2.) Postfix to infix translator (NOTE: This is NOT the evaluator you have already coded!) Create a java class called PostfixToInfixTranslator that includes a main method. The code you write should prompt the user for an expression in postfix notation and use ArrayStack to output the equivalent expression in infix. See the following session: Enter a postfix expression: 3 4 + 2 * In infix notation that is: ((3+4)*2) Translate another expression [y/n]? y Enter a postfix expression: 4 2 + 3 5 1 - * + In infix notation that is: ((4+2)+(3*(5-1))) Translate another expression [y/n]? n NOTE: The postfix expression has to accept spaces, double digit numbers, and negative numbers. So .charAt() is not a good approach. Maybe a scanner on a string?

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StackADT, EmptyCollectionException, and ArrayStack code already below, please use these.

public interface StackADT //Provide layout of what we want something to do w/o supplying the way it gets done = interface { public void push( T element) ; public T pop () throws EmptyCollectionException; public T peek() throws EmptyCollectionException; public boolean isEmpty(); public int size(); }

public class EmptyCollectionException extends Exception { private static final long serialVersionUID = 358083002087971606L;

public EmptyCollectionException() { super(); } public EmptyCollectionException(String msg) { super(msg); } }

import java.util.Arrays;

public class ArrayStack implements StackADT //We agree to supply all functions that we say exist StackADT Functions correlate to ArrayStack { private final static int DEFAULT_CAPACITY = 100; private int top; private T[] stack; public ArrayStack() { this(DEFAULT_CAPACITY); } @SuppressWarnings("unchecked") public ArrayStack(int initialCapacity) { top = 0; stack = (T[]) new Object [initialCapacity]; //Have to cast Object Array to T array in order for placeholder to work. Java just works like that. } public void push( T element) { if(size() == stack.length) expandCapacity(); stack [top] = element; ++top; } public T pop() throws EmptyCollectionException { if(isEmpty()) throw new EmptyCollectionException(); --top; T result = stack[top]; stack [top] = null; // Top before that ends up being empty; Makes it clean return result; } public T peek() throws EmptyCollectionException { if(isEmpty()) throw new EmptyCollectionException(); return stack[top -1]; } public int size() { return top; } public boolean isEmpty() { // Even shorter return top ==0; // Short // boolean a = top == 0; // return a; // long way // if(top==0) // return true; // else // return false; } public void expandCapacity() { stack = Arrays.copyOf(stack, stack.length * 2); //Would values stay the same but only place in memory changes? Hexa decimal #'s? } public String toString() { String output = "ArrayStack-> [ "; for(int i =top-1; i >=0 ; i--) { output+= stack[i].toString() + " "; } output+= "]"; return output; } }

WHAT I ALREADY HAVE CODED IN TRANSLATOR, PLEASE HELP. I don't know how to allow spaces, double digits, and negative numbers, and also how to loop the scanner input.

import java.util.Scanner; import java.util.Stack;

public class PostfixToInfixTranslator {

public static void main(String[] args) throws EmptyCollectionException { PostfixToInfixTranslator obj = new PostfixToInfixTranslator(); Scanner sc = new Scanner(System.in); System.out.print("Postfix : "); String postfix = sc.next(); System.out.println("Infix : " +obj.convert(postfix));

}

private boolean isOperator(char c) { if(c == '+' || c== '-' || c== '*' || c=='/' || c=='^') return true; return false; } public String convert(String postfix) throws EmptyCollectionException { ArrayStack s = new ArrayStack<>(); for(int i =0; i < postfix.length(); i++) { char c = postfix.charAt(i); if(isOperator(c)) { String b = s.pop(); String a = s.pop(); s.push("("+a+c+b+")"); } else s.push(""+c); } return s.pop(); }

}