我查看了您的 eval 方法，这是问题的来源:

Then, I looked at your eval method, which is where the problem is coming from, according to the error message I'm receiving:

Exception in thread "main" java.lang.RuntimeException: Stack Underflow at Stack.pop(Postfix.java:74) at Postfix.eval(Postfix.java:221) at Postfix.main(Postfix.java:112)

注意行 Postfix.java:221，它表示调用产生错误的方法的行.如果您在调用该行之前输出字符 c，您会注意到 c 是 ( 字符，这意味着您的 eval 方法正在将 ( 识别为运算符，并试图在其后弹出两个操作数，从而导致下溢.

Note the line Postfix.java:221, which indicates the line that called the method which created the error. If you output your character c right before that line is called, you'll notice that c is the ( character, which means your eval method is recognizing ( as an operator, and is attempting to pop two operands after it, causing your underflow.

通过一些 System.out.println() 调用并查看您的错误，可以很容易地确定所有这些.我会把实际的修复留给你，但至少你现在希望有一个前进的方向.

All of this is fairly simple to determine with some System.out.println() calls, and looking at your error. I'll leave the actual fixing to you, but at least you've hopefully got a direction to head in now.