RPN Calculator

This project provides a command line implementation of an RPN calculator.

What is RPN?

RPN, or "Reverse Polish Notation", is a postfix notation for mathematical operations. The common infix notation places the operator between the operands:

1 + 2

RPN instead places the operator after the operands:

1 2 +

This postfix notation eliminates the need to use parentheses to specify the order of operations. For example, in the following expression using infix notation:

1 + 2 * 3

The multiplication is performed before the addition. To specify that the addition must be performed first, the expression would need to be written as:

(1 + 2) * 3

With RPN, the first expression becomes:

1 2 3 * +

Whereas the second expression would be:

1 2 + 3 *

Implementation and operation

RPN calculators can be easily implemented using a stack. Each number entered into the calculator is pushed onto the stack. Each binary operator (e.g. '+', '*') pops the top two numbers from the stack, performs the calculation, then pushes the result back on the stack.

For example, with the input:

1 2 3 * +

The stack manipulations would appear as follows:

  1      2      3      *      +
  v      v      v      v      v

              +---+
              | 3 |
       +---+  +---+  +---+
       | 2 |  | 2 |  | 6 |
+---+  +---+  +---+  +---+  +---+
| 1 |  | 1 |  | 1 |  | 1 |  | 7 |
+---+  +---+  +---+  +---+  +---+

Likewise, with the input:

1 2 + 3 *

The stack manipulations would appear as:

  1      2      +      3      *
  v      v      v      v      v

       +---+         +---+
       | 2 |         | 3 |
+---+  +---+  +---+  +---+  +---+
| 1 |  | 1 |  | 3 |  | 3 |  | 9 |
+---+  +---+  +---+  +---+  +---+

Goal

Create an RPN calculator that takes a string input and performs the corresponding action.

If the input is a number, push the number on the stack.
If the input is an operator, perform the operation.

The RPN calculator must maintain its stack between inputs. Assume all numbers are integers.

Steps

Update the enter() method so that numbers are pushed onto the stack.
- Strings should be parsed with IntegerParser.
Update the enter() method to handle the addition operator: +. enter() should throw an error if there are not enough operands on the stack.
Update the enter() method to handle the multiplication operator: *. enter() should throw an error if there are not enough operands on the stack.
Update the enter() method so that unknown operations throw an error.

Bonus steps