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Thread: Infix to Prefix

  1. #1
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    Default Infix to Prefix

    Hi i keep receiving the error:

    Exception in thread "main" java.util.EmptyStackException
    at java.util.Stack.peek(Stack.java:85)
    at java.util.Stack.pop(Stack.java:67)
    at calc.Main.calculate(Main.java:147)
    at calc.Main.main(Main.java:251)
    Java Result: 1

    I keep trying to google and read on how to fix it, but i deadlocked. Help pls?

    Heres my code:

     
    /*
     * To change this template, choose Tools | Templates
     * and open the template in the editor.
     */
     
    package calc;
     
    import java.util.*;
    import jpb.*;
     
    public class Main {
     
        private integerStack myStack;
        private String infix;
        private String postfix = "";
     
        //creates an empty stack used for declaring methods
        public Main(String in) {
            infix = in;
    	int size = in.length();
    	myStack = new integerStack(size);
        }
     
        //method to convert infix expression to postfix expression
        public String convert() {
            for (int j = 0; j < infix.length(); j++) {
                char ch = infix.charAt(j);
                switch (ch) {
                    case '+':
                    case '-':
                        checkPrecedence(ch, 1);
                        break;
                    case '*':
                    case '/':
                        checkPrecedence(ch, 2);     // pop operators
                        break;
                    case '%':
                        checkPrecedence(ch,3);
                        break;
                    case '(':
                        myStack.push(ch);           // push it
                        break;
                    case ')':
                        parentheses(ch);            // pop operators
                        break;
                    default:                        // must be an operand
                        postfix = postfix + ch;     // append to output
                        break;
                }
            }
            while (!myStack.isEmpty()) {
                postfix = postfix + myStack.pop();
    	}
            System.out.println(postfix);
    	return postfix;                             // return postfix
        }
     
        //method to order each operator encountered by the correct precedence
        public void checkPrecedence(char opThis, int p1) {
            while (!myStack.isEmpty()) {
                char opTop = myStack.pop();
                if (opTop == '(') {
                    myStack.push(opTop);
                    break;
                 }                                           // it's an operator
                 else {                                      // precedence of new op
                    int p2;
                    if (opTop == '+' || opTop == '-')
                        p2 = 1;
                    else if (opTop=='*' || opTop=='/')
                        p2=2;
                    else
                        p2=3;
                    // if precendence of new op less than precendence of old save newly-popped op
                    if (p2 < p1){
                        myStack.push(opTop);
                        break;
                    }
                    //else append to postfix
                    else
                        postfix = postfix + opTop;
                }
            }
    	myStack.push(opThis);
        }
     
        //method to grant expressions inside parentheses top precedence
        public void parentheses(char ch){
            while (!myStack.isEmpty()) {
                char paren = myStack.pop();
                if (paren == '(')
                    break;
                else
                    postfix = postfix + paren;
    	}
        }
     
        //method to calculate postfix expression
        public static int calculate(String input) throws Exception {
            int finalResult;
            Stack <Integer> solution = new Stack<Integer>();
            int op1,op2,result;
     
            for(int i=0;i<input.length();i++){
                char c=input.charAt(i);
                try {
                    switch (c){
                        case '+':
                            op1=solution.pop();
                            op2=solution.pop();
                            result=op1 + op2;
                            solution.push(result);
                            break;
                        case '-':
                            op1=solution.pop();
                            op2=solution.pop();
                            result=op1 - op2;
                            solution.push(result);
                            break;
                        case '*':
                            op1=solution.pop();
                            op2=solution.pop();
                            result=op1 * op2;
                            solution.push(result);
                            break;
                        case '/':
                            op1=solution.pop();
                            op2=solution.pop();
                            result=op1 / op2;
                            solution.push(result);
                            break;
                        case '%':
                            op1=solution.pop();
                            op2=solution.pop();
                            result=op1 % op2;
                            solution.push(result);
                            break;
                        default:
                            solution.push(Integer.parseInt(Character.toString(c)));
                            break;
                    }
                }
     
                catch(EmptyStackException e){
                    System.exit(-1);
                }
     
            }        
            finalResult = solution.pop();
            return finalResult;
     
     
        }
     
        //method to check if number of operators is more than number of operands
        public static boolean checkOps(String input){
            int countNumbers=0;
            int countOperators=0;
     
            for(int i=0;i<input.length();i++){
                char ops=input.charAt(i);
                switch(ops){
                    case '+':case '-':case '*':case '/': case '%':
                        countOperators++;break;
                    default: countNumbers++;
                }
            }
            if(countNumbers<=countOperators)
                return false;
            else
                return true;
        }
     
        //method to check if infix contains floating point numbers.
        public static boolean checkFloat(String input){
            char [] c=input.toCharArray();
     
            for (int i=0;i<c.length;i++){
                if (c[i]=='.'){
                return false;}
            }
            return true;
        }
     
        //method to check if infix expression is in correct form
        public static boolean checkInfix(String input){
            char[] c=input.toCharArray();
     
            for(int i=0;i<c.length;i++){
                if(Character.isDigit(c[i]) && Character.isDigit(c[i+1])){
                    return false;
                }
                else if (c[i]=='(' && !Character.isDigit(c[i+1]))
                    return false;
                else
                    return true;
            }
            return true;
        }
     
        public static void main(String[] args) throws Exception {
            while(true){
                SimpleIO.prompt("Enter an infix expression to evaluate: \n");
                String input = SimpleIO.readLine();
     
