JAVA-Why/Where am I getting an Array Index out of bound error while trying to covert an infix to post fix expression?

Question

Here is my code for converting infix to posfix expression. The problem is in the conversion method. I am getting error on line 21.

Error-ab+ Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 3 >= 0

at java.util.Vector.elementAt(Unknown Source)
at infixpostfix.conversion(infixpostfix.java:22)
at infixpostfix.main(infixpostfix.java:77)

for the current input.

I think that this error appears because I am trying to access i when its exceeding the length of my string. But how is it possible when my i's value is being governed by the for loop?

Code:-

import java.util.Stack;
public class infixpostfix {

    Stack<Character> st = new Stack<Character>();

    void conversion(String e){


        for(int i = 0 ; i < e.length() ; i++){

            if(isOp(e.charAt(i)) ){

                if(st.isEmpty()){

                    st.push(e.charAt(i));
                }

                else{
                    while(!st.isEmpty() && checkPrec(st.peek())<=checkPrec(e.charAt(i))){
                        System.out.println(st.pop());
                    }
                    st.push(st.elementAt(i));
                }
            }

            else{

                System.out.print(e.charAt(i));
            }

        }

        while(!st.isEmpty()){

            System.out.print(st.pop());

        }
    }

    int checkPrec(char o){

        switch(o){

        case '+':
        case '-':

            return 1;

        case '*':
        case '/':

            return 2;

        default:

            return -1;
        }
    }

    boolean isOp(char c){

        if(c=='+' || c=='-' || c=='/' || c=='*'){

            return true;
        }

        else{

            return false;
        }
    }

    public static void main(String args[]){

        infixpostfix obj = new infixpostfix();

        obj.conversion("a+b-c/d*f");
    }
}

Answer 1

From what I understand, i represents an index in the string. However, here:

st.push(st.elementAt(i));

you are using at as an index to your vector. My guess is what you meat was:

st.push(e.charAt(i));