java堆栈下溢 [英] java stack Underflow
问题描述
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.
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