                //if user enters float show error message and prompt for another expression
                while(!checkFloat(input)){
                    System.out.println("Error in Expression! Cannot accept "
                            + "floating numbers \n");
                    SimpleIO.prompt("Enter an infix expression to evaluate: \n");
                    input = SimpleIO.readLine();
                }
     
                //if user enters incorrect formatted infix show error message and prompt for another expression
                while(!checkInfix(input)){
                    System.out.println("Error in Expression! No operator between "
                            + "operands. Also last token must be an operand \n");
                    SimpleIO.prompt("Enter an infix expression to evaluate: \n");
                    input = SimpleIO.readLine();
                }
     
                //if user enters too many operators show error message and prompt for another expression
                while(!checkOps(input)){
                    System.out.println("Error in Expression! Operator cannot "
                            + "be preceded by an operator \n");
                    SimpleIO.prompt("Enter an infix expression to evaluate: \n");
                    input = SimpleIO.readLine();
                }
     
                //takes a value for x and creates new infix with entered value
                char[] c=input.toCharArray();
                String input2="";
     
                for(int i=0;i<c.length;i++){
                    if(c[i]=='x'){
                        SimpleIO.prompt("Enter a value for x: ");
                        c[i]=SimpleIO.readLine().charAt(0);
     
                        for(int j=0;j<c.length;j++){
                            input2+=c[j];
                        }
     
                        if(!Character.isDigit(c[i])){
                            System.out.println("You did not enter an integer "
                                    + "value for x ");
                            System.exit(-1);
                        }
                    }
                }
     
                String output;
                Main theTrans = new Main(input2);
                output = theTrans.convert();
                int result= calculate(output);
     
                System.out.println("Postfix is: " + output );
                System.out.println("Postfix evaluated is: " + result +'\n');
                SimpleIO.prompt("Evaluate another expression? (y/n)");
     
                if (SimpleIO.readLine().equals("n")){
                    System.exit(0);
                }
            }
        }
     
        //class to build stack which host integers and char
        class integerStack {
            private char[] stackArray;
    	private int top;
    	public integerStack(int size) {
                stackArray = new char[size];
    	    top = -1;
    	}
     
            public void push(char j) {
                stackArray[++top] = j;
    	}
     
    	public char pop() {
                return stackArray[top--];
    	}
     
    	public char peek() {
                return stackArray[top];
            }
     
    	public boolean isEmpty() {
                return (top == -1);
            }
        }
    }


  2. #2
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    Default Re: Infix to Prefix

    sorry the error occurs at:

    finalResult = solution.pop(); //line 147
     
    //........
     
    int result= calculate(output); //line 251

  3. #3
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    Default Re: Infix to Prefix

    Yeah and the problem is, your stack is empty and you want to pop it.
    You must implement, isEmpty() and isFull() for stack class.
    And call isFull() before pushing data and isEmpty() before poping.

    Also, do you have clear understanding of stack concept?

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    Default Re: Infix to Prefix

    Quote Originally Posted by Mr.777 View Post
    You must implement isFull() for stack class.
    ???

    Since when have Stacks had an isFull method?
    Improving the world one idiot at a time!

  5. #5
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    Default Re: Infix to Prefix

    Quote Originally Posted by Junky View Post
    ???

    Since when have Stacks had an isFull method?
    What if OP had not implemented the circular stack?
    And what if his stack is, let's say of size 10 and there are already 10 values in it????

  6. #6
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    Default Re: Infix to Prefix

    The exception provides a lot of information - you are trying to pop a stack that is empty.

    The Stack class does not contain an isFull method, and implementing one would not solve your problem. A circular stack is beside the point - you are accessing the J2SE class Stack which is the problem (at least the problem as it manifests itself)

    Your code is not an SSCCE, so I cannot run it. That being said, inspect the calculate method and make sure your code adds to the stack as appropriate. Add some println's in there, or use a debugger, to step through the code and make sure it behaves properly.

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    Default Re: Infix to Prefix

    Quote Originally Posted by copeg View Post
    The Stack class does not contain an isFull method, and implementing one would not solve your problem.
    Why stack class can not have a isFull() method? Kindly give me some reasons. As far as i know, if you are playing with a fixed size stack, you must check it somehow, whether stack is full or not, so that you could push() data into it.

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    Default Re: Infix to Prefix

    Quote Originally Posted by Mr.777 View Post
    Why stack class can not have a isFull() method? Kindly give me some reasons. As far as i know, if you are playing with a fixed size stack, you must check it somehow, whether stack is full or not, so that you could push() data into it.
    It can, but like Junky and I said the java.util.Stack class does not contain this method ( Stack (Java Platform SE 6) ), and this is where the exception is being thrown - from a call to pop on a java.util.Stack instance.

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    Default Re: Infix to Prefix

    Well copeg, i was not talking about java.util.Stack, i know it doesn't implement isFUll(), infact if i ever implement a stack, i always use isFull() method.
    Anyways, thanks Copeg and Junky

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