EmptyStackException

java, stack

Solution

`String in = input.next();` reads you one word, then you are trying to tokenise that word. Perhaps you meant `String in = input.nextLine();`

http://docs.oracle.com/javase/1.5.0/docs/api/java/util/Scanner.html#next() http://docs.oracle.com/javase/1.5.0/docs/api/java/util/Scanner.html#nextLine()

Also, you have these two lines in your code.

if (!stack.isEmpty())
  // throw new StackUnderflowException("Empty Stack");

This is plain wrong. Without its curly braces, the `if` affects the next statement. It is not the comment - it is the following if.

This:

if (!stack.isEmpty())
// throw new StackUnderflowException("Empty Stack");

// Base case: stack has 1 element, which is the answer => finished
if (stack.size() == 1)
    System.out.printf("Finished, Answer: %s\n",stack.peek());

is equivalent to this:

if (!stack.isEmpty())
    if (stack.size() == 1)
        System.out.printf("Finished, Answer: %s\n",stack.peek());

and this:

if (!stack.isEmpty() && stack.size() == 1){
    System.out.printf("Finished, Answer: %s\n",stack.peek());
}

Moral: always use curly brackets with an `if` AND don't comment out assertions. Even if you do comment out assertions, comment them completely, and not one half of them, epecially when the other half is an un-bracketed if.

Third, your logic is flawed. You do this:

push all symbols to the stack, then pop the top three and consider them an operator and two numbers. This will work with some inputs if you use a queue instead.

4 5 * 6 -

By your logic, this will pop `* 6 -` and crash. If you use a queue, it will work in this case

4 5 * 6 - 
20 6 -
14

But not is this case:

(1+1)*(1+1)
express as RPN
1 1 + 1 1 + *
2 1 1 + *

Next, you pop 2 1 1 and crash.

Instead, what you should do:

Read the input. For each symbol:
  if it is a number,
    push it on the stack.
  else,
    pop two numbers from the stack,
    perform the operation and
     push the result.

Problem

This EmptyStackException continues to pop up. Obliviously there is nothing in my stack, but the first element that the User inputs. However, I am not sure where the code is flawed. (many spots) but I just need to fix this error. ``` import java.util.*; public class stacks2 { public static void main (String []args){ System.out.printf("Enter a math equation in reverse polish notation:\n"); //Create stack of Strings Stack<String> rpnStack = new Stack<String>(); //Create Scanner Scanner input = new Scanner(System.in); //String in = input.next(); while(input != null) { String in = input.next(); // Tokenize string based on spaces. StringTokenizer st = new StringTokenizer(in, " ", true); while (st.hasMoreTokens()) { rpnStack.push(st.nextToken()); } //Send stack to Calculation Method calculate(rpnStack); } } public static void calculate(Stack<String> stack) { // Base case: stack is empty => Error, or finished if (!stack.isEmpty()) // throw new StackUnderflowException("Empty Stack"); // Base case: stack has 1 element, which is the answer => finished if (stack.size() == 1) System.out.printf("Finished, Answer: %s\n",stack.peek()); // Recursive case: stack more elements on it. if (stack.size() > 1){ String temp1 = stack.peek(); stack.pop(); String temp2 = stack.peek(); stack.pop(); String temp3 = stack.peek(); stack.pop(); if (temp3.equals("+")){ float resultant = Float.parseFloat(temp1) + Float.parseFloat(temp2); stack.push(String.valueOf(resultant)); //System.out.println(resultant); calculate(stack); } if (temp3.equals("-")){ float resultant = Float.parseFloat(temp1) - Float.parseFloat(temp2); stack.push(String.valueOf(resultant)); //System.out.println(resultant); calculate(stack); } else if (temp3.equals("*")){ float resultant = Float.parseFloat(temp1) * Float.parseFloat(temp2); stack.push(String.valueOf(resultant)); //System.out.println(resultant); calculate(stack); } else if (temp3.equals("/")){ float resultant = Float.parseFloat(temp1) / Float.parseFloat(temp2); stack.push(String.valueOf(resultant)); //System.out.println(resultant); calculate(stack); } else{ System.out.printf("Something severely has gone wrong."); } } } } ``` the input and error: ``` :~ Home$ java stacks2 Enter a math equation in reverse polish notation: 4 5 * 6 - Finished, Answer: 4 Exception in thread "main" java.util.EmptyStackException at java.util.Stack.peek(Stack.java:85) at stacks2.calculate(stacks2.java:41) at stacks2.main(stacks2.java:22) ``` clearly this is only taking the first element which makes me think my while loop at 17 is the cause. Any insight?

Original